SUMMIT INVESTORS GE IX/VC IV (UK), L.P.

SEC source: 0001193125-26-349311 · ownership.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 13,852,778

Underlying: Series A Common Stock, par value $0.001 per share · 5,000,000 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F5 · balance_after: Consists of (i) 8,187,302 shares of Series B Common Stock held by Fund IX-A, (ii) 5,112,037 shares of Series B Common Stock held by Fund IX-B, (iii) 504,825 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 43,030 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 5,584 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001193125-26-349311 · ownership.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001193125-26-349311 · ownership.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 5,000,000

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-113988 · ownership.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 18,852,778

Underlying: Series A Common Stock, par value $0.001 per share · 4,000,000 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F5 · balance_after: Consists of (i) 11,142,414 shares of Series B Common Stock held by Fund IX-A, (ii) 6,957,168 shares of Series B Common Stock held by Fund IX-B, (iii) 687,035 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 58,561 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 7,600 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-113988 · ownership.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-113988 · ownership.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 4,000,000

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-25-023115 · form4-08272025_080802.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 22,852,778

Underlying: Series A Common Stock, par value $0.001 per share · 975,000 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F5 · balance_after: Consists of (i) 13,506,504 shares of Series B Common Stock held by Fund IX-A, (ii) 8,433,272 shares of Series B Common Stock held by Fund IX-B, (iii) 832,803 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 70,986 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 9,213 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-25-023115 · form4-08272025_080802.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-25-023115 · form4-08272025_080802.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 975,000

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-25-022457 · form4-08202025_080825.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · shareholder

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · shareholder

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · Owner

Summit Partners Entrepreneur Advisors Fund II, L.P. · CIK 0001660981 · Owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Direct

Disclosed balance after: 33,524,715

F1 · shares: Represents 5,148,494 shares sold by Summit Partners Growth Equity Fund IX-A, L.P., 3,214,648 shares sold by Summit Partners Growth Equity Fund IX-B, L.P., 35,938 shares sold by Summit Investors GE IX/VC IV, LLC, 4,804 shares sold by Summit Partners Entrepreneur Advisors Fund II, L.P. and 3,759 shares sold by Summit Investors GE IX/VC IV (UK), L.P.

F2 · balance_after: The reported securities are held as follows: 20,529,149 shares in the name of Summit Partners Growth Equity Fund IX-A, L.P.; 12,818,117 shares in the name of Summit Partners Growth Equity Fund IX-B, L.P.; 143,301 shares in the name of Summit Investors GE IX/VC IV, LLC; 19,157 shares in the name of Summit Partners Entrepreneur Advisors Fund II, L.P.; and 14,991 shares in the name of Summit Investors GE IX/ VC IV (UK), L.P.

F3 · ownership.direct: Summit Partners, L.P. is the managing member of Summit Partners GE IX, LLC, which is general partner of Summit Partners GE IX, L.P., which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. and Summit Partners Growth Equity Fund IX-B, L.P. Summit Master Company, LLC is (i) the sole member of Summit Partners Entrepreneur Advisors GP II, LLC, which is the general partner of Summit Partners Entrepreneur Advisors Fund II, L.P. and (ii) the general partner of Summit Partners L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC, and the general partner of Summit Investors GE IX/VC IV (UK), L.P.

F4 · ownership.direct: Summit Master Company, LLC, as the sole member of Summit Partners Entrepreneur Advisors GP II, LLC and the managing member of Summit Investors Management, LLC, has delegated investment decisions, including voting and dispositive power, to Summit Partners, L.P. and its investment committee responsible for voting and investment decisions with respect to LifeStance Health Group, Inc. Summit Partners, L.P., through a four-person investment committee, currently composed of Peter Y. Chung, Mark A. deLaar, Darren M. Black and Craig D. Frances, has voting and dispositive authority over the shares held by each of these entities and therefore beneficially owns such shares. Each of the Summit Funds, Mr. Chung, Mr. deLaar, Mr. Black and Dr. Frances disclaim beneficial ownership of the shares, except, in each case, to the extent of such person's or entity's pecuniary interest therein.

