Description of the Business and Basis of Presentation	6 Months Ended
Jun. 30, 2026
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Description of the Business and Basis of Presentation	1. DESCRIPTION OF THE BUSINESS AND BASIS OF PRESENTATION Description of the Business Onterris, Inc. (“Onterris” or the “Company”), formerly known as Montrose Environmental Group, Inc., is a corporation formed in November 2013, under the laws of the State of Delaware. The Company changed its name to Onterris on April 17, 2026. The Company has approximately 120 offices across the United States, Canada, and Australia and approximately 3,500 employees as of June 30, 2026. Onterris is an environmental services company serving the recurring environmental needs of a diverse client base, including Fortune 500 companies and federal, state and local governments. Historically, the Company had three reportable segments: Assessment, Permitting and Response, Measurement and Analysis, and Remediation and Reuse. Effective in the first quarter of 2026, the Company realigned its reportable segments to reflect updates made to its organizational structure and operating model. As a result of these changes, the Company aggregated the Assessment, Permitting and Response and Remediation and Reuse segments into a newly created Consulting and Treatment segment based on their similar characteristics, including similar long-term financial performance, the nature of services provided, internal processes for delivering those services, and types of customers. The Company's Measurement and Analysis and corporate segments were not affected by the realignment. See Note 19 – Segment Information for more details. Consulting and Treatment segment provides environmental consulting, engineering, and implementation services to help clients assess, manage, and mitigate environmental risks across the lifecycle of their operations and projects. The Company supports environmental assessments, regulatory permitting, toxicology consulting, emergency preparedness, and response and environmental audits and permits, while also delivering engineering, design, and implementation solutions to treat contaminated water and remediate soil. The Company works closely with clients to navigate regulatory requirements, address environmental liabilities, respond to operational disruptions, and develop practical, sustainable solutions for new industrial developments, facility upgrades and ongoing operations. The Company provides these services as a consultant and service provider and does not own the properties, facilities, or underlying environmental liabilities associated with client projects. Measurement and Analysis segment is one of the largest providers of environmental testing and laboratory services in North America. The Company's highly credentialed teams test and analyze air, water and soil to determine concentrations of contaminants, as well as the toxicological impact of contaminants on flora, fauna and human health. The Company's offerings include source and ambient air testing and monitoring, leak detection, and advanced multi-media laboratory services, including air, soil, stormwater, wastewater and drinking water analysis. Corporate activities not directly related to segment performance, including general corporate expenses, interest and taxes, are reported separately. Prior period information has been recast herein to reflect the above change in segment reporting. The recast segment information constitutes a reclassification and has no impact on reported net income (loss) or earnings (loss) per share for preceding periods. This change does not restate information previously reported in the consolidated statements of financial position, the consolidated statements of operations and comprehensive income (loss), consolidated statements of convertible and redeemable series A-2 preferred stock and stockholders’ equity or consolidated statements of cash flows for the Company for preceding periods. Basis of Presentation The unaudited condensed consolidated financial statements include the operations of the Company and its wholly-owned subsidiaries. These unaudited condensed consolidated financial statements are presented in conformity with accounting principles generally accepted in the United States (U.S. GAAP) and have been prepared pursuant to the rules and regulations of the U.S. Securities and Exchange Commission (SEC) that permit reduced disclosure for interim periods. The unaudited condensed consolidated financial statements include all accounts of the Company and, in the opinion of management, include all recurring adjustments and normal accruals necessary for a fair statement of the Company’s financial position, results of operations and cash flows for the dates and periods presented. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements as of and for the year ended December 31, 2025. Results for interim periods are not necessarily indicative of the results to be expected during the remainder of the current year or for any future period. All intercompany transactions, accounts and profits, have been eliminated in the unaudited condensed consolidated financial statements. Certain prior period amounts, which are not material, have been reclassified to conform to current period presentation in the notes to the condensed consolidated financial statements. The realigned segment information does not modify or update the disclosures included in the Company's Annual Report on Form 10-K for the year ended December 31, 2025 in any way, nor does it reflect any subsequent information or events, other than as required to reflect the change in segments as described above.

