Organization and Operations	12 Months Ended
Dec. 31, 2021
Organization Consolidation And Presentation Of Financial Statements [Abstract]
ORGANIZATION AND OPERATIONS	1. ORGANIZATION AND OPERATIONS Business Sensei Biotherapeutics, Inc. (the “Company” or “Sensei”) is a biopharmaceutical company that was incorporated in 1999 as a Maryland corporation until incorporated in Delaware on December 1, 2017. The Company is engaged in the discovery, development and delivery of next generation immunotherapies with an initial focus on treatments for cancer. On May 18, 2020, the Company acquired Alvaxa Biosciences, Inc. (“Alvaxa”) in a cash and stock purchase (“Stock Purchase Agreement”). Under the terms of the Stock Purchase Agreement, the Company acquired Alvaxa’s existing camelid nanobodies and other biomaterials (“Biomaterials”), expertise in nanobody discovery, as well as a license agreement with a research organization. The former majority shareholder of Alvaxa is the Company’s current Chief Research and Development Officer. Under the Stock Purchase Agreement, the Company paid $197 thousand to settle liabilities assumed from Alvaxa and issued 304,376 shares of the Company’s common stock to the shareholders of Alvaxa. The Company has evaluated the acquisition under ASC 805, Business Combinations and determined this transaction to be an asset acquisition. The 304,376 shares of common stock was valued at $541 thousand in total, based on a valuation determined with the assistance of a third party. The Company determined that substantially all the value acquired in the transaction related to the Biomaterials and represents in-process research and development (“IPR&D”). The liabilities of $197 thousand assumed were related to previously incurred employee costs as well as contractually required vendor payments. The consideration transferred in this transaction was recorded as an expense in the Alvaxa IPR&D line item within the Statement of Operations during the year ended December 31, 2020. Liquidity and capital resources Since its inception, the Company has devoted substantially all of its resources to advancing development of its portfolio of programs, establishing and protecting its intellectual property, conducting research and development activities, organizing and staffing the Company, business planning, raising capital and providing general and administrative support for these operations. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry including, but not limited to, technical risks associated with the successful research, development and manufacturing of product candidates, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Current and future programs will require significant research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. Since its inception, the Company has incurred substantial losses and had a net loss of $36.8 million for the year ended December 31, 2021. As of December 31, 2021, the Company had an accumulated deficit of $149.2 million. The Company expects to generate operating losses and negative operating cash flows for the foreseeable future. On February 3, 2021, the Company completed its initial public offering, or IPO, in which the Company issued and sold 8,030,295 shares of its common stock, including 1,030,243 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, at a public offering price of $19.00 per share, for aggregate gross proceeds of $152.6 million. The Company received $138.5 million in net proceeds after deducting underwriting discounts and offering expenses payable by the Company. The Company expects that its cash, cash equivalents and marketable securities, as of December 31, 2021 of $147.6 million will be sufficient to fund its operations for at least the next twelve months from the date of issuance of these financial statements. The Company will need additional financing to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through a combination of equity offerings, debt financings, collaborations, strategic alliances and licensing arrangements. The Company may be unable to raise additional funds or enter into such other agreements when needed on favorable terms or at all. The inability to raise capital as and when needed would have a negative impact on the Company’s financial condition and its ability to pursue its business strategy. The Company will need to generate significant revenue to achieve profitability, and it may never do so. Reverse stock split On January 29, 2021, the Company effected a reverse stock split of the Company’s common stock on a 48-for-1 basis (the “Reverse Stock Split”). In connection with the Reverse Stock Split, the conversion ratio for the Company’s Series AA and Series BB convertible preferred stock was proportionately adjusted such that the common stock issuable upon conversion of such preferred stock was decreased in proportion to the Reverse Stock Split. Accordingly, all common stock share and per share amounts, as well as all preferred stock conversion ratios, for all periods presented in these financial statements have been retroactively adjusted, to reflect this reverse stock split and adjustment of the Series AA and BB convertible preferred stock conversion ratios.

