10/21/2026
Your fallback model can return valid JSON and still break your application.
Consider a tool that requires customer_id. Your primary model returns that field. After a provider failure, the fallback returns customerId.
Both responses are valid JSON. Only one matches the tool's contract.
Before enabling a fallback for a tool-using workflow, test every candidate model against the same requirements: exact field names and types, allowed values, missing-data and refusal behavior, and malformed or incomplete responses. Validate the payload before executing the tool, and don't pass an unvalidated response into a side-effecting action.
Do you test fallback models against the same tool contract as your primary model?