0001193125-26-220584 | 10-Q | XBRL Viewer

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Stock-based Compensation - Summary of Stock Options Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2026	Dec. 31, 2025
Share-Based Payment Arrangement [Abstract]
Number of Options, Outstanding as of December 31, 2025	7,960,728
Number of Options, Granted	1,315,750
Number of Options, Exercised	(159,331)
Number Of Options, Forfeited	(116,162)
Number of Options, Outstanding as of March 31, 2026	9,000,985	7,960,728
Number of Options, Options exercisable March 31, 2026	3,948,929
Number of Options, Vested and expected to vest March 31 2026	9,000,985
Weighted Average Exercise Price, Outstanding as of December 31, 2025	$ 6.77
Weighted Average Exercise Price, Granted	27.11
Weighted Average Exercise Price, Exercised	3.75
Weighted Average Exercise Price, Forfeited	15.15
Weighted Average Exercise Price, Outstanding as of March 31, 2026	9.68	$ 6.77
Weighted Average Exercise Price, Options exercisable March 31, 2026	5.94
Weighted Average Exercise Price, Vested and expected to vest March 31, 2026	$ 9.68
Weighted-Average Remaining Contractual Term, Outstanding	7 years 10 months 24 days	8 years
Weighted-Average Remaining Contractual Term, Options exercisable March 31, 2026	7 years 2 months 12 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest March 31, 2026	7 years 10 months 24 days
Intrinsic Value, Outstanding	$ 21,225	$ 162,277
Intrinsic Value, Options exercisable March 31, 2026	14,257
Intrinsic Value, Vested and expected to vest March 31, 2026	$ 21,225

Nature of the Business and Basis of Presentation	3 Months Ended
Mar. 31, 2026
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the business and basis of presentation Upstream Bio, Inc. was incorporated in April 2021, under the laws of the State of Delaware, and along with its consolidated subsidiary (collectively, the “Company” or “Upstream”), is focused on developing treatments for inflammatory diseases, with an initial focus on severe respiratory disorders. Since its inception, the Company has devoted substantially all of its efforts to raising capital and incurring research and development expenses related to advancing verekitug, a clinical-stage monoclonal antibody that targets and inhibits the Thymic Stromal Lymphopoietin receptor. Risks and uncertainties The Company is subject to risks and uncertainties common to early-stage companies in the biopharmaceutical industry, including, but not limited to, the successful development of verekitug, the development of new technological innovations by competitors, dependence on key personnel, the ability to attract and retain qualified employees, protection of proprietary technology, compliance with governmental regulations and the ability to secure additional capital to fund operations and commercial success of verekitug. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be maintained, that any therapeutic products developed will obtain required regulatory approval or that any approved or consumer products will be commercially viable. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will generate significant product sales. Liquidity The Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the issuance of these condensed consolidated financial statements. The accompanying condensed consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. To date, the Company has financed its operations primarily through equity financings and the proceeds from its initial public offering (“IPO”). As of March 31, 2026, the Company has received an aggregate of $668.8 million from such transactions, including gross proceeds of $400.0 million from the issuance and sale of its redeemable convertible preferred stock and $268.8 million in net proceeds from the closing of its IPO. The Company incurred net losses of $40.6 million and $27.3 million for the three months ended March 31, 2026 and 2025, respectively. As of March 31, 2026, the Company had an accumulated deficit of $374.8 million The Company has incurred recurring losses and negative cash flows from operations since its inception and expects to continue to incur losses and negative cash flows for the foreseeable future as it continues the research and development of verekitug. The Company expects its cash, cash equivalents and short-term investments will be sufficient to fund its operating expenses and capital expenditure requirements for at least the next twelve months from the date of issuance of these condensed consolidated 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 product revenue, if ever, the Company expects to fund its operations through equity offerings or debt financings, credit or loan facilities, potentially other capital resources, or a combination of one or more of these funding sources. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. The Company’s failure to raise capital as and when needed could have a negative impact on its financial condition and its ability to pursue its business strategies. If adequate funds are not available to the Company, the Company may be required to delay, reduce or eliminate clinical programs, obtain funds through arrangements with collaborators on terms unfavorable to the Company or pursue merger or acquisition strategies. There can be no assurances the Company will be able to obtain additional funding. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all. Basis of presentation The accompanying unaudited condensed consolidated financial statements reflect the operations of the Company. Intercompany balances and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) for interim financial reporting and as required by Regulation S-X, Rule 10-01. Any reference in these notes to applicable guidance is meant to refer to the authoritative U.S. GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”). The condensed consolidated interim financial statements have been prepared on the same basis as the audited annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair statement of the Company’s financial position as of March 31, 2026 and the results of operations for the three months ended March 31, 2026 and 2025. The condensed balance sheet as of December 31, 2025 was derived from audited annual financial statements but does not include all disclosures required by U.S. GAAP. The results of operations for the interim periods are not necessarily indicative of results to be expected for the year ending December 31, 2026, any other interim periods, or any future year or period.

