X0202 SCHEDULE 13G 0001602227 XXXXXXXX LIVE Common Stock, par value $0.0001 per share 10/14/2025 0001782303 Boundless Bio, Inc. 10170A100 11099 North Torrey Pines Road Suite 150 La Jolla CA 92037 Rule 13d-1(c) Zachary Hornby X1 2426857 0 2426857 0 2426857 N 9.8 IN Boundless Bio, Inc. 11099 North Torrey Pines Road, Suite 150, La Jolla, CA 92037 This statement is filed on behalf of Zachary Hornby (the "Reporting Person"). c/o Boundless Bio, Inc., 11099 North Torrey Pines Road, Suite 150, La Jolla, CA 92037 The Reporting Person is a citizen of the United States. Y The ownership information presented herein represents beneficial ownership of Common Stock of the Issuer as of August 17, 2026, based upon 22,747,985 shares of Common Stock outstanding as of August 5, 2026, as disclosed in the Issuer's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on August 7, 2026. The Reporting Person is the beneficial owner of 2,426,857 shares of Common Stock, which consist of (i) 456,410 shares of Common Stock held directly by the Reporting Person, and (ii) 1,970,447 shares of Common Stock underlying stock options that are vested or will vest within 60 days of August 17, 2026. 9.8% 2,426,857 0 2,426,857 0 Y Y Y Y Y N By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under § 240.14a-11. Zachary Hornby /s/ Zachary Hornby 08/27/2026