X0609 4 2026-07-01 0001782303 Boundless Bio, Inc. BOLD 0001666306 ARCH Venture Fund IX, L.P. false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 0 0 1 0 0001666307 ARCH Venture Fund IX Overage, L.P. false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 0 0 1 0 0001757017 ARCH Venture Fund X Overage, L.P. false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 0 0 1 0 0001723173 ARCH Venture Partners IX, L.P. false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 0 0 1 0 0001723172 ARCH Venture Partners IX Overage, L.P. false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 0 0 1 0 0001768905 ARCH Venture Partners X Overage, L.P. false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 0 0 1 0 0001723174 ARCH Venture Partners IX, LLC false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 0 0 1 0 0001769012 ARCH Venture Partners X, LLC false 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO IL 60631 0 0 1 0 0 Common Stock 2026-07-01 4 M 0 16000 1.03 A 16000 I See footnote Common Stock 2026-07-01 4 S 0 16000 2.49 D 0 I See footnote Common Stock 683759 I See footnotes Common Stock 828570 I See footnotes Common Stock 1181766 I See footnotes Stock Option (right to buy) 1.03 2026-07-01 4 M 0 16000 0 D 2035-06-22 Common Stock 16000 0 I See footnote Represents a cashless stock option exercise by Kristina Burow. The net proceeds of such exercise were remitted to ARCH Venture Fund IX, L.P. (ARCH IX), ARCH Venture Fund IX Overage, L.P. (ARCH IX Overage), and ARCH Venture Fund X, L.P. (AVF X Overage LP). The stock option was granted to Kristina Burow in connection with her service on the board of directors of the Issuer. Represents shares held directly by ARCH Venture Fund IX, L.P. (ARCH IX). ARCH Venture Partners IX, L.P. (AVP IX LP) is the sole general partner of ARCH IX. Represents shares held directly by ARCH Venture Fund IX Overage, L.P. (ARCH IX Overage). ARCH Venture Partners IX Overage, L.P. (AVP IX Overage LP) is the sole general partner of ARCH IX Overage. ARCH Venture Partners IX, LLC (AVP IX LLC) is the sole general partner of each of AVP IX LP and AVP IX Overage LP. Keith Crandell, Robert Nelsen and Clinton Bybee are managing directors of AVP IX LLC (the AVP IX MDs). AVP IX LP and AVP IX Overage LP may be deemed to beneficially own the shares held by ARCH IX and ARCH IX Overage, respectively, AVP IX LLC may be deemed to beneficially own the shares held by ARCH IX and ARCH IX Overage, and each of the AVP IX MDs may be deemed to share the power to direct the disposition and vote of the shares held by ARCH IX and ARCH IX Overage. AVP IX LP, AVP IX Overage LP, AVP IX LLC, and the AVP IX MDs each disclaim beneficial ownership except to to the extent of any pecuniary interest therein, if any. Represents shares held directly by ARCH Venture Fund X Overage, L.P. (ARCH X Overage). ARCH Venture Partners X Overage, L.P. (AVP X Overage LP) is the sole general partner of ARCH X Overage. ARCH Venture Partners X, LLC (AVP X LLC) is the sole general partner of AVP X Overage LP. Keith Crandell, Robert Nelsen, Kristina Burow and Steven Gillis are members of the investment committee of AVP X LLC (the AVP X Committee Members). AVP X Overage LP may be deemed to beneficially own the shares held by ARCH X Overage, AVP X LLC may be deemed to beneficially own the shares held by ARCH X Overage, and each of the AVP X Committee Members may be deemed to share the power to direct the disposition and vote of the shares held by ARCH X Overage. AVP X Overage LP, AVP X LLC, and the AVP X Committee Members each disclaim beneficial ownership except to the extent of any pecuniary interest therein, if any. The stock option vested in substantially equal monthly installments over the 11 months following June 23, 2025. The final unvested portion vested on June 15, 2026, the date of the Issuer's 2026 annual meeting of stockholders. This Form 4 is one of two reports relating to the same transaction being filed jointly by ARCH IX, AVP IX LP, ARCH IX Overage, AVP IX Overage LP, AVP IX LLC, ARCH X Overage, AVP X Overage LP, AVP X LLC, Robert Nelsen, Keith Crandell, Kristina Burow, Steve Gillis and Clint Bybee. /s/ ARCH Venture Fund X Overage, L.P., By: ARCH Venture Partners X Overage, L.P., its General Partner, By: ARCH Venture Partners X, LLC, its General Partner, By: Mark McDonnell, attorney in-fact 2026-07-06 /s/ ARCH Venture Fund IX, L.P., By: ARCH Venture Partners IX, L.P., its General Partner, By: ARCH Venture Partners IX, LLC, its General Partner, By: Mark McDonnell, attorney in-fact 2026-07-06 /s/ ARCH Venture Fund IX Overage, L.P., By: ARCH Venture Partners IX Overage, L.P., its General Partner, By: ARCH Venture Partners IX, LLC, its General Partner, By: Mark McDonnell attorney in-fact 2026-07-06 /s/ ARCH Venture Partners X Overage, L.P. By: ARCH Venture Partners X, LLC, its General Partner, By: Mark McDonnell, attorney in-fact 2026-07-06 /s/ ARCH Venture Partners X, LLC, By: Mark McDonnell, attorney in-fact 2026-07-06 /s/ ARCH Venture Partners IX, L.P., By: ARCH Venture Partners IX, LLC, its General Partner, By: Mark McDonnell, attorney in-fact 2026-07-06 /s/ ARCH Venture Partners IX Overage, L.P., By: ARCH Venture Partners IX, LLC, its General Partner, By: Mark McDonnell, attorney in-fact 2026-07-06 /s/ ARCH Venture Partners IX, LLC, By: Mark McDonnell, attorney in-fact 2026-07-06