Nature of the Business and Basis of Presentation

3 Months Ended

Mar. 31, 2026

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Nature of the Business and Basis of Presentation

1. Nature of the business and basis of presentation

Upstream Bio, Inc. was incorporated in April 2021, under the laws of the State of Delaware, and along with its consolidated subsidiary (collectively, the “Company” or “Upstream”), is focused on developing treatments for inflammatory diseases, with an initial focus on severe respiratory disorders. Since its inception, the Company has devoted substantially all of its efforts to raising capital and incurring research and development expenses related to advancing verekitug, a clinical-stage monoclonal antibody that targets and inhibits the Thymic Stromal Lymphopoietin receptor.

Risks and uncertainties

The Company is subject to risks and uncertainties common to early-stage companies in the biopharmaceutical industry, including, but not limited to, the successful development of verekitug, the development of new technological innovations by competitors, dependence on key personnel, the ability to attract and retain qualified employees, protection of proprietary technology, compliance with governmental regulations and the ability to secure additional capital to fund operations and commercial success of verekitug. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be maintained, that any therapeutic products developed will obtain required regulatory approval or that any approved or consumer products will be commercially viable. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will generate significant product sales.

Liquidity

The Company has evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about the Company’s ability to continue as a going concern within one year from the issuance of these condensed consolidated financial statements.

The accompanying condensed consolidated financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities and commitments in the ordinary course of business. To date, the Company has financed its operations primarily through equity financings and the proceeds from its initial public offering (“IPO”). As of March 31, 2026, the Company has received an aggregate of $668.8 million from such transactions, including gross proceeds of $400.0 million from the issuance and sale of its redeemable convertible preferred stock and $268.8 million in net proceeds from the closing of its IPO. The Company incurred net losses of $40.6 million and $27.3 million for the three months ended March 31, 2026 and 2025, respectively. As of March 31, 2026, the Company had an accumulated deficit of $374.8 million

The Company has incurred recurring losses and negative cash flows from operations since its inception and expects to continue to incur losses and negative cash flows for the foreseeable future as it continues the research and development of verekitug. The Company expects its cash, cash equivalents and short-term investments will be sufficient to fund its operating expenses and capital expenditure requirements for at least the next twelve months from the date of issuance of these condensed consolidated 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 product revenue, if ever, the Company expects to fund its operations through equity offerings or debt financings, credit or loan facilities, potentially other capital resources, or a combination of one or more of these funding sources. The terms of any financing may adversely affect the holdings or the rights of the Company’s stockholders. The Company’s failure to raise capital as and when needed could have a negative impact on its financial condition and its ability to pursue its business strategies. If adequate funds are not available to the Company, the Company may be required to delay, reduce or eliminate clinical programs, obtain funds through arrangements with collaborators on terms unfavorable to the Company or pursue merger or acquisition strategies. There can be no assurances the Company will be able to obtain additional funding. Although management continues to pursue these plans, there is no assurance that the Company will be successful in obtaining sufficient funding on terms acceptable to the Company to fund continuing operations, if at all.

Basis of presentation

The accompanying unaudited condensed consolidated financial statements reflect the operations of the Company. Intercompany balances and transactions have been eliminated in consolidation. The accompanying unaudited condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) for interim financial reporting and as required by Regulation S-X, Rule 10-01. Any reference in these notes to applicable guidance is meant to refer to the authoritative U.S. GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Update (“ASU”) of the Financial Accounting Standards Board (“FASB”).

The condensed consolidated interim financial statements have been prepared on the same basis as the audited annual financial statements and, in the opinion of management, reflect all adjustments, which include only normal recurring adjustments, necessary for a fair statement of the Company’s financial position as of March 31, 2026 and the results of operations for the three months ended March 31, 2026 and 2025. The condensed balance sheet as of December 31, 2025 was derived from audited annual financial statements but does not include all disclosures required by U.S. GAAP. The results of operations for the interim periods are not necessarily indicative of results to be expected for the year ending December 31, 2026, any other interim periods, or any future year or period.

