Exhibit 99.1

JOINT FILING AGREEMENT

Pursuant to Rule 13d-1(k)(1) promulgated pursuant to the Securities Exchange Act of 1934, as amended, the undersigned agree that the attached Schedule 13D is being filed on behalf of each of the undersigned.

Date: October 20, 2025


	Atlas Venture Fund XI, L.P.

	By:	Atlas Venture Associates XI, L.P., its general partner
	By:	Atlas Venture Associates XI, LLC, its general partner

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

	Atlas Venture Associates XI, L.P.

	By:	Atlas Venture Associates XI, LLC, its general partner

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

	Atlas Venture Associates XI, LLC

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

	Atlas Venture Opportunity Fund II, L.P.

	By:	Atlas Venture Associates Opportunity II, L.P., its general partner
	By:	Atlas Venture Associates Opportunity II, LLC, its general partner

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

	Atlas Venture Associates Opportunity II, L.P.

	By:	Atlas Venture Associates Opportunity II, LLC, its general partner

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer


	Atlas Venture Associates Opportunity II, LLC

	By: /s/ Ommer Chohan
	Name:	Ommer Chohan
	Title:	Chief Financial Officer

Sionna Therapeutics, Inc. SCHEDULE 13D/A