validationMessage property
Custom instructions for the AI on how to judge this answer (injected into the system prompt).
Use this to spell out rules the model should follow when accepting or rejecting, e.g. "only accept if the name is exactly 3 characters" or "must be a valid email format". When set, the model uses this as a strict validation rule; it is not shown verbatim to the user unless the model paraphrases it in a rejection reason.
Implementation
final String? validationMessage;