The previous copilot_model_api_mode() checked the catalog's
supported_endpoints first and picked /chat/completions when a model
supported both endpoints. This is wrong — GPT-5+ models should use
the Responses API even when the catalog lists both.
Replicate opencode's shouldUseCopilotResponsesApi() logic:
- GPT-5+ models (gpt-5.4, gpt-5.3-codex, etc.) → Responses API
- gpt-5-mini → Chat Completions (explicit exception)
- Everything else (gpt-4o, claude, gemini, etc.) → Chat Completions
- Model ID pattern is the primary signal, catalog is secondary
The catalog fallback now only matters for non-GPT-5 models that might
exclusively support /v1/messages (e.g. Claude via Copilot).
Models are auto-detected from the live catalog at
api.githubcopilot.com/models — no hardcoded list required for
supported models, only a static fallback for when the API is
unreachable.
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