SEC source: 0000950170-25-109399 · ownership.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 23,827,778

Underlying: Series A Common Stock, par value $0.001 per share · 7,000,000 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F6 · balance_after: Consists of (i) 14,082,751 shares of Series B Common Stock held by Fund IX-A, (ii) 8,793,072 shares of Series B Common Stock held by Fund IX-B, (iii) 868,334 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 74,015 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 9,606 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-109399 · ownership.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 500,000

F5 · balance_after: Consists of (i) 307,809 shares of Series A Common Stock held by Fund IX-A and (ii) 192,191 shares of Series A Common Stock held by Fund IX-B, in each case to facilitate charitable giving.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-109399 · ownership.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 7,000,000

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-082604 · ownership.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 30,827,778

Underlying: Series A Common Stock, par value $0.001 per share · 4,000,000 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F5 · balance_after: Consists of (i) 18,219,907 shares of Series B Common Stock held by Fund IX-A, (ii) 11,376,255 shares of Series B Common Stock held by Fund IX-B, (iii) 1,123,429 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 95,759 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 12,428 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-082604 · ownership.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-082604 · ownership.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 4,000,000

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-075445 · ownership.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 34,827,778

Underlying: Series A Common Stock, par value $0.001 per share · 4,000,000 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F5 · balance_after: Consists of (i) 20,583,996 shares of Series B Common Stock held by Fund IX-A, (ii) 12,852,359 shares of Series B Common Stock held by Fund IX-B, (iii) 1,269,198 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 108,184 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 14,041 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-075445 · ownership.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-075445 · ownership.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 4,000,000

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-040470 · ownership.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 38,827,778

Underlying: Series A Common Stock, par value $0.001 per share · 2,000,000 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F5 · balance_after: Consists of (i) 22,948,085 shares of Series B Common Stock held by Fund IX-A, (ii) 14,328,463 shares of Series B Common Stock held by Fund IX-B, (iii) 1,414,967 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 120,609 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 15,654 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-040470 · ownership.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-25-040470 · ownership.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 2,000,000

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-24-135422 · ownership.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 40,827,778

Underlying: Series A Common Stock, par value $0.001 per share · 2,100,000 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On December 9, 2024, the reporting persons directed the sale of an aggregate of 2,100,000 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On December 9, 2024, the reporting persons directed the sale of an aggregate of 2,100,000 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On December 9, 2024, the reporting persons directed the sale of an aggregate of 2,100,000 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F5 · balance_after: Consists of (i) 24,130,130 shares of Series B Common Stock held by Fund IX-A, (ii) 15,066,515 shares of Series B Common Stock held by Fund IX-B, (iii) 1,487,851 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 126,822 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 16,460 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-24-135422 · ownership.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-24-135422 · ownership.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 2,100,000

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On December 9, 2024, the reporting persons directed the sale of an aggregate of 2,100,000 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-24-127985 · ownership.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 42,927,778

Underlying: Series A Common Stock, par value $0.001 per share · 3,500,000 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F6 · balance_after: Consists of (i) 25,371,277 shares of Series B Common Stock held by Fund IX-A, (ii) 15,841,470 shares of Series B Common Stock held by Fund IX-B, (iii) 1,564,379 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 133,345 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 17,307 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 3) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-24-127985 · ownership.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnotes

Disclosed balance after: 346,707

F5 · balance_after: Consists of (i) 213,439 shares of Series A Common Stock held by Fund IX-A and (ii) 133,268 shares of Series A Common Stock held by Fund IX-B.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 3) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0000950170-24-127985 · ownership.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 3,500,000

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 3) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the securities held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-023518 · form4-09172024_080927.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 46,427,778

Underlying: Series A Common Stock, par value $0.001 per share · 113,561 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 13, 2024, the reporting persons directed the sale of an aggregate of 113,561 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 13, 2024, the reporting persons directed the sale of an aggregate of 113,561 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 13, 2024, the reporting persons directed the sale of an aggregate of 113,561 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F5 · balance_after: Consists of (i) 27,439,856 shares of Series B Common Stock held by Fund IX-A, (ii) 17,133,061 shares of Series B Common Stock held by Fund IX-B, (iii) 1,691,926 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 144,217 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 18,718 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-023518 · form4-09172024_080927.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-023518 · form4-09172024_080927.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 113,561

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 13, 2024, the reporting persons directed the sale of an aggregate of 113,561 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-023447 · form4-09162024_090941.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 46,541,339

Underlying: Series A Common Stock, par value $0.001 per share · 107,205 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 12, 2024, the reporting persons directed the sale of an aggregate of 107,205 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 12, 2024, the reporting persons directed the sale of an aggregate of 107,205 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 12, 2024, the reporting persons directed the sale of an aggregate of 107,205 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F5 · balance_after: Consists of (i) 27,506,973 shares of Series B Common Stock held by Fund IX-A, (ii) 17,174,968 shares of Series B Common Stock held by Fund IX-B, (iii) 1,696,064 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 144,570 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 18,764 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-023447 · form4-09162024_090941.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-023447 · form4-09162024_090941.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 107,205

