Material Event (Form 8-K)

Item 8.01. Other Events.

On May 22, 2026, Sensei Biotherapeutics, Inc. (the "Company") announced an update regarding the expected reconstitution of the board of directors of the Company (the "Board") in connection with the Company's 2026 Annual Meeting of Stockholders to be held on June 10, 2026 (the "Annual Meeting"), as described in the Company's definitive proxy statement filed with the Securities and Exchange Commission (the "SEC") on April 27, 2026 (the "Definitive Proxy Statement").

Due to personal reasons, Karen Vousden, Ph.D. has requested that her appointment to the Board be delayed until later in 2026. The Board intends to consider appointing Dr. Vousden in the third or fourth quarter of 2026. If the Required Company Stockholder Matters (as defined in the Definitive Proxy Statement) are approved at the Annual Meeting, the Board intends to appoint Bob Holmen as chair of the Nominating and Corporate Governance Committee, effective June 12, 2026, when the other board appointments described in the Definitive Proxy Statement would become effective. The Board has also approved a Board size of five directors as of that date.