Exhibit 5.1
July 10, 2026
Decoy Therapeutics Inc.
2450 Holcombe Blvd., Suite X
Houston, Texas 77021
Re: Registration Statement on Form S-1
Ladies and Gentlemen:
We have acted as counsel to Decoy Therapeutics Inc., a Delaware corporation (the “Company”), in connection with preparing and filing with the U.S. Securities and Exchange Commission (the “Commission”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), a Registration Statement on Form S-1 (the “Registration Statement”), in connection with the registration under the Securities Act for resale from time to time by the selling stockholders named in the related prospectus included in the Registration Statement (the “Selling Stockholders”) of up to 3,766,500 shares (the “Shares”) of the Company’s common stock, par value $0.0001 per share (the “Common Stock”). The Shares consist of (i) up to 592,217 shares of Common Stock issuable upon the exercise of pre-funded warrants (the “Pre-Funded Warrants”), (ii) up to 2,961,085 shares of Common Stock issuable upon the exercise of Series A common warrants, Series B common warrants and Series C common warrants (collectively, the “Milestone Warrants”) and (iii) up to 213,198 shares of Common Stock issuable upon the exercise of warrants issued to the placement agent and its designee (the “Placement Agent Warrants” and, together with the Pre-Funded Warrants and the Milestone Warrants, the “Warrants”), in each case issued by the Company in a private placement pursuant to or in connection with that certain Securities Purchase Agreement, dated as of June 26, 2026 (the “Purchase Agreement”), by and between the Company and the purchaser named therein, in each case as described in the Registration Statement and the related prospectus. This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or the related prospectus, other than as expressly stated herein with respect to the issuance of the Shares.
In connection with this opinion letter, we examined originals or copies of such records, instruments, certificates, opinions, memoranda and other documents as in our judgment are necessary or appropriate to enable us to render the opinions expressed below. We have not independently sought to verify factual matters. In conducting our examination, we assumed, without independent investigation, the genuineness and authenticity of all signatures on original documents, the accuracy, completeness and authenticity of all certificates, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted as copies, and the authenticity of the originals of such copies, the accuracy and completeness of all records made available to us by the Company, and the due authorization, execution and delivery by the Selling Stockholders of all documents where authorization, execution and delivery are prerequisites to the legal effectiveness of such documents. In addition, in connection with this opinion letter, we have assumed that the Shares will be offered in the manner and on the terms identified or referred to in the Registration Statement and the accompanying prospectus, including all supplements and amendments thereto.
Our opinions herein are limited solely to the matters set forth herein. The law covered by the opinions expressed herein is limited to the Delaware General Corporation Law as in effect on the date hereof (the “DGCL”). We are not admitted to practice in the State of Delaware and, with respect to the opinions set forth below, with your permission, we have (i) limited our review to standard compilations available to us of the DGCL, which we have assumed to be accurate and complete, and (ii) not reviewed case law. We express no opinion as to whether the laws of any jurisdiction are applicable to the subject matter hereof. We are not rendering any opinion with respect to federal law, including federal securities laws, or state blue sky securities laws. It is understood that this opinion letter is to be used only in connection with the offer and resale of the Shares while the Registration Statement is effective under the Securities Act.