Insider Trading Arrangements	3 Months Ended
Insider Trading Arrangements	Mar. 31, 2026
Trading Arrangements, by Individual
Rule 10b5-1 Arrangement Adopted	false
Non-Rule 10b5-1 Arrangement Adopted	false
Rule 10b5-1 Arrangement Terminated	false
Non-Rule 10b5-1 Arrangement Terminated	false
Rule 10b5 1 Arr Modified Flag	false
Non Rule 10b5 1 Arr Modified Flag	false

Stock-based compensation - Schedule of Classification of Stock-based Compensation in Condensed Consolidated Statements of Operations and Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation	$ 4,647	$ 2,635
General and Administrative
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation	2,713	2,155
Research and Development
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]
Stock-based compensation	$ 1,934	$ 480

Accrued expenses and other current liabilities

3 Months Ended

Mar. 31, 2026

Payables and Accruals [Abstract]

Accrued expenses and other current liabilities

6. Accrued expenses and other current liabilities

Accrued expenses and other current liabilities consisted of the following (in thousands):


	March 31,		December 31,
	2026		2025
Accrued external research and development expenses	$	7,759	$	5,057
Accrued employee compensation and benefits		2,215		4,192
Accrued consultant and professional fees		1,261		757
	$	11,235	$	10,006

Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2026

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Schedule of Financial Assets and Liabilities Measured at Fair Value

The following tables present information about the Company’s financial assets and liabilities measured at fair value on a recurring basis and indicate the level of the fair value hierarchy used to determine such fair values (in thousands):


	Fair Value Measurements at March 31, 2026
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents:
Money market funds	$	95,894	$	—	$	—	$	95,894
Short-term investments:
U.S. treasury securities		—		149,847		—		149,847
Corporate debt securities		—		42,756		—		42,756
Government agency bonds		—		5,252		—		5,252
	$	95,894	$	197,855	$	—	$	293,749


	Fair Value Measurements at December 31, 2025
	Level 1		Level 2		Level 3		Total
Assets:
Cash equivalents:
Money market funds	$	85,866	$	—	$	—	$	85,866
U.S. treasury securities		—		14,991		—		14,991
Short-term investments:
U.S. treasury securities		—		159,455		—		159,455
Corporate debt securities		—		75,216		—		75,216
Government agency bonds		—		5,260		—		5,260
	$	85,866	$	254,922	$	—	$	340,788

Schedule of Short-Term Investments

Short-term investments consisted of the following (in thousands):


	March 31, 2026
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term investments:
U.S. treasury securities	$	149,745	$	142	$	(40	)	$	149,847
Corporate debt securities		42,758		29		(31	)		42,756
Government agency bonds		5,252		—		—			5,252
Total short-term investments:	$	197,755	$	171	$	(71	)	$	197,855


	December 31, 2025
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Short-term investments:
U.S. treasury securities	$	159,033	$	422	$	—		$	159,455
Corporate debt securities		75,117		102		(3	)		75,216
Government agency bonds		5,253		7		—			5,260
Total short-term investments:	$	239,403	$	531	$	(3	)	$	239,931

Schedule of Contractual Maturities of Short-term Investments

The contractual maturities of the Company’s short-term investments in available-for-sale securities held were as follows (in thousands):


	March 31, 2026		December 31, 2025
Due within one year	$	186,796	$	221,506
Due after one year through two years		11,059		18,425
Total available-for-sale securities	$	197,855	$	239,931

Schedule of Assumptions Used in Option-Pricing Model to Determine Fair Value of Series B Preferred Stock Tranche Right Liability

The following table presents, on a weighted-average basis, the assumptions used in the Black-Scholes option-pricing model to determine the fair value of stock options granted:


	Three Months Ended March 31,
	2026			2025
Expected volatility		91.2	%		92.1	%
Expected dividends		0	%		0	%
Expected term (in years)		6.0			6.0
Risk-free rate		3.86	%		4.31	%

Fair Value Measurements - Schedule of Contractual Maturities of Short-term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Fair Value Disclosures [Abstract]
Due within one year	$ 186,796	$ 221,506
Due after one year through two years	11,059	18,425
Total available-for-sale securities	$ 197,855	$ 239,931

