SCHEDULE 13D/A 0000950170-25-016399 0001117003 XXXXXXXX LIVE 2 Common Stock, $0.0001 par value per share 04/03/2025 false 0001681087 878972108 Tectonic Therapeutic, Inc.

490 Arsenal Way Suite 210 Watertown MA 02472

Lauren Crockett (781) 290-0770 One Marina Park Drive 8th Floor Boston MA 02210 0001117003 N Terrance McGuire b AF N X1 0 657976 0 657976 657976 N 6.3 IN Consists of (i) 20,262 shares of Common Stock (as defined in Item 1 of the Original Schedule 13D (as defined below)) held by PFC I (as defined in Item 2(a) of the Original Schedule 13D), (ii) 64,652 shares of Common Stock held by PFC II (as defined in Item 2(a) of the Original Schedule 13D) and (iii) 573,062 shares of Common Stock held by PP IX (as defined in Item 2(a) of the Original Schedule 13D). The aggregate amount beneficially owned does not include 11,760 shares of Common Stock underlying stock options granted to Mr. McGuire in his capacity as a director of the Issuer because such options are not exercisable as of the date of filing this Amendment (as defined in Item 1 below) or within 60 days thereafter. PFCM I (as defined in Item 2(a) of the Original Schedule 13D) is the general partner of PFC I. Each of Mr. Flint and Mr. McGuire, a member of the Issuer's (as defined in Item 1 of the Original Schedule 13D) board of directors, are managing members of PFCM I. Each of PFCM I, Mr. Flint and Mr. McGuire may be deemed to have shared voting, investment and dispositive power with respect to the shares held by PFC I. PFCM II (as defined in Item 2(a) of the Original Schedule 13D) is the general partner of PFC II. Each of Mr. Flint and Mr. McGuire, a member of the Issuer's board of directors, are managing members of PFCM II. Each of PFCM II, Mr. Flint and Mr. McGuire may be deemed to have shared voting, investment and dispositive power with respect to the shares held by PFC II. PP GP IX (as defined in Item 2(a) of the Original Schedule 13D) is the general partner of PP IX. The PP GP IX Managing Members (as defined in Item 2(c) of the Original Schedule 13D) are the managing members of PP GP IX, and Mr. McGuire, a member of the Issuer's board of directors, is an interest holder in PP GP IX. Each of PP GP IX, the PP GP IX Managing Members and Mr. McGuire may be deemed to have shared voting, investment and dispositive power with respect to the shares held by PP IX. Percentage based on 18,662,672 shares of Common Stock outstanding as of March 1, 2025, as set forth in the Issuer's Annual Report on Form 10-K for the year ended December 31, 2024, filed with the United States Securities and Exchange Commission (the "Commission") on March 20, 2025 (the "Form 10-K"). 0001779755 N Polaris Partners IX, L.P. b WC N DE 0 573062 0 573062 573062 N 3.1 PN All shares are held of record by PP IX. PP GP IX is the general partner of PP IX. The PP GP IX Managing Members are the managing members of PP GP IX, and Mr. McGuire, a member of the Issuer's board of directors, is an interest holder in PP GP IX. Each of PP GP IX, the PP GP IX Managing Members and Mr. McGuire may be deemed to have shared voting, investment and dispositive power with respect to the shares held by PP IX. Percentage based on 18,662,672 shares of Common Stock outstanding as of March 1, 2025, as set forth in the Form 10-K. 0001877150 N Polaris Partners GP IX, L.L.C. b AF N DE 0 573062 0 573062 573062 N 3.1 OO All shares are held of record by PP IX. PP GP IX is the general partner of PP IX. The PP GP IX Managing Members are the managing members of PP GP IX, and Mr. McGuire, a member of the Issuer's board of directors, is an interest holder in PP GP IX. Each of PP GP IX, the PP GP IX Managing Members and Mr. McGuire may be deemed to have shared voting, investment and dispositive power with respect to the shares held by PP IX. Percentage based on 18,662,672 shares of Common Stock outstanding as of March 1, 2025, as set forth in the Form 10-K. Y Polaris Founders Capital Fund I, L.P. b WC N DE 0 20262 0 20262 20262 N 0.1 PN All shares are held of record by PFC I. PFCM I is the general partner of PFC I. Each of Mr. Flint and Mr. McGuire, a member of the Issuer's board of directors, are managing members of PFCM I. Each of PFCM I, Mr. Flint and Mr. McGuire may be deemed to have shared voting, investment and dispositive power with respect to the shares held by PFC I. Percentage based on 18,662,672 shares of Common Stock outstanding as of March 1, 2025, as set forth in the Form 10-K. Y Polaris Founders Capital Fund Management Co. I, L.L.C. b AF N DE 0 20262 0 20262 20262 N 0.1 OO All shares are held of record by PFC I. PFCM I is the