Copilot serves Grok, Gemini, and MAI Code only on /responses, but the endpoint fallback only routed gpt-5+ there, so they hit /chat/completions whenever the live model metadata was missing.
The catalog only carries the generic @ai-sdk/openai-compatible package for this provider, and config-defined models that do not reference a catalog model are merged after the Copilot plugin's SDK remap, so they fall through to the generic SDK instead of the bundled one required for CAPI URL, headers, and error handling. Explicit config still wins.
Advertised supported_endpoints metadata still takes precedence.
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