| Document and Entity Information | Mar. 29, 2021 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001566373 |
| Document Type | 8-K |
| Document Period End Date | Mar. 29, 2021 |
| Entity Registrant Name | F-STAR THERAPEUTICS, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-37718 |
| Entity Tax Identification Number | 52-2386345 |
| Entity Address, Address Line One | Eddeva B920 Babraham Research Campus |
| Entity Address, City or Town | Cambridge |
| Entity Address, Country | GB |
| Entity Address, Postal Zip Code | CB22 3AT |
| City Area Code | 44-1223 |
| Local Phone Number | 497400 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common stock, $0.0001 par value |
| Trading Symbol | FSTX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | true |
