| Cover Page - USD ($) $ in Millions | 12 Months Ended | |
| Dec. 31, 2021 | Mar. 01, 2022 |
| Cover [Abstract] | | |
| Document Type | 10-K/A | |
| Amendment Flag | true | |
| Amendment Description | F-star Therapeutics, Inc. (“F-star”, “our” or “we”) is filing this Amendment No. 1 on Form 10-K/A (this “Amendment”) to its Annual Report on Form 10-K for the fiscal year ended December 31, 2021 (the “Original Form 10-K”), which was filed with the Securities and Exchange Commission on March 15, 2022. This Amendment replaces in its entirety Part II, Item 9A “Controls and Procedures” (“Item 9A”) of the Original Form 10-K, which inadvertently did not include the conclusion by our Chief Executive Officer and Chief Financial Officer of the effectiveness of our disclosure controls and procedures, as required by Item 307 of Regulation S-K. The new Item 9A provided by this Amendment includes a statement regarding the conclusion of our Chief Executive Officer and Chief Financial Officer that our disclosure controls and procedures were effective as of December 31, 2021. This Amendment does not amend the Original Form 10-K in any other way. Specifically, it does not update the Original Form 10-K to reflect events occurring after March 15, 2022. | |
| Document Period End Date | Dec. 31, 2021 | |
| Document Fiscal Year Focus | 2021 | |
| Document Fiscal Period Focus | FY | |
| Trading Symbol | FSTX | |
| Title of 12(b) Security | Common Stock, $0.0001 par value | |
| Security Exchange Name | NASDAQ | |
| Entity Registrant Name | F-STAR THERAPEUTICS, INC. | |
| Entity Central Index Key | 0001566373 | |
| Current Fiscal Year End Date | --12-31 | |
| Entity Well-known Seasoned Issuer | No | |
| Entity Current Reporting Status | Yes | |
| Entity Interactive Data Current | Yes | |
| Entity Voluntary Filers | No | |
| Entity Filer Category | Non-accelerated Filer | |
| Entity Small Business | true | |
| Entity Emerging Growth Company | false | |
| Entity Shell Company | false | |
| Document Annual Report | true | |
| Document Transition Report | false | |
| Entity File Number | 001-37718 | |
| Entity Incorporation, State or Country Code | DE | |
| Entity Tax Identification Number | 52-2386345 | |
| Entity Address, Address Line One | B920 Babraham Research Campus | |
| Entity Address, City or Town | Cambridge | |
| Entity Address, Postal Zip Code | CB22 3AT | |
| Entity Address, Country | GB | |
| City Area Code | 44 | |
| Local Phone Number | 1223-497400 | |
| Entity Public Float | $ 176.8 | |
| Entity Common Stock, Shares Outstanding | | 21,064,788 |
| Documents Incorporated by Reference | The registrant filed a definitive proxy statement pursuant to Regulation 14A relating to the 2022 Annual Meeting of Stockholders on April 22, 2022. Portions of such definitive proxy statement are incorporated by reference into Part III of this Annual Report on Form 10-K to the extent stated herein. | |
| ICFR Auditor Attestation Flag | false | |
| Auditor Name | RSM US LLP | |
| Auditor Firm ID | 49 | |
| Auditor Location | Boston, Massachusetts | |
