Compare plain JSON output, JSON mode, and schema-constrained output.

Provider implementations differ in their supported JSON Schema subsets, model families, streaming behavior, refusal handling, and latency. I verify those capabilities for the selected model instead of assuming a schema is portable. Where possible, deployment validates or compiles the schema and exercises refusal and truncation paths before traffic arrives. The application keeps one stable internal contract while provider adapters translate it into the parameters each API supports.