Onterris, Inc. (“Onterris” or the “Company”), formerly known as Montrose Environmental Group, Inc., is a corporation formed in November 2013, under the laws of the State of Delaware. The Company changed its name to Onterris on April 17, 2026. The Company has approximately 120 offices across the United States, Canada, and Australia and approximately 3,500 employees as of June 30, 2026.

Onterris is an environmental services company serving the recurring environmental needs of a diverse client base, including Fortune 500 companies and federal, state and local governments. Historically, the Company had three reportable segments: Assessment, Permitting and Response, Measurement and Analysis, and Remediation and Reuse. Effective in the first quarter of 2026, the Company realigned its reportable segments to reflect updates made to its organizational structure and operating model. As a result of these changes, the Company aggregated the Assessment, Permitting and Response and Remediation and Reuse segments into a newly created Consulting and Treatment segment based on their similar characteristics, including similar long-term financial performance, the nature of services provided, internal processes for delivering those services, and types of customers. The Company's Measurement and Analysis and corporate segments were not affected by the realignment. See Note 19 – Segment Information for more details.

Consulting and Treatment segment provides environmental consulting, engineering, and implementation services to help clients assess, manage, and mitigate environmental risks across the lifecycle of their operations and projects. The Company supports environmental assessments, regulatory permitting, toxicology consulting, emergency preparedness, and response and environmental audits and permits, while also delivering engineering, design, and implementation solutions to treat contaminated water and remediate soil. The Company works closely with clients to navigate regulatory requirements, address environmental liabilities, respond to operational disruptions, and develop practical, sustainable solutions for new industrial developments, facility upgrades and ongoing operations. The Company provides these services as a consultant and service provider and does not own the properties, facilities, or underlying environmental liabilities associated with client projects.

Measurement and Analysis segment is one of the largest providers of environmental testing and laboratory services in North America. The Company's highly credentialed teams test and analyze air, water and soil to determine concentrations of contaminants, as well as the toxicological impact of contaminants on flora, fauna and human health. The Company's offerings include source and ambient air testing and monitoring, leak detection, and advanced multi-media laboratory services, including air, soil, stormwater, wastewater and drinking water analysis.

Corporate activities not directly related to segment performance, including general corporate expenses, interest and taxes, are reported separately.

Prior period information has been recast herein to reflect the above change in segment reporting. The recast segment information constitutes a reclassification and has no impact on reported net income (loss) or earnings (loss) per share for preceding periods. This change does not restate information previously reported in the consolidated statements of financial position, the consolidated statements of operations and comprehensive income (loss), consolidated statements of convertible and redeemable series A-2 preferred stock and stockholders’ equity or consolidated statements of cash flows for the Company for preceding periods.

Basis of Presentation

The unaudited condensed consolidated financial statements include the operations of the Company and its wholly-owned subsidiaries. These unaudited condensed consolidated financial statements are presented in conformity with accounting principles generally accepted in the United States (U.S. GAAP) and have been prepared pursuant to the rules and regulations of the U.S. Securities and Exchange Commission (SEC) that permit reduced disclosure for interim periods. The unaudited condensed consolidated financial statements include all accounts of the Company and, in the opinion of management, include all recurring adjustments and normal accruals necessary for a fair statement of the Company’s financial position, results of operations and cash flows for the dates and periods presented. These unaudited condensed consolidated financial statements should be read in conjunction with the Company’s audited financial statements as of and for the year ended December 31, 2025. Results for interim periods are not necessarily indicative of the results to be expected during the remainder of the current year or for any future period. All intercompany transactions, accounts and profits, have been eliminated in the unaudited condensed consolidated financial statements.

Certain prior period amounts, which are not material, have been reclassified to conform to current period presentation in the notes to the condensed consolidated financial statements. The realigned segment information does not modify or update the disclosures included in the Company's Annual Report on Form 10-K for the year ended December 31, 2025 in any way, nor does it reflect any subsequent information or events, other than as required to reflect the change in segments as described above.