**Exhibit 10.3 **
**JOINDER TO **
**REGISTRATION RIGHTS AGREEMENT **
The undersigned hereby agrees to become a party to the Registration Rights Agreement, dated as of September 25, 2025, by and among American Exceptionalism Acquisition Corp. A, a Cayman Islands exempted company, AEXA Sponsor LLC, a Cayman Islands limited liability company and the other parties thereto (together with the Sponsor, the “Holders” and, each, a “Holder”) (as amended from time to time, the “Agreement”), hereby accepts and agrees to be subject to, and comply with the terms, conditions and provisions of, the Agreement as a Holder thereunder, and shall be entitled to the rights and benefits and subject to the duties and obligations of a Holder thereunder.
Date: September 1, 2026
[Signature page follows]
| DIRECTOR: | ||
| By: | /s/ Michael Teng | |
| Name: Michael Teng |
[Signature Page to Joinder to Registration Rights Agreement]
| THE COMPANY: | ||
| AMERICAN EXCEPTIONALISM CORP. A | ||
| By: | /s/ Steven Trieu | |
| Name: Steven Trieu | ||
| Title: Chief Executive Officer | ||
| THE SPONSOR: | ||
| AEXA SPONSOR LLC A | ||
| By: | /s/ Steven Trieu | |
| Name: Steven Trieu | ||
| Title: Chief Executive Officer |
[Signature Page to Joinder to Registration Rights Agreement]