Item 8.01. Other Events. On April 8, 2026, Tecogen Inc., a Delaware corporation (the “ Company”) agreed to extend the term of the Advisory Agreement dated January 18, 2018, as amended on July 22, 2019 and July 19, 2023, between the Company and Mr. John N. Hatsopoulos (the “ Advisory Agreement”) through March 31, 2028. The extension of the term of the Advisory Agreement was approved by Unanimous Written Consent of the Company’s Board of Directors on April 7, 2026. A copy of the letter agreement extending the Advisory Agreement is attached hereto as Exhibit 99.1.

Item 9.01. Financial Statements and Exhibits. (d) Exhibits Exhibit Description 99.1 Letter Agreement dated April 8, 2026 extending the term of the Advisory Agreement between Tecogen Inc. and John N. Hatsopoulos. ──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────── SIGNATURES Pursuant to the requirements of the Securities Exchange Act of 1934, the Company has duly caused this report to be signed on its behalf by the undersigned, hereunto duly authorized. TECOGEN INC. By: /s/ Abinand Rangesh April 8, 2026 Abinand Rangesh, Chief Executive Officer