Treat Gemini Structured Output as a Versioned API to Prevent Agent Breakage

When applications rely on structured JSON output from Gemini, that output effectively becomes an API contract, meaning field changes or new enum values can silently break downstream consumers. Developers are advised to embed schema version identifiers directly in persisted output values rather than relying solely on deployment configuration. Google's Gemini SDK supports JSON Schema alongside Zod validation, enabling stricter runtime checks at the application boundary. Structured generation handles syntax and shape validation, but semantic and policy-level checks — such as verifying referenced evidence IDs exist or confirming an action is permitted — remain the application's responsibility. Different validation failures require different handling strategies, and policy violations in particular should be blocked rather than re-prompted until the model produces a compliant answer.