From Chat GPT:
Great question. For a Thailand Retirement Extension (Non-Immigrant O or O-A Visa), the Thai immigration rules typically require the banked money (e.g., 800,000 THB for a retirement extension) to be in a Thai bank account in the applicant’s name only, not a joint account.
That said:
Some immigration offices may allow a joint account if the applicant is the primary account holder and provides proof of full access/control.
However, in most cases, strict interpretation means the money should be solely in the retiree’s name to avoid delays or rejection.