Summary of Significant Accounting Policies	12 Months Ended
Dec. 31, 2025
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	(2) Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S.) (U.S. GAAP or GAAP). Principles of Consolidation The accompanying consolidated financial statements include the accounts of Synlogic and its wholly owned subsidiaries. All intercompany accounts and transactions have been eliminated in consolidation. Use of Estimates The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting period. On an on-going basis, the Company’s management evaluates its estimates, including those related to accrued expenses and contingencies. The Company bases its estimates on historical experience and on various other assumptions that are believed to be reasonable, the results of which form the basis for making judgments about the carrying values of assets and liabilities. Actual results could differ from those estimates. Changes in estimates are reflected in reported results in the period in which they become known. Cash Equivalents The Company considers all highly liquid investment instruments with a remaining maturity when purchased of three months or less to be cash equivalents. Investments qualifying as cash equivalents consist of money market funds, including money market funds held in a sweep account. Cash equivalents are stated at cost plus accrued interest, which approximates fair value. The amount of cash equivalents included in cash and cash equivalents was approximately $14.7 million and $18.9 million at December 31, 2025 and 2024, respectively. Concentration of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk include amounts held as cash, cash equivalents and restricted cash. The Company uses high quality, accredited financial institutions to maintain its balances, and accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. The Company has no financial instruments with off-balance sheet risk of loss. Fair Value The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. Accounting Standards Codification (ASC) Topic 820, Fair Value Measurements and Disclosures, establishes a fair value hierarchy for those instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). The hierarchy consists of three levels: • Level 1 – Utilize observable inputs such as quoted prices in active markets for identical assets or liabilities; • Level 2 – Utilize data points that are either directly or indirectly observable, such as quoted prices, interest rates and yield curves; • Level 3 – Utilize unobservable data points in which there is little or no market data, which require the Company to develop its own assumptions for the asset or liability. The Company evaluates transfers between levels at the end of each reporting period. There were no transfers of assets or liabilities between Level 1, Level 2 or Level 3 during the years ended December 31, 2025 and 2024. Impairment of Long‑Lived Assets Long‑lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is impairment, the amount of impairment is calculated as the difference between the carrying value and fair value of the asset. As a result of the corporate restructuring, further discussed in Note 7, Restructuring and Other Charges, there were various impairment charges recorded during the year ended December 31, 2024. Warrants The Company accounts for issued warrants either as a liability or equity in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity (“ASC 480-10”) or ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock (“ASC 815-40”). The assessment considers whether the warrants are freestanding financial instruments pursuant to ASC 480, meet the definition of a liability pursuant to ASC 480, and whether the warrants meet all of the requirements for equity classification under ASC 815, including whether the warrants are indexed to the company’s own stock and whether the warrant holders could potentially require “net cash settlement” in a circumstance outside of the company’s control, among other conditions for equity classification. This assessment, which requires the use of professional judgment, is conducted at the time of warrant issuance and as of each subsequent quarterly period end date while the warrants are outstanding. Warrants that are equity-classified instruments and recorded in additional paid-in capital at issuance are not subject to remeasurement. The purchase warrants issued in October 2023 are liability classified and recorded at fair value using the Black-Scholes option-pricing model at issuance, with any subsequent changes in fair value recognized in the consolidated statements of operations. The Company periodically evaluates changes in facts and circumstances that could impact the classification of warrants. Equity‑Based Compensation The Company measures equity-based compensation to employees, non-employees and directors based on the grant date fair value of the awards and recognizes the associated expense in the financial statements over the requisite service period of the award, which is generally the vesting period. The fair value of each option and purchase rights under the employee stock purchase plan (ESPP) is estimated on the date of grant using the Black‑Scholes option‑pricing model. Expected volatility for the Company’s common stock is determined based on an average of the historical volatility of the Company and the historical volatility of a peer‑group of similar public companies. The expected term of options granted to employees is calculated using the simplified method, which represents the average of the contractual term of the option and the weighted-average vesting period of the option. The expected term of purchase rights for the ESPP is based on the duration of an offering period. The assumed dividend yield is based upon the Company’s expectation of not paying dividends in the foreseeable future. The risk‑free interest rate is based upon the U.S. Treasury yield curve commensurate with the expected term at the time of grant or remeasurement. Forfeitures are recognized as they occur. The Company classifies equity-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified. Income Taxes The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial reporting and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies. Uncertain tax positions represent tax positions for which reserves have been established. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more likely than not to be sustained, the tax position is then assessed to determine the amount of benefit to be recognized in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties. Net Loss Per Share Basic net loss per share is computed using the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is computed using the sum of the weighted-average number of shares of common stock outstanding during the period and if dilutive, the weighted-average number of potential shares of common stock, including unvested restricted common stock, outstanding stock options and potential shares issuable under the ESPP. Segment Information Operating segments are defined as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company operates in one operating segment: discovery and development of Synthetic Biotics. The Company’s principal executive officer, as chief operating decision maker, manages and allocates resources to the operations of the Company on a total company basis. Prior to the corporate restructuring, all of the Company’s equipment, leasehold improvements and other fixed assets were physically located within the United States, and all agreements with its partners were denominated in U.S. dollars, except where noted. Treasury Stock Treasury stock purchases are accounted for under the cost method whereby the entire cost of the acquired stock is recorded as treasury stock. Recently Issued and Adopted Accounting Pronouncements From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (FASB) or other accounting standard setting bodies that the Company adopts as of the specified effective date. Unless otherwise discussed below, recently issued pronouncements that are or will be applicable to the Company did not have, or are not expected to have, a material impact on its present or future consolidated financial statements or disclosures. In December 2023, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2023-09, Income Taxes (Topics 740): Improvements to Income Tax Disclosures, to enhance the disclosure requirements for income taxes. Upon adoption, companies will be required to disclose additional specified categories in the rate reconciliation. Companies will also be required to disclose the amount of income taxes paid disaggregated by jurisdiction, among other disclosure requirements. The standard is effective for public business entities for annual periods beginning after December 15, 2024. The Company adopted the standard during our 2025 annual period and included the required disclosures prospectively in Note 11, Income Taxes.

