<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>106</ContextCount>
  <ElementCount>232</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>34</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>75000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>75010 - Statement - Condensed consolidated balance sheets (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/StatementCondensedConsolidatedBalanceSheetsUnaudited</Role>
      <ShortName>Condensed consolidated balance sheets (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>75020 - Statement - Condensed consolidated balance sheets (Unaudited) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/StatementCondensedConsolidatedBalanceSheetsUnauditedParenthetical</Role>
      <ShortName>Condensed consolidated balance sheets (Unaudited) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>75030 - Statement - Condensed consolidated statements of operations and comprehensive loss (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/StatementCondensedConsolidatedStatementsOfOperationsAndComprehensiveLossUnaudited</Role>
      <ShortName>Condensed consolidated statements of operations and comprehensive loss (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>75050 - Statement - Condensed Consolidated Statements of Stockholders' Deficit (Unaudited)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/StatementCondensedConsolidatedStatementsOfStockholdersDeficitUnaudited</Role>
      <ShortName>Condensed Consolidated Statements of Stockholders' Deficit (Unaudited)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>75060 - Statement - Condensed Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/StatementCondensedConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>995455 - Disclosure - Nature of the Business and Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureNatureOfTheBusinessAndBasisOfPresentation1</Role>
      <ShortName>Nature of the Business and Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>995465 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>995475 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureFairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>995485 - Disclosure - Prepaid Expenses and Other Current Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosurePrepaidExpensesAndOtherCurrentAssets1</Role>
      <ShortName>Prepaid Expenses and Other Current Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>995495 - Disclosure - Property and Equipment, Net</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosurePropertyAndEquipmentNet1</Role>
      <ShortName>Property and Equipment, Net</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>995505 - Disclosure - Accrued expenses and other current liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilities</Role>
      <ShortName>Accrued expenses and other current liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>995515 - Disclosure - Leases</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureLeases</Role>
      <ShortName>Leases</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>995535 - Disclosure - Common Stock</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureCommonStock</Role>
      <ShortName>Common Stock</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>995545 - Disclosure - Stock-based Compensation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureStockbasedCompensation</Role>
      <ShortName>Stock-based Compensation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>995555 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureIncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>995565 - Disclosure - Net loss per share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureNetLossPerShare1</Role>
      <ShortName>Net loss per share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995575 - Disclosure - Commitments and contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureCommitmentsAndContingencies</Role>
      <ShortName>Commitments and contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995585 - Disclosure - License Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureLicenseAgreements1</Role>
      <ShortName>License Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995595 - Disclosure - Revenue</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureRevenue</Role>
      <ShortName>Revenue</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995615 - Disclosure - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureSegmentReporting1</Role>
      <ShortName>Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995635 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://upstreambio.com/20260331/taxonomy/role/DisclosureSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>995645 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureFairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://upstreambio.com/20260331/taxonomy/role/DisclosureFairValueMeasurements</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>995655 - Disclosure - Prepaid Expenses and Other Current Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosurePrepaidExpensesAndOtherCurrentAssetsTables</Role>
      <ShortName>Prepaid Expenses and Other Current Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://upstreambio.com/20260331/taxonomy/role/DisclosurePrepaidExpensesAndOtherCurrentAssets1</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>995665 - Disclosure - Property and Equipment, Net (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosurePropertyAndEquipmentNetTables</Role>
      <ShortName>Property and Equipment, Net (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://upstreambio.com/20260331/taxonomy/role/DisclosurePropertyAndEquipmentNet1</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>995675 - Disclosure - Accrued expenses and other current liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilitiesTables</Role>
      <ShortName>Accrued expenses and other current liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://upstreambio.com/20260331/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilities</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>995695 - Disclosure - Net Loss per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureNetLossPerShareTables</Role>
      <ShortName>Net Loss per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>995705 - Disclosure - Stock-based Compensation (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureStockbasedCompensationTables</Role>
      <ShortName>Stock-based Compensation (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://upstreambio.com/20260331/taxonomy/role/DisclosureStockbasedCompensation</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>995715 - Disclosure - Segment Reporting (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureSegmentReportingTables</Role>
      <ShortName>Segment Reporting (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://upstreambio.com/20260331/taxonomy/role/DisclosureSegmentReporting1</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>995725 - Disclosure - Nature of the Business and Basis of Presentation - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureNatureOfTheBusinessAndBasisOfPresentationAdditionalInformationDetails</Role>