general partner of PFC I. Each of Mr. Flint and Mr. McGuire, a member of the Issuer's board of directors, are managing members of PFCM I. Each of PFCM I, Mr. Flint and Mr. McGuire may be deemed to have shared voting, investment and dispositive power with respect to the shares held by PFC I. Percentage based on 18,662,672 shares of Common Stock outstanding as of March 1, 2025, as set forth in the Form 10-K. Y Polaris Founders Capital Fund II, L.P. b WC N DE 0 64652 0 64652 64652 N 0.3 PN All shares are held of record by PFC II. PFCM II is the general partner of PFC II. Each of Mr. Flint and Mr. McGuire, a member of the Issuer's board of directors, are managing members of PFCM II. Each of PFCM II, Mr. Flint and Mr. McGuire may be deemed to have shared voting, investment and dispositive power with respect to the shares held by PFC II. Percentage based on 18,662,672 shares of Common Stock outstanding as of March 1, 2025, as set forth in the Form 10-K. Y Polaris Founders Capital Fund Management Co. II, L.L.C. b AF N DE 0 64652 0 64652 64652 N 0.3 OO All shares are held of record by PFC II. PFCM II is the general partner of PFC II. Each of Mr. Flint and Mr. McGuire, a member of the Issuer's board of directors, are managing members of PFCM II. Each of PFCM II, Mr. Flint and Mr. McGuire may be deemed to have shared voting, investment and dispositive power with respect to the shares held by PFC II. Percentage based on 18,662,672 shares of Common Stock outstanding as of March 1, 2025, as set forth in the Form 10-K. 0001295676 N Jonathan Flint b AF N X1 0 84914 0 84914 84914 N 0.5 IN Consists of (i) 20,262 shares of Common Stock held by PFC I and (ii) 64,652 shares of Common Stock held by PFC II. PFCM I is the general partner of PFC I. Each of Mr. Flint and Mr. McGuire, a member of the Issuer's board of directors, are managing members of PFCM I. Each of PFCM I, Mr. Flint and Mr. McGuire may be deemed to have shared voting, investment and dispositive power with respect to the shares held by PFC I. PFCM II is the general partner of PFC II. Each of Mr. Flint and Mr. McGuire, a member of the Issuer's board of directors, are managing members of PFCM II. Each of PFCM II, Mr. Flint and Mr. McGuire may be deemed to have shared voting, investment and dispositive power with respect to the shares held by PFC II. Percentage based on 18,662,672 shares of Common Stock outstanding as of March 1, 2025, as set forth in the Form 10-K. Common Stock, $0.0001 par value per share Tectonic Therapeutic, Inc. 490 Arsenal Way Suite 210 Watertown MA 02472 Explanatory Note: This Amendment No. 2 (this "Amendment") amends and supplements the Schedule 13D originally filed by the Reporting Persons with the Commission on June 25, 2024, as amended by Amendment No. 1 filed with the Commission on February 7, 2025 (collectively, the "Original Schedule 13D"). Only those items that are hereby reported are amended; all other items reports in the Original Schedule 13D remain unchanged. Information given in response to each item shall be deemed incorporated by reference in all other items, as applicable. Capitalized terms not defined in this Amendment have the meanings ascribed to them in the Original Schedule 13D. See Items 7-11 and 13 of the cover pages of this Amendment for each Reporting Person and the corresponding comments. See Items 7-11 and 13 of the cover pages of this Amendment for each Reporting Person and the corresponding comments. On April 3, 2025, PP IX entered into Stock Purchase Agreements with each of Timothy A. Springer ("Dr. Springer") and TAS Partners LLC ("TAS"), pursuant to which Dr. Springer purchased 87,500 shares of Common Stock from PP IX at a purchase price of $17.10 per share, or $1,496,250 in the aggregate, and TAS purchased 412,500 shares of Common Stock at a purchase price of $17.10 per share, or $7,053,750 in the aggregate. The closing occurred on April 7, 2025. April 7, 2025 Terrance McGuire /s/ Lauren Crockett* 04/07/2025 Polaris Partners IX, L.P. /s/ Lauren Crockett 04/07/2025 Polaris Partners GP IX, L.L.C. /s/ Lauren Crockett 04/07/2025 Polaris Founders Capital Fund I, L.P. /s/ Gregg Rubin 04/07/2025 Polaris Founders Capital Fund Management Co. I, L.L.C. /s/ Gregg Rubin 04/07/2025 Polaris Founders Capital Fund II, L.P. /s/ Harold Friedman 04/07/2025 Polaris Founders Capital Fund Management Co. II, L.L.C. /s/ Harold Friedman 04/07/2025 Jonathan Flint /s/ Lauren Crockett* 04/07/2025 [* This Amendment was executed pursuant to a Power of Attorney. Note that copies of the applicable Powers of Attorney are already on file with the appropriate agencies.]