Accrued expenses and other current liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Payables and Accruals [Abstract]
Accrued external research and development expenses	$ 7,759	$ 5,057
Accrued employee compensation and benefits	2,215	4,192
Accrued consultant and professional fees	1,261	757
Accrued expenses and other current liabilities	$ 11,235	$ 10,006

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Prepaid Expense and Other Assets, Current [Abstract]
Prepaid research and development expense	$ 16,819	$ 6,204
Interest receivable	1,573	1,653
Prepaid insurance	591	881
Prepaid employee-related costs	110	155
Other	1,573	727
Prepaid expenses and other current assets	$ 20,666	$ 9,620

Fair Value Measurements - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2026	Dec. 31, 2025
Debt Securities, Available-for-Sale [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset,Transfers, Net	$ 0	$ 0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Transfers, Net, Total	$ 0	$ 0

Leases - Additional Information (Details) $ in Millions	3 Months Ended
Leases - Additional Information (Details) $ in Millions	Mar. 31, 2026 USD ($) ft²
Leases [Abstract]
Lease description	In July 2024, the Company entered into an operating lease agreement for office space located at 890 Winter Street in Waltham, Massachusetts. The lease commenced in September 2024 and the Company began paying monthly rent starting one month thereafter.
Area of real estate property \| ft²	16,801
Operating lease, term of contract	3 years
Rent payment, year one	$ 0.7
Rent payment, year two	0.8
Rent payment, year three	$ 0.8

Summary of Significant Accounting Policies (Policies)	3 Months Ended
Summary of Significant Accounting Policies (Policies)	Mar. 31, 2026
Accounting Policies [Abstract]
Use of Estimates	Use of estimates The preparation of condensed consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenue and expenses, and the disclosure of contingent assets and liabilities as of and during the reporting period. The Company bases estimates and assumptions on historical experience when available and on various factors that it believes to be reasonable under the circumstances. The Company assesses estimates on an ongoing basis; however, actual results could materially differ from those estimates. Significant estimates and assumptions reflected within these condensed consolidated financial statements include, but are not limited to, prepaid and accrued research and development expenses, including those related to contract research organizations (“CROs”), contract manufacturing organizations (“CMOs”) and other third-party vendors and the valuation of the Company’s stock-based awards. Changes in estimates are recorded in the period in which they become known.
Recently Issued Accounting Pronouncements Not Yet Adopted	Recently issued accounting pronouncements not yet adopted In November 2024, the FASB issued ASU 2024-03, Disaggregation of Income Statement Expenses, which requires more detailed information about specified categories of expenses (purchases of inventory, employee compensation, depreciation, amortization, and depletion) included in certain expense captions presented on the face of the income statement. This ASU is effective for fiscal years beginning after December 15, 2026, and for interim periods within fiscal years beginning after December 15, 2027. Early adoption is permitted. The amendments may be applied either (1) prospectively to financial statements issued for reporting periods after the effective date of this ASU or (2) retrospectively to all prior periods presented in the financial statements. The Company is currently evaluating the impact of adopting this ASU on its consolidated financial statements and related disclosures.

Property and Equipment, Net - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 49,000	$ 43,000
Maximum
Property, Plant and Equipment [Line Items]
Depreciation and amortization	$ 100,000	$ 100,000

Net Loss per Share - Summary of Basic and Diluted Net Loss per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Numerator:
Net loss	$ (40,591)	$ (27,270)
Denominator:
Weighted-average common shares outstanding, basic	54,332,319	53,617,723
Weighted-average common shares outstanding, diluted	54,332,319	53,617,723
Net loss per share attributable to common stockholders, basic	$ (0.75)	$ (0.51)
Net loss per share attributable to common stockholders, diluted	$ (0.75)	$ (0.51)

