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0011 - GitHub Copilot Backend

Status

Accepted

Context

GitHub Copilot subscriptions (personal and enterprise) can serve OpenAI-family models and are a common second subscription source alongside ChatGPT/Codex. The flow is specified by opencode's plugin/github-copilot implementation, which we verified: device flow with client ID Ov23li8tweQw6odWQebz, long-lived OAuth token used directly as the bearer token (no exchange, no refresh), API base https://api.githubcopilot.com (or https://copilot-api.<ghe-domain> for enterprise), required headers (X-GitHub-Api-Version: 2026-06-01, x-initiator, Openai-Intent, Copilot-Vision-Request when the body carries images, X-Interaction-Id per session), and a live /models catalog filtered by usability (policy.state != "disabled", prompt/output limits present).

Decision

Implement Copilot as a second backend behind the Backend trait:

  • Onboarding via the GitHub device flow; per-account optional enterprise domain captured at add time.
  • The GitHub OAuth token is stored as the account's static credential; the token manager's refresh step is a no-op for this backend.
  • Required Copilot headers are injected per request; image detection walks the JSON body for image_url/input_image parts to set Copilot-Vision-Request.
  • Catalog fetched from /models on boot and on account add, stored per ADR 0007.
  • 429 -> cooling (ADR 0004); 401/403 -> AuthError (re-login).

z.ai support was explored (its coding plan is API-key based: api.z.ai/api/coding/paas/v4, ZHIPU_API_KEY) and deferred: the Backend trait is the seam, and the keys-based flow is simpler than either implemented backend.

Consequences

  • Copilot accounts participate in the same flat pool and sticky map as codex accounts.
  • x-initiator is always user from the proxy; opencode's own plugin sets agent for subagent sessions, a distinction the proxy cannot see through the OpenAI-compatible surface. Accepted as a limitation.
  • Enterprise and personal Copilot accounts can be mixed in one pool; the enterprise domain lives on the account.