Document and Entity Information |
Feb. 11, 2026 |
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| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0002022626 |
| Document Type | 8-K |
| Document Period End Date | Feb. 11, 2026 |
| Entity Registrant Name | UPSTREAM BIO, INC. |
| Entity Incorporation State Country Code | DE |
| Entity File Number | 001-42366 |
| Entity Tax Identification Number | 38-4187694 |
| Entity Address, Address Line One | 890 Winter Street |
| Entity Address, Address Line Two | Suite 200 |
| Entity Address, City or Town | Waltham |
| Entity Address, State or Province | MA |
| Entity Address, Postal Zip Code | 02451 |
| City Area Code | (781) |
| Local Phone Number | 208-2466 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Common Stock, $0.001 par value per share |
| Trading Symbol | UPB |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | true |
| Entity Ex Transition Period | false |
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- Definition Boolean flag that is true when the XBRL content amends previously-filed or accepted submission. No definition available.
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- Definition Area code of city No definition available.
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- Definition Cover page. No definition available.
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- Definition For the EDGAR submission types of Form 8-K: the date of the report, the date of the earliest event reported; for the EDGAR submission types of Form N-1A: the filing date; for all other submission types: the end of the reporting or transition period. The format of the date is YYYY-MM-DD. No definition available.
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- Definition The type of document being provided (such as 10-K, 10-Q, 485BPOS, etc). The document type is limited to the same value as the supporting SEC submission type, or the word 'Other'. No definition available.
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- Definition Address Line 1 such as Attn, Building Name, Street Name No definition available.
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- Definition Address Line 2 such as Street or Suite number No definition available.
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- Definition Name of the City or Town No definition available.
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- Definition Code for the postal or zip code No definition available.
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- Definition Name of the state or province. No definition available.
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- Definition A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Indicate if registrant meets the emerging growth company criteria. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Indicate if an emerging growth company has elected not to use the extended transition period for complying with any new or revised financial accounting standards. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Commission file number. The field allows up to 17 characters. The prefix may contain 1-3 digits, the sequence number may contain 1-8 digits, the optional suffix may contain 1-4 characters, and the fields are separated with a hyphen. No definition available.
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- Definition Two-character EDGAR code representing the state or country of incorporation. No definition available.
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- Definition The exact name of the entity filing the report as specified in its charter, which is required by forms filed with the SEC. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition The Tax Identification Number (TIN), also known as an Employer Identification Number (EIN), is a unique 9-digit value assigned by the IRS. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Local phone number for entity. No definition available.
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- Definition Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Title of a 12(b) registered security. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Name of the Exchange on which a security is registered. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as soliciting material pursuant to Rule 14a-12 under the Exchange Act. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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- Definition Trading symbol of an instrument as listed on an exchange. No definition available.
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- Definition Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as written communications pursuant to Rule 425 under the Securities Act. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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