[BUG]: GitHub Copilot Responses API routing for GPT-5+

[christso]

Problem

When using the github-copilot provider, GPT-5+ models are currently routed through the Chat API. This prevents users from enabling Responses-API-only controls (eg. reasoningEffort, reasoningSummary) for those models, even when they are supported by the model and useful for agent behavior tuning.

This is especially noticeable for github-copilot/gpt-5.* where users expect to be able to configure reasoning parameters similarly to other OpenAI-compatible providers.

Proposed Solution

Add an opt-in provider option to route supported Copilot models through the Responses API:

• New config flag: provider.github-copilot.options.useResponsesApi: true
• Behavior:
  • Always route codex* models through sdk.responses(modelID) (existing behavior)
  • If useResponsesApi is enabled and the model is GPT-5 or later, route through sdk.responses(modelID)
  • Otherwise, default to sdk.chat(modelID) (no behavior change by default)

Example config:

{
  "provider": {
    "github-copilot": {
      "options": {
        "useResponsesApi": true
      }
    }
  }
}

Future (Deprecation)

Once GitHub Copilot / VS Code enables Responses API usage broadly (the VS Code setting is currently labeled Experimental), this option can likely be deprecated and/or defaulted on for supported models to reduce configuration surface area.

References

Current status of supported API route for provider github-copilot:

Model	API Route
gpt-4.1	chat
gpt-4o	chat
gpt-5	responses
gpt-5-codex	responses
gpt-5-mini	chat
gpt-5.1	responses
gpt-5.1-codex	responses
gpt-5.1-codex-max	responses
gpt-5.1-codex-mini	responses
gpt-5.2	responses

• VS Code UI (settings) relevant to this feature:
  • GitHub > Copilot > Chat: Use Responses Api (Experimental)
  • GitHub > Copilot > Chat: Responses Api Reasoning Effort (Experimental)
  • GitHub > Copilot > Chat: Responses Api Reasoning Summary (Experimental)

Screenshot of the VS Code settings panel showing the above Copilot settings.

Environment

• Repo: sst/opencode (dev)
• Platform observed: Windows (but the behavior is provider/model routing specific, not OS-specific)

[github-actions[bot]]

This issue might be a duplicate of existing issues. Please check:

• #4943: github copilot: gpt-5-mini not supported via responses api

Feel free to ignore if none of these address your specific case.