Error responses included parameter names, valid alternatives, doc links, and structured decline codes

Three error scenarios were tested with both models. In each case, the API response included enough structured information for the agent to understand the error and determine next steps without human help. The error responses were consistent between runs. The invalid currency error named the bad parameter ("param": "currency"), listed all valid currency codes, and linked to the request log. The declined card error returned code, decline_code, advice_code, and network_decline_code as separate fields, and left the PaymentIntent in requires_payment_method status (retryable). The missing customer error returned resource_missing with the invalid ID quoted back. Both models recovered autonomously from all three scenarios.