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 12, 2024, the reporting persons directed the sale of an aggregate of 107,205 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-023354 · form4-09132024_080920.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 46,648,544

Underlying: Series A Common Stock, par value $0.001 per share · 2,942 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 11, 2024, the reporting persons directed the sale of an aggregate of 2,942 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 11, 2024, the reporting persons directed the sale of an aggregate of 2,942 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 11, 2024, the reporting persons directed the sale of an aggregate of 2,942 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F5 · balance_after: Consists of (i) 27,570,334 shares of Series B Common Stock held by Fund IX-A, (ii) 17,214,529 shares of Series B Common Stock held by Fund IX-B, (iii) 1,699,971 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 144,903 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 18,807 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-023354 · form4-09132024_080920.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-023354 · form4-09132024_080920.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 2,942

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 11, 2024, the reporting persons directed the sale of an aggregate of 2,942 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022891 · form4-09052024_080933.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 46,651,486

Underlying: Series A Common Stock, par value $0.001 per share · 6,039 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 3, 2024, the reporting persons directed the sale of an aggregate of 6,039 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 3, 2024, the reporting persons directed the sale of an aggregate of 6,039 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 3, 2024, the reporting persons directed the sale of an aggregate of 6,039 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F5 · balance_after: Consists of (i) 27,572,073 shares of Series B Common Stock held by Fund IX-A, (ii) 17,215,614 shares of Series B Common Stock held by Fund IX-B, (iii) 1,700,078 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 144,912 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 18,809 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022891 · form4-09052024_080933.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022891 · form4-09052024_080933.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 6,039

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On September 3, 2024, the reporting persons directed the sale of an aggregate of 6,039 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022793 · form4-09042024_080932.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 46,657,525

Underlying: Series A Common Stock, par value $0.001 per share · 1,300 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 30, 2024, the reporting persons directed the sale of an aggregate of 1,300 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 30, 2024, the reporting persons directed the sale of an aggregate of 1,300 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 30, 2024, the reporting persons directed the sale of an aggregate of 1,300 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F5 · balance_after: Consists of (i) 27,575,642 shares of Series B Common Stock held by Fund IX-A, (ii) 17,217,843 shares of Series B Common Stock held by Fund IX-B, (iii) 1,700,298 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 144,931 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 18,811 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022793 · form4-09042024_080932.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022793 · form4-09042024_080932.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 1,300

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 30, 2024, the reporting persons directed the sale of an aggregate of 1,300 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022689 · form4-09032024_080950.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 46,658,825

Underlying: Series A Common Stock, par value $0.001 per share · 16,931 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 29, 2024, the reporting persons directed the sale of an aggregate of 16,931 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 29, 2024, the reporting persons directed the sale of an aggregate of 16,931 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 29, 2024, the reporting persons directed the sale of an aggregate of 16,931 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F5 · balance_after: Consists of (i) 27,576,410 shares of Series B common stock held by Fund IX-A, (ii) 17,218,323 shares of Series B Common Stock held by Fund IX-B, (iii) 1,700,345 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 144,935 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 18,812 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022689 · form4-09032024_080950.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022689 · form4-09032024_080950.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 16,931

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 29, 2024, the reporting persons directed the sale of an aggregate of 16,931 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022618 · form4-08302024_080808.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 46,675,756

Underlying: Series A Common Stock, par value $0.001 per share · 27,445 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 28, 2024, the reporting persons directed the sale of an aggregate of 27,445 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 28, 2024, the reporting persons directed the sale of an aggregate of 27,445 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 28, 2024, the reporting persons directed the sale of an aggregate of 27,445 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F5 · balance_after: Consists of (i) 27,586,416 shares of Series B common stock held by Fund IX-A, (ii) 17,224,571 shares of Series B Common Stock held by Fund IX-B, (iii) 1,700,962 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 144,988 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 18,819 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022618 · form4-08302024_080808.xml · nonDerivative · entry 2

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 0

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022618 · form4-08302024_080808.xml · nonDerivative · entry 1

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 27,445

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 28, 2024, the reporting persons directed the sale of an aggregate of 27,445 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.

