Statement of Changes in Beneficial Ownership (Form 4)

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b). Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person * ARCH Venture Fund IX, L.P. 2. Issuer Name and Ticker or Trading Symbol Boundless Bio, Inc. [BOLD] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director __X__ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 8755 W. HIGGINS ROAD, SUITE 1025 3. Date of Earliest Transaction (Month/Day/Year) 07/01/2026 (Street) CHICAGO, IL 60631 4. If Amendment, Date Original Filed (Month/Day/Year) 6. Individual or Joint/Group Filing (Check Applicable Line) ___ Form filed by One Reporting Person _X_ Form filed by More than One Reporting Person (City) (State) (Zip) Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned

1.Title of Security (Instr. 3) 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 07/01/2026 M 16,000 A $1.03 16,000 I See footnote(1) Common Stock 07/01/2026 S 16,000 D $2.49 0 I See footnote(1) Common Stock 683,759 I See footnotes(2)(4) Common Stock 828,570 I See footnotes(3)(4) Common Stock 1,181,766 I See footnotes(5)(6)

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
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Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)

1. Title of Derivative Security (Instr. 3)

2. Conversion or Exercise Price of Derivative Security

3. Transaction Date (Month/Day/Year)

3A. Deemed Execution Date, if any (Month/Day/Year)

4. Transaction Code (Instr. 8)

5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5)

6. Date Exercisable and Expiration Date (Month/Day/Year)

7. Title and Amount of Underlying Securities (Instr. 3 and 4)

8. Price of Derivative Security (Instr. 5)

9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)

10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4)

11. Nature of Indirect Beneficial Ownership (Instr. 4)

Code

V

(A)

(D)

Date Exercisable

Expiration Date

Title

Amount or Number of Shares

Stock Option (right to buy)

$1.03

07/01/2026

M

16,000

(7)

06/22/2035

Common Stock

16,000

$ 0

0

I

See footnote(1)

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
ARCH Venture Fund IX, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
ARCH Venture Fund IX Overage, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
ARCH Venture Fund X Overage, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
ARCH Venture Partners IX, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
ARCH Venture Partners IX Overage, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
ARCH Venture Partners X Overage, L.P. 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
ARCH Venture Partners IX, LLC 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X
ARCH Venture Partners X, LLC 8755 W. HIGGINS ROAD, SUITE 1025 CHICAGO, IL 60631		X

Signatures

/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		07/06/2026
**Signature of Reporting Person		Date
/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		07/06/2026
**Signature of Reporting Person		Date
/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		07/06/2026
**Signature of Reporting Person		Date
/s/ ARCH Venture Partners X Overage, L.P. By: ARCH Venture Partners X, LLC, its General Partner, By: Mark McDonnell, attorney in-fact		07/06/2026
**Signature of Reporting Person		Date
/s/ ARCH Venture Partners X, LLC, By: Mark McDonnell, attorney in-fact		07/06/2026
**Signature of Reporting Person		Date
/s/ ARCH Venture Partners IX, L.P., By: ARCH Venture Partners IX, LLC, its General Partner, By: Mark McDonnell, attorney in-fact		07/06/2026
**Signature of Reporting Person		Date
/s/ ARCH Venture Partners IX Overage, L.P., By: ARCH Venture Partners IX, LLC, its General Partner, By: Mark McDonnell, attorney in-fact		07/06/2026
**Signature of Reporting Person		Date
/s/ ARCH Venture Partners IX, LLC, By: Mark McDonnell, attorney in-fact		07/06/2026
**Signature of Reporting Person		Date

Explanation of Responses:

*	If the form is filed by more than one reporting person, see Instruction 4(b)(v).
**	Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1)	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.
(2)	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.
(3)	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.
(4)	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.
(5)	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.
(6)	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.
(7)	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.
Remarks: 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.

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