Stock-based Compensation - Additional Information (Details) - USD ($)		3 Months Ended		12 Months Ended
	Aug. 19, 2024	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025	Jan. 01, 2026
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Weighted-average grant-date fair value of options granted		$ 20.78	$ 6.64
Aggregate intrinsic value of stock options exercised		$ 3,100,000	$ 200,000
Unrecognized compensation cost related to unvested stock options		$ 43,400,000
Weighted average period in years of recognition for unvested stock options		2 years 7 months 6 days
2024 Stock Option and Incentive Plan and 2021 Stock Option and Grant Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock reserved for future issuance		14,012,842
2024 Stock Option and Incentive Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock reserved for future issuance	3,180,000	4,670,401			2,711,887
Increase in percentage of outstanding number of common stock		5.00%
2024 Employee Stock Purchase Plan
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Common stock reserved for future issuance	488,467				0
Increase in number of shares available for issuance	976,934
Shares issued		26,197
Increase in percentage of outstanding number of common stock		1.00%
Restricted Stock Units
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Weighted average period in years of recognition for unvested stock options		3 years 8 months 12 days
Fair value of stock units vested		$ 200,000		$ 0
Unrecognized compensation cost related to RSUs		$ 9,100,000

Revenue - Additional Information (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Oct. 31, 2021	Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
Disaggregation of Revenue [Line Items]
Collaboration revenue		$ 1,034	$ 566
Related party accounts receivable		1,034		$ 668
Maruho Agreement
Disaggregation of Revenue [Line Items]
License agreement, termination written notice period	60 days
Maruho Agreement \| Maruho
Disaggregation of Revenue [Line Items]
Cost reimbursements received		700	600
Collaboration revenue		1,000	$ 600
Related party accounts receivable		$ 1,000		$ 700

Commitments and Contingencies - Additional Information (Details) - USD ($)	Mar. 31, 2026	Dec. 31, 2025
Commitments and Contingencies Disclosure [Abstract]
Liabilities recorded for loss contingencies	$ 0	$ 0

Document and Entity Information - shares	3 Months Ended
Document and Entity Information - shares	Mar. 31, 2026	May 07, 2026
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2026
Document Fiscal Year Focus	2026
Document Fiscal Period Focus	Q1
Entity Registrant Name	UPSTREAM BIO, INC.
Trading Symbol	UPB
Entity Central Index Key	0002022626
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Common Stock, Shares Outstanding		54,419,986
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
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
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false
Title of 12(b) Security	Common Stock, par value $0.001 per share
Security Exchange Name	NASDAQ

Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Income Tax Disclosure [Abstract]
Effective federal tax rate	0.00%	0.00%
Effective state tax rate	0.00%	0.00%
Income tax expense or benefit	$ 0	$ 0

Net Loss per Share - Schedule of Dilutive Securities Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive securities excluded from calculation of diluted net loss per share	9,342,441	7,927,513
Stock Options to Purchase Common Stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive securities excluded from calculation of diluted net loss per share	9,000,985	7,927,513
Unvested restricted common stock awards
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive securities excluded from calculation of diluted net loss per share	341,456	0

Redeemable Convertible Preferred Stock - Additional Information (Details) - $ / shares	Mar. 31, 2026	Dec. 31, 2025
Conversion of Stock [Line Items]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0

Stock-based compensation - Summary of RSU activity (Details) - Restricted Stock Units	3 Months Ended
	Mar. 31, 2026 $ / shares shares
Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]
Number of Stock Units, Unvested and outstanding as of December 31, 2025 \| shares	0
Number of Stock Units, Granted \| shares	376,250
Number of Stock Units, Vested \| shares	(22,905)
Number of Stock Units, Forfeited \| shares	(11,889)
Number of Stock Units, Unvested and outstanding as of March 31, 2026 \| shares	341,456
Weighted Average Grant Date Fair Value Per Share, Unvested and outstanding as of December 31, 2025 \| $ / shares	$ 0
Weighted Average Grant Date Fair Value Per Share, Granted \| $ / shares	26.65
Weighted Average Grant Date Fair Value Per Share, Vested \| $ / shares	27.07
Weighted Average Grant Date Fair Value Per Share, Forfeited \| $ / shares	27.07
Weighted Average Grant Date Fair Value Per Share, Unvested and outstanding as of March 31, 2026 \| $ / shares	$ 26.6

Segment Reporting - Additional Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2026 USD ($) Segment
Segment Reporting [Abstract]
Number of operating segments	1
Number of reportable segments	1
Segment reporting, expense information used by codm, description	The Company’s chief executive officer, who is the CODM, manages the Company on a consolidated basis and utilizes consolidated net loss as a basis for resource allocation and decision making.
Segment Reporting, CODM, Individual Title and Position or Group Name [Extensible Enumeration]	Chief Executive Officer
Cash, cash equivalents and short-term investments \| $	$ 294.6