SEC source: 0001415889-24-022531 · form4-08292024_080816.xml · derivative · entry 3

SUMMIT PARTNERS L P · CIK 0000830588 · 10% owner

Summit Partners Growth Equity Fund IX-A, L.P. · CIK 0001634415 · 10% owner

Summit Partners Growth Equity Fund IX-B, L.P. · CIK 0001634426 · 10% owner

Summit Partners Co-Invest (Kiwi), LP · CIK 0001830877 · 10% owner

SUMMIT INVESTORS GE IX/VC IV, LLC · CIK 0001654074 · 10% owner

SUMMIT INVESTORS GE IX/VC IV (UK), L.P. · CIK 0001846709 · 10% owner

Joint disclosure: these amounts are not allocated separately to each reporting owner.

10b5-1 filing: No plan indicated

Ownership: Indirect · See footnote

Disclosed balance after: 46,703,201

Underlying: Series A Common Stock, par value $0.001 per share · 67,436 shares

Expires not disclosed

Exercise/conversion can include RSU settlement. Transaction price and option strike are different fields; a missing price is not zero. Derivative and underlying rows can describe the same event.

F1 · derivative.exercise_price: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 27, 2024, the reporting persons directed the sale of an aggregate of 67,436 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.exercise_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 27, 2024, the reporting persons directed the sale of an aggregate of 67,436 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F1 · derivative.expiration_date: The Series B common stock, par value $0.001 per share ("Series B Common Stock"), is convertible into an equal number of shares of Series A common stock, par value $0.001 per share ("Series A Common Stock"), at any time, at the holder's election, and has no expiration date. Each share of Series B Common Stock will automatically convert into Series A Common Stock upon the earlier of (i) the date specified by 66-2/3% of the outstanding shares of Series B Common Stock, voting as a single series; (ii) following the seventh anniversary of the closing of Klaviyo, Inc.'s (the "Company") initial public offering; or (iii) any transfer, whether or not for value, except for certain permitted transfers described in the Company's certificate of incorporation. On August 27, 2024, the reporting persons directed the sale of an aggregate of 67,436 shares of their Series B Common Stock, resulting in the automatic conversion of the shares into Series A Common Stock upon execution of the sale.

F5 · balance_after: Consists of (i) 27,602,637 shares of Series B common stock held by Fund IX-A, (ii) 17,234,699 shares of Series B Common Stock held by Fund IX-B, (iii) 1,701,962 shares of Series B Common Stock held by Kiwi Co-Invest Fund, (iv) 145,073 shares of Series B Common Stock held by Fund IX/VC IV, and (v) 18,830 shares of Series B Common Stock held by Fund IX/VC IV (UK).

F2 · ownership.nature: Summit Partners, L.P. is the manager of Summit Partners GE IX, LLC, which is the general partner of Summit Partners GE IX, LP, which is the general partner of Summit Partners Growth Equity Fund IX-A, L.P. ("Fund IX-A") and Summit Partners Growth Equity Fund IX-B, L.P. ("Fund IX-B"). Summit Partners, L.P. is the managing member of Summit Partners Co-Invest Kiwi GP, LLC, which is the general partner of Summit Partners Co-Invest (Kiwi), L.P. ("Kiwi Co-Invest Fund"). Summit Master Company, LLC is the general partner of Summit Partners, L.P., which is the manager of Summit Investors Management, LLC, which is the manager of Summit Investors GE IX/VC IV, LLC ("Fund IX/VC IV") and the general partner of Summit Investors GE IX/VC IV (UK), L.P. ("Fund IX/VC IV (UK)").

F3 · ownership.nature: (continued from footnote 2) Summit Master Company, LLC, as the managing member of Summit Investors Management, LLC, has delegated voting and investment decisions with respect to the Series B Common Stock held by Fund IX/VC IV and Fund IX/VC IV (UK) to Summit Partners, L.P.

F4 · ownership.nature: The investment committee of Summit Partners, L.P., is currently composed of Peter Y. Chung, Scott C. Collins, and Peter L. Rottier, who may be deemed to have voting and dispositive authority over, and therefore beneficial ownership of, the reported securities. Each of the foregoing entities, Mr. Chung, Mr. Collins, and Mr. Rottier disclaims beneficial ownership of the shares, except to the extent of such person's or entity's pecuniary interest therein and the filing of this statement shall not be deemed an admission that any of the foregoing entities or persons are, for purposes of Section 16 of the Act or otherwise, the beneficial owner of any equity securities covered by this statement.