      <ShortName>Nature of the Business and Basis of Presentation - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>995745 - Disclosure - Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureFairValueMeasurementsScheduleOfFinancialAssetsAndLiabilitiesMeasuredAtFairValueDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>995755 - Disclosure - Fair Value Measurements - Schedule of Short-Term Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureFairValueMeasurementsScheduleOfShorttermInvestmentsDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Short-Term Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>995765 - Disclosure - Fair Value Measurements - Schedule of Contractual Maturities of Short-term Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureFairValueMeasurementsScheduleOfContractualMaturitiesOfShorttermInvestmentsDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of Contractual Maturities of Short-term Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>995775 - Disclosure - Fair Value Measurements - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureFairValueMeasurementsAdditionalInformationDetails</Role>
      <ShortName>Fair Value Measurements - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>995795 - Disclosure - Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosurePrepaidExpensesAndOtherCurrentAssetsScheduleOfPrepaidExpensesAndOtherCurrentAssetsDetails</Role>
      <ShortName>Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>995805 - Disclosure - Property and Equipment, Net - Schedule of Property Plant and Equipment, Net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosurePropertyAndEquipmentNetScheduleOfPropertyPlantAndEquipmentNetDetails</Role>
      <ShortName>Property and Equipment, Net - Schedule of Property Plant and Equipment, Net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>995815 - Disclosure - Property and Equipment, Net - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosurePropertyAndEquipmentNetAdditionalInformationDetails</Role>
      <ShortName>Property and Equipment, Net - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>995825 - Disclosure - Accrued expenses and other current liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureAccruedExpensesAndOtherCurrentLiabilitiesScheduleOfAccruedExpensesAndOtherCurrentLiabilitiesDetails</Role>
      <ShortName>Accrued expenses and other current liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>995835 - Disclosure - Leases - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureLeasesAdditionalInformationDetails</Role>
      <ShortName>Leases - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>995845 - Disclosure - Redeemable Convertible Preferred Stock - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureRedeemableConvertiblePreferredStockAdditionalInformationDetails</Role>
      <ShortName>Redeemable Convertible Preferred Stock - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>995855 - Disclosure - Common Stock - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureCommonStockAdditionalInformationDetails</Role>
      <ShortName>Common Stock - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>995865 - Disclosure - Stock-based Compensation - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureStockbasedCompensationAdditionalInformationDetails</Role>
      <ShortName>Stock-based Compensation - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>995875 - Disclosure - Stock-based compensation - Summary of RSU activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureStockbasedCompensationSummaryOfRsuActivityDetails</Role>
      <ShortName>Stock-based compensation - Summary of RSU activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>995885 - Disclosure - Stock-based Compensation - Schedule of Assumptions Used in Black-Scholes Option-pricing Model to Determine the Fair Value of Stock Options Granted (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureStockbasedCompensationScheduleOfAssumptionsUsedInBlackscholesOptionpricingModelToDetermineTheFairValueOfStockOptionsGrantedDetai</Role>
      <ShortName>Stock-based Compensation - Schedule of Assumptions Used in Black-Scholes Option-pricing Model to Determine the Fair Value of Stock Options Granted (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>995895 - Disclosure - Stock-based Compensation - Summary of Stock Options Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureStockbasedCompensationSummaryOfStockOptionsActivityDetails</Role>
      <ShortName>Stock-based Compensation - Summary of Stock Options Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>995905 - Disclosure - Stock-based compensation - Schedule of Classification of Stock-based Compensation in Condensed Consolidated Statements of Operations and Comprehensive Loss (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureStockbasedCompensationScheduleOfClassificationOfStockbasedCompensationInCondensedConsolidatedStatementsOfOperationsAndComprehensive</Role>
      <ShortName>Stock-based compensation - Schedule of Classification of Stock-based Compensation in Condensed Consolidated Statements of Operations and Comprehensive Loss (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>995915 - Disclosure - Income Taxes - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureIncomeTaxesAdditionalInformationDetails</Role>
      <ShortName>Income Taxes - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>995925 - Disclosure - Net Loss per Share - Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureNetLossPerShareSummaryOfBasicAndDilutedNetLossPerShareAttributableToCommonStockholdersDetails</Role>
      <ShortName>Net Loss per Share - Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>995935 - Disclosure - Net Loss per Share - Schedule of Dilutive Securities Excluded from Calculation of Diluted Net Loss Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureNetLossPerShareScheduleOfDilutiveSecuritiesExcludedFromCalculationOfDilutedNetLossPerShareDetails</Role>
      <ShortName>Net Loss per Share - Schedule of Dilutive Securities Excluded from Calculation of Diluted Net Loss Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>995945 - Disclosure - Commitments and Contingencies - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureCommitmentsAndContingenciesAdditionalInformationDetails</Role>
      <ShortName>Commitments and Contingencies - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>995955 - Disclosure - License Agreements - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureLicenseAgreementsAdditionalInformationDetails</Role>
      <ShortName>License Agreements - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>995965 - Disclosure - Revenue - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureRevenueAdditionalInformationDetails</Role>
      <ShortName>Revenue - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>995985 - Disclosure - Segment Reporting - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureSegmentReportingAdditionalInformationDetails</Role>
      <ShortName>Segment Reporting - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="upb-20260331.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>995995 - Disclosure - Segment Reporting - Schedule of Segment Net Loss, Including Disaggregated Components of Operating Expenses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://upstreambio.com/20260331/taxonomy/role/DisclosureSegmentReportingScheduleOfSegmentNetLossIncludingDisaggregatedComponentsOfOperatingExpensesDetails</Role>
      <ShortName>Segment Reporting - Schedule of Segment Net Loss, Including Disaggregated Components of Operating Expenses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="upb-20260331.htm">upb-20260331.htm</File>
    <File>upb-20260331.xsd</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="442">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="31">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>false</HasPresentationLinkbase>
  <HasCalculationLinkbase>false</HasCalculationLinkbase>
</FilingSummary>