Segment Reporting - Schedule of Segment Net Loss, Including Disaggregated Components of Operating Expenses (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Segment Reporting Information [Line Items]
Collaboration revenue	$ 1,034	$ 566
Research and development:
Personnel expenses	7,201	3,705
Manufacturing costs	4,973	2,129
Professional fees and other	1,630	1,492
Total research and development expenses	36,565	25,797
General and administrative:
Personnel expenses	4,937	4,232
Professional fees	2,034	1,496
Other	1,113	1,054
Total general and administrative expenses	8,084	6,782
Total operating expenses	44,649	32,579
Loss from operations	(43,615)	(32,013)
Other income (expense):
Interest income	3,054	4,743
Other expense, net	(30)	(0)
Total other income, net	3,024	4,743
Net loss	(40,591)	(27,270)
COPD Indication
Research and development:
Verekitug program	10,962	5,687
Asthma Indication
Research and development:
Verekitug program	10,710	10,021
CRSwNP Indication
Research and development:
Verekitug program	$ 1,089	$ 2,763

Condensed consolidated balance sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Current assets:
Cash and cash equivalents	$ 96,724	$ 101,578
Short-term investments	197,855	239,931
Accounts receivable	1,034	668
Prepaid expenses and other current assets	20,666	9,620
Total current assets	316,279	351,797
Property and equipment, net	510	559
Operating lease right-of-use assets	1,071	1,222
Restricted cash	194	194
Total assets	318,054	353,772
Current liabilities:
Accounts payable	1,710	2,726
Accrued expenses and other current liabilities	11,235	10,006
Operating lease liabilities, current portion	724	720
Total current liabilities	13,669	13,452
Operating lease liabilities, net of current portion	390	549
Total liabilities	14,059	14,001
Commitments and contingencies (Note 13)
Stockholders' deficit:
Preferred stock, $0.001 par value; 10,000,000 shares authorized at March 31, 2026 and December 31, 2025; no shares issued and outstanding at March 31, 2026 and December 31, 2025
Common stock, $0.001 par value; 500,000,000 shares authorized at March 31, 2026 and December 31, 2025; 54,419,986 and 54,237,750 shares issued and outstanding at March 31, 2026 and December 31, 2025, respectively	54	54
Additional paid-in capital	678,655	673,410
Accumulated other comprehensive income	100	530
Accumulated deficit	(374,814)	(334,223)
Total stockholders' deficit	303,995	339,771
Total liabilities, redeemable convertible preferred stock and stockholders' deficit	$ 318,054	$ 353,772

Condensed consolidated statements of operations and comprehensive loss (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Income Statement [Abstract]
Collaboration revenue	$ 1,034	$ 566
Operating expenses:
Research and development	36,565	25,797
General and administrative	8,084	6,782
Total operating expenses	44,649	32,579
Loss from operations	(43,615)	(32,013)
Other income (expense):
Interest income	3,054	4,743
Other expense, net	(30)	(0)
Total other income, net	3,024	4,743
Net loss	$ (40,591)	$ (27,270)
Net loss per share attributable to common stockholders, basic	$ (0.75)	$ (0.51)
Net loss per share attributable to common stockholders, diluted	$ (0.75)	$ (0.51)
Weighted-average common shares outstanding, basic	54,332,319	53,617,723
Weighted-average common shares outstanding, diluted	54,332,319	53,617,723
Comprehensive loss:
Net loss	$ (40,591)	$ (27,270)
Unrealized gain (loss) on investments, net of tax	(430)	393
Total other comprehensive income (loss)	(430)	393
Comprehensive loss	$ (41,021)	$ (26,877)

Property and Equipment, Net - Schedule of Property Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 903	$ 903
Less: Accumulated depreciation and amortization	(393)	(344)
Property and equipment, net	510	559
Office Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	559	559
Computer Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	202	202
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	87	87
Lab Equipment
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 55	$ 55

Fair Value Measurements - Schedule of Short-Term Investments (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	$ 197,755	$ 239,403
Gross Unrealized Gains	171	531
Gross Unrealized Losses	(71)	(3)
Fair Value	197,855	239,931
US Treasury Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	149,745	159,033
Gross Unrealized Gains	142	422
Gross Unrealized Losses	(40)	0
Fair Value	149,847	159,455
Corporate Debt Securities
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	42,758	75,117
Gross Unrealized Gains	29	102
Gross Unrealized Losses	(31)	(3)
Fair Value	42,756	75,216
U.S. Government Agency Bonds
Debt Securities, Available-for-Sale [Line Items]
Amortized Cost	5,252	5,253
Gross Unrealized Gains	0	7
Gross Unrealized Losses	0	0
Fair Value	$ 5,252	$ 5,260