The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S.) (U.S. GAAP or GAAP).

Use of Estimates

The preparation of financial statements in accordance with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets, liabilities, revenues and expenses, and related disclosure of contingent assets and liabilities at the date of the consolidated financial statements and the reported amounts of expenses during the reporting period. On an on-going basis, the Company’s management evaluates its estimates, including those related to accrued expenses and contingencies. The Company bases its estimates on historical experience and on various other assumptions that are believed to be reasonable, the results of which

Cash Equivalents

The Company considers all highly liquid investment instruments with a remaining maturity when purchased of three months or less to be cash equivalents. Investments qualifying as cash equivalents consist of money market funds, including money market funds held in a sweep account. Cash equivalents are stated at cost plus accrued interest, which approximates fair value. The amount of cash equivalents included in cash and cash equivalents was approximately $14.7 million and $18.9 million at December 31, 2025 and 2024, respectively.

Concentration of Credit Risk

Financial instruments that potentially subject the Company to concentrations of credit risk include amounts held as cash, cash equivalents and restricted cash. The Company uses high quality, accredited financial institutions to maintain its balances, and accordingly, such funds are subject to minimal credit risk. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. The Company has no financial instruments with off-balance sheet risk of loss.

Fair Value

The Company is required to disclose information on all assets and liabilities reported at fair value that enables an assessment of the inputs used in determining the reported fair values. Accounting Standards Codification (ASC) Topic 820, Fair Value Measurements and Disclosures, establishes a fair value hierarchy for those instruments measured at fair value that distinguishes between assumptions based on market data (observable inputs) and the Company’s own assumptions (unobservable inputs). The hierarchy consists of three levels:

•

Level 1 – Utilize observable inputs such as quoted prices in active markets for identical assets or liabilities;

•

Level 2 – Utilize data points that are either directly or indirectly observable, such as quoted prices, interest rates and yield curves;

•

Level 3 – Utilize unobservable data points in which there is little or no market data, which require the Company to develop its own assumptions for the asset or liability.

The Company evaluates transfers between levels at the end of each reporting period. There were no transfers of assets or liabilities between Level 1, Level 2 or Level 3 during the years ended December 31, 2025 and 2024.

Impairment of Long‑Lived Assets

Long‑lived assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of the asset may not be recoverable. When such events occur, the Company compares the carrying amounts of the assets to their undiscounted expected future cash flows. If this comparison indicates that there is impairment, the amount of impairment is calculated as the difference between the carrying value and fair value of the asset. As a result of the corporate restructuring, further discussed in Note 7, Restructuring and Other Charges, there were various impairment charges recorded during the year ended December 31, 2024.

Warrants

The Company accounts for issued warrants either as a liability or equity in accordance with ASC 480-10, Accounting for Certain Financial Instruments with Characteristics of both Liabilities and Equity (“ASC 480-10”) or ASC 815-40, Accounting for Derivative Financial Instruments Indexed to, and Potentially Settled in, a Company’s Own Stock (“ASC 815-40”). The assessment considers whether the warrants are freestanding financial instruments pursuant to ASC 480, meet the definition of a liability pursuant to ASC 480, and whether the warrants meet all of the requirements for equity classification under ASC 815, including whether the warrants are indexed to the company’s own stock and whether the warrant holders could potentially require “net cash settlement” in a circumstance outside of the company’s control, among other conditions for equity classification. This assessment, which requires the use of professional judgment, is conducted at the time of warrant issuance and as of each subsequent quarterly period end date while the warrants are outstanding.

