Treat pre-output Responses API 400 validation failures as unsupported so OpenAI-compatible providers that reject /v1/responses payloads can retry through Chat Completions. Also prefer a valid tool-call JSON object after leading prose or incidental JSON to reduce false misformat warnings.
Route Agent Zero turns through a LiteLLM transport layer that prefers the Responses API while preserving chat-completions fallback for providers without compatible endpoints.
Persist Responses metadata in history and agent state so provider-state continuation, local replay, native function-call execution, and stored-response cleanup survive normal chat workflows.
Normalize prompt caching by provider: OpenAI and Azure use prompt_cache_key and prompt_cache_retention, while Anthropic, Gemini, Bedrock, OpenRouter, and compatible chat providers keep block-level cache_control breakpoints and cached tool definitions.