Organization and Operations

12 Months Ended

Dec. 31, 2021

Organization Consolidation And Presentation Of Financial Statements [Abstract]

1. ORGANIZATION AND OPERATIONS

Business

Sensei Biotherapeutics, Inc. (the “Company” or “Sensei”) is a biopharmaceutical company that was incorporated in 1999 as a Maryland corporation until incorporated in Delaware on December 1, 2017. The Company is engaged in the discovery, development and delivery of next generation immunotherapies with an initial focus on treatments for cancer.

On May 18, 2020, the Company acquired Alvaxa Biosciences, Inc. (“Alvaxa”) in a cash and stock purchase (“Stock Purchase Agreement”). Under the terms of the Stock Purchase Agreement, the Company acquired Alvaxa’s existing camelid nanobodies and other biomaterials (“Biomaterials”), expertise in nanobody discovery, as well as a license agreement with a research organization. The former majority shareholder of Alvaxa is the Company’s current Chief Research and Development Officer. Under the Stock Purchase Agreement, the Company paid $197 thousand to settle liabilities assumed from Alvaxa and issued 304,376 shares of the Company’s common stock to the shareholders of Alvaxa. The Company has evaluated the acquisition under ASC 805, Business Combinations and determined this transaction to be an asset acquisition.

The 304,376 shares of common stock was valued at $541 thousand in total, based on a valuation determined with the assistance of a third party. The Company determined that substantially all the value acquired in the transaction related to the Biomaterials and represents in-process research and development (“IPR&D”). The liabilities of $197 thousand assumed were related to previously incurred employee costs as well as contractually required vendor payments. The consideration transferred in this transaction was recorded as an expense in the Alvaxa IPR&D line item within the Statement of Operations during the year ended December 31, 2020.

Liquidity and capital resources

Since its inception, the Company has devoted substantially all of its resources to advancing development of its portfolio of programs, establishing and protecting its intellectual property, conducting research and development activities, organizing and staffing the Company, business planning, raising capital and providing general and administrative support for these operations. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry including, but not limited to, technical risks associated with the successful research, development and manufacturing of product candidates, development by competitors of new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Current and future programs will require significant research and development efforts, including extensive preclinical and clinical testing and regulatory approval prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure. Even if the Company’s drug development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales.

Since its inception, the Company has incurred substantial losses and had a net loss of $36.8 million for the year ended December 31, 2021. As of December 31, 2021, the Company had an accumulated deficit of $149.2 million. The Company expects to generate operating losses and negative operating cash flows for the foreseeable future.

On February 3, 2021, the Company completed its initial public offering, or IPO, in which the Company issued and sold 8,030,295 shares of its common stock, including 1,030,243 shares pursuant to the full exercise of the underwriters’ option to purchase additional shares, at a public offering price of $19.00 per share, for aggregate gross proceeds of $152.6 million. The Company received $138.5 million in net proceeds after deducting underwriting discounts and offering expenses payable by the Company.

The Company expects that its cash, cash equivalents and marketable securities, as of December 31, 2021 of $147.6 million will be sufficient to fund its operations for at least the next twelve months from the date of issuance of these financial statements. The Company will need additional financing to support its continuing operations and pursue its growth strategy. Until such time as the Company can generate significant revenue from product sales, if ever, it expects to finance its operations through a combination of equity offerings, debt financings, collaborations, strategic alliances and licensing arrangements. The Company may be unable to raise additional funds or enter into such other agreements when needed on favorable terms or at all. The inability to raise capital as and when needed would have a negative impact on the Company’s financial condition and its ability to pursue its business strategy. The Company will need to generate significant revenue to achieve profitability, and it may never do so.

Reverse stock split

On January 29, 2021, the Company effected a reverse stock split of the Company’s common stock on a 48-for-1 basis (the “Reverse Stock Split”). In connection with the Reverse Stock Split, the conversion ratio for the Company’s Series AA and Series BB convertible preferred stock was proportionately adjusted such that the common stock issuable upon conversion of such preferred stock was decreased in proportion to the Reverse Stock Split. Accordingly, all common stock share and per share amounts, as well as all preferred stock conversion ratios, for all periods presented in these financial statements have been retroactively adjusted, to reflect this reverse stock split and adjustment of the Series AA and BB convertible preferred stock conversion ratios.