Warrants that are equity-classified instruments and recorded in additional paid-in capital at issuance are not subject to remeasurement. The purchase warrants issued in October 2023 are liability classified and recorded at fair value using the Black-Scholes option-pricing model at issuance, with any subsequent changes in fair value recognized in the consolidated statements of operations. The Company periodically evaluates changes in facts and circumstances that could impact the classification of warrants.

Equity‑Based Compensation

The Company measures equity-based compensation to employees, non-employees and directors based on the grant date fair value of the awards and recognizes the associated expense in the financial statements over the requisite service period of the award, which is generally the vesting period. The fair value of each option and purchase rights under the employee stock purchase plan (ESPP) is estimated on the date of grant using the Black‑Scholes option‑pricing model. Expected volatility for the Company’s common stock is determined based on an average of the historical volatility of the Company and the historical volatility of a peer‑group of similar public companies. The expected term of options granted to employees is calculated using the simplified method, which represents the average of the contractual term of the option and the weighted-average vesting period of the option. The expected term of purchase rights for the ESPP is based on the duration of an offering period. The assumed dividend yield is based upon the Company’s expectation of not paying dividends in the foreseeable future. The risk‑free interest rate is based upon the U.S. Treasury yield curve commensurate with the expected term at the time of grant or remeasurement. Forfeitures are recognized as they occur.

The Company classifies equity-based compensation expense in its consolidated statements of operations and comprehensive loss in the same manner in which the award recipient’s payroll costs are classified or in which the award recipients’ service payments are classified.

Income Taxes

The Company accounts for income taxes using the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been recognized in the financial statements or in the Company’s tax returns. Deferred taxes are determined based on the difference between the financial reporting and tax basis of assets and liabilities using enacted tax rates in effect in the years in which the differences are expected to reverse. Changes in deferred tax assets and liabilities are recorded in the provision for income taxes. The Company assesses the likelihood that its deferred tax assets will be recovered from future taxable income and, to the extent it believes based upon the weight of available evidence, that it is more likely than not that all or a portion of deferred tax assets will not be realized, a valuation allowance is established through a charge to income tax expense. Potential for recovery of deferred tax assets is evaluated by estimating the future taxable profits expected and considering prudent and feasible tax planning strategies.

Uncertain tax positions represent tax positions for which reserves have been established. The Company accounts for uncertainty in income taxes recognized in the consolidated financial statements by applying a two-step process to determine the amount of tax benefit to be recognized. First, the tax position must be evaluated to determine the likelihood that it will be sustained upon external examination by the taxing authorities. If the tax position is deemed more likely than not to be sustained, the tax position is then assessed to determine the amount of benefit to be recognized in the financial statements. The amount of the benefit that may be recognized is the largest amount that has a greater than 50% likelihood of being realized upon ultimate settlement. The provision for income taxes includes the effects of any resulting tax reserves, or unrecognized tax benefits, that are considered appropriate as well as the related net interest and penalties.

Net Loss Per Share

Basic net loss per share is computed using the weighted-average number of shares of common stock outstanding during the period. Diluted net loss per share is computed using the sum of the weighted-average number of shares of common stock outstanding during the period and if dilutive, the weighted-average number of potential shares of common stock, including unvested restricted common stock, outstanding stock options and potential shares issuable under the ESPP.

Segment Information

Operating segments are defined as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company operates in one operating segment: discovery and development of Synthetic Biotics. The Company’s principal executive officer, as chief operating decision maker, manages and allocates resources to the operations of the Company on a total company basis. Prior to the corporate restructuring, all of the Company’s equipment, leasehold improvements and other fixed

Recently Issued and Adopted Accounting Pronouncements

From time to time, new accounting pronouncements are issued by the Financial Accounting Standards Board (FASB) or other accounting standard setting bodies that the Company adopts as of the specified effective date. Unless otherwise discussed below, recently issued pronouncements that are or will be applicable to the Company did not have, or are not expected to have, a material impact on its present or future consolidated financial statements or disclosures.

In December 2023, the Financial Accounting Standards Board, or FASB, issued Accounting Standards Update, or ASU, 2023-09, Income Taxes (Topics 740): Improvements to Income Tax Disclosures, to enhance the disclosure requirements for income taxes. Upon adoption, companies will be required to disclose additional specified categories in the rate reconciliation. Companies will also be required to disclose the amount of income taxes paid disaggregated by jurisdiction, among other disclosure requirements. The standard is effective for public business entities for annual periods beginning after December 15, 2024. The Company adopted the standard during our 2025 annual period and included the required disclosures prospectively in Note 11, Income Taxes.