Pay vs Performance Disclosure - USD ($) $ in Thousands	3 Months Ended
Pay vs Performance Disclosure - USD ($) $ in Thousands	Mar. 31, 2026	Mar. 31, 2025
Pay vs Performance Disclosure
Net Income (Loss)	$ (40,591)	$ (27,270)

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Cash flows from operating activities:
Net loss	$ (40,591)	$ (27,270)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	49	43
Stock-based compensation expense	4,647	2,635
Net amortization of premiums and accretion of discounts on short-term investments	(754)	(1,735)
Non-cash lease expense	151	134
Changes in operating assets and liabilities:
Accounts receivable	(366)	47
Prepaid expenses and other assets	(11,046)	(13,745)
Accounts payable	(1,016)	711
Accrued expenses and other current liabilities	1,229	(1,851)
Operating lease liabilities	(155)	(134)
Net cash used in operating activities	(47,852)	(41,165)
Cash flows from investing activities:
Purchases of short-term investments	(14,422)	(266,056)
Maturities of short-term investments	56,822	52,675
Net cash provided by (used in) investing activities	42,400	(213,381)
Cash flows from financing activities:
Payments of initial public offering costs	0	(34)
Proceeds from exercises of stock options	598	0
Net cash provided by (used in) financing activities	598	(34)
Net decrease in cash, cash equivalents and restricted cash	(4,854)	(254,580)
Cash, cash equivalents and restricted cash at beginning of period	101,772	326,086
Cash, cash equivalents and restricted cash at end of period	96,918	71,506
Cash, cash equivalents and restricted cash at end of period:
Cash and cash equivalents	96,724	71,312
Restricted cash	194	194
Total cash, cash equivalents and restricted cash at end of period	$ 96,918	$ 71,506

Condensed Consolidated Statements of Stockholders' Deficit (Unaudited) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Beginning balance at Dec. 31, 2024	$ 469,852	$ 53	$ 660,604	$ (190,780)	$ (25)
Beginning balance, shares at Dec. 31, 2024		53,603,398
Issuance of common stock upon exercise of stock options, shares		37,497
Stock-based compensation expense	2,635		2,635
Unrealized gain (loss) on available-for-sale securities, net of tax	393				393
Net loss	(27,270)			(27,270)
Ending balance at Mar. 31, 2025	445,610	$ 53	663,239	(218,050)	368
Ending balance, shares at Mar. 31, 2025		53,640,895
Beginning balance at Dec. 31, 2025	339,771	$ 54	673,410	(334,223)	530
Beginning balance, shares at Dec. 31, 2025		54,237,750
Issuance of common stock upon exercise of stock options	598		598
Issuance of common stock upon exercise of stock options, shares		159,331
Vesting of restricted stock units, shares		22,905
Stock-based compensation expense	4,647		4,647
Unrealized gain (loss) on available-for-sale securities, net of tax	(430)				(430)
Net loss	(40,591)			(40,591)
Ending balance at Mar. 31, 2026	$ 303,995	$ 54	$ 678,655	$ (374,814)	$ 100
Ending balance, shares at Mar. 31, 2026		54,419,986

Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2026	Dec. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shor-term investments	$ 197,855	$ 239,931
Fair Value on Recurring Basis
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	293,749	340,788
Fair Value on Recurring Basis \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	95,894	85,866
Fair Value on Recurring Basis \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets	197,855	254,922
Fair Value on Recurring Basis \| Money Market Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	95,894	85,866
Fair Value on Recurring Basis \| Money Market Funds \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	95,894	85,866
Fair Value on Recurring Basis \| US Treasury Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		14,991
Shor-term investments	149,847	159,455
Fair Value on Recurring Basis \| US Treasury Securities \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents		14,991
Shor-term investments	149,847	159,455
Fair Value on Recurring Basis \| U.S. Government Agency Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shor-term investments	5,252	5,260
Fair Value on Recurring Basis \| U.S. Government Agency Bonds \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shor-term investments	5,252	5,260
Fair Value on Recurring Basis \| Corporate Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shor-term investments	42,756	75,216
Fair Value on Recurring Basis \| Corporate Debt Securities \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Shor-term investments	$ 42,756	$ 75,216