Sec Form 3 Filing - Muller Elizabeth @ DEEP FISSION, INC. - 2025-09-05

FORM 3 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Muller Elizabeth 2. Issuer Name and Ticker or Trading Symbol DEEP FISSION, INC. [ NONE] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) President and Chief Executive (Last) (First) (Middle) C/O DEEP FISSION, INC., 2831 GARBER STREET 3. Date of Earliest Transaction (MM/DD/YY) 09/05/2025 (Street) BERKELEY, CA94705 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ 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 (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 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

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 (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 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 (MM/DD/YY) 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 Employee Stock Option (right to buy) ( 1 ) $ 51.2 ( 2 ) 08/28/2035 Common Stock 811,699 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Muller Elizabeth C/O DEEP FISSION, INC. 2831 GARBER STREET BERKELEY, CA94705 X President and Chief Executive Signatures /s/ Elizabeth Muller 09/09/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Received pursuant to the terms of the Agreement and Plan of Merger and Reorganization dated as of September 5, 2025, by and among the Issuer (f/k/a Surfside Acquisition Inc.), Deep Fission Acquisition Co. and Deep Fission Inc. ("Legacy DF") in exchange for options to acquire shares of Legacy DF common stock. The number of shares of Legacy DF common stock subject to such options (the "option shares") was converted into an option to purchase the number of shares of the Issuer's common stock equal to the option shares multiplied by 17.32142 (the "Conversion Ratio"), rounded down to the nearest whole share, and the exercise price of each option was divided by the Conversion Ratio, rounded up to the nearest whole cent. The options were acquired by the Reporting Person prior to the Reporting Person becoming an executive officer of the Issuer. The Reporting Person was appointed as a director and executive officer of the Issuer effective immediately after the effective time of the Merger. ( 2 )The shares subject to this option vest over a 4-year period, with 25% of the underlying shares vesting on June 1, 2025 and 1/48th of the shares underlying shares vesting monthly over the 36-month period thereafter, subject to the executive's continued employment through each vesting date.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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