Amendments to Bylaws (Form 8-K)

On June 12, 2025, Sensei Biotherapeutics, Inc. (the "Company") filed a Certificate of Amendment to its Amended and Restated Certificate of Incorporation (the "Amendment") with the Secretary of State of the State of Delaware to effect a one-for-twenty (1-for-20)reverse stock split (the "Reverse Stock Split") of its outstanding common stock and a reduction in the total number of authorized shares of its common stock from 250,000,000 to 12,500,000 (the "Shares Reduction"). The Amendment will be effective at 5:00 p.m. Eastern Time on June 16, 2025. A series of alternate amendments to effect (i) a reverse stock split and (ii) a reduction in the total number of authorized shares of the Company's common stock was approved by the Company's stockholders at the Company's 2025 Annual Meeting of Stockholders held on May 21, 2025, and the specific one-for-twenty (1-for-20)Reverse Stock Split and corresponding Shares Reduction was subsequently approved by the Company's board of directors on June 3, 2025.

The Amendment provides that, at the effective time of the Amendment, every twenty (20) shares of the Company's issued and outstanding common stock will automatically be combined into one (1) issued and outstanding share of common stock and the authorized shares of the Company's common stock will be reduced from 250,000,000 to 12,500,000, without any change in par value per share. The Reverse Stock Split will affect all shares of the Company's common stock outstanding immediately prior to the effective time of the Amendment. As a result of the Reverse Stock Split, proportionate adjustments will be made to the per share exercise price and/or the number of shares issuable upon the exercise or vesting of all stock options issued by the Company and outstanding immediately prior to the effective time of the Amendment, which will result in a proportionate decrease in the number of shares of the Company's common stock reserved for issuance upon exercise or vesting of such stock options and a proportionate increase in the exercise price of all such stock options. In addition, all outstanding restricted stock unit awards and warrants will be proportionately adjusted, and the number of shares reserved for issuance under the Company's equity compensation plans immediately prior to the effective time of the Amendment will be reduced proportionately.

No fractional shares will be issued as a result of the Reverse Stock Split. Stockholders of record who would otherwise be entitled to receive a fractional share will receive a cash payment in lieu thereof. The Reverse Stock Split will affect all stockholders proportionately and will not affect any stockholder's percentage ownership of the Company's common stock (except to the extent that the Reverse Stock Split results in any stockholder owning only a fractional share).

The Company's common stock will begin trading on The Nasdaq Capital Market on a split-adjusted basis when the market opens on June 17, 2025. The new CUSIP number for the Company's common stock following the Reverse Stock Split is 81728A207.

The foregoing description is qualified in its entirety by the Amendment, which is filed as Exhibit 3.1 to this Current Report on Form 8-Kand is incorporated herein by reference.