OpenAI via ChatGPT subscription (Codex proxy)

Unofficial and ToS-risky. This routes inference to your ChatGPT subscription through the Codex backend, the way codex login does. It is not a supported way for a third-party app to bill a user's subscription. Anthropic and Google closed their equivalents in 2026; OpenAI may follow, and accounts that abuse it can be limited or banned. Decide with eyes open. The supported alternative is a normal OpenAI API key (api_kind = "openai_compatible", auth_env = "OPENAI_API_KEY").

Why the Apps SDK does not work for this

The Apps SDK OAuth authenticates users into your MCP server and issues tokens for your tools. It does not grant a token to call OpenAI inference on the user's plan. The only path that does is the Codex one below.

How it works here

router (api_kind="openai_codex")
  → codex_backend.make_codex_async_call_provider
      → POST {base_url}/responses           (https://chatgpt.com/backend-api/codex/responses)
        Authorization: Bearer <access_token from ~/.codex/auth.json, auto-refreshed>
        chatgpt-account-id: <account_id>
        body: Responses API schema, stream=true (SSE)
      ← SSE aggregated back into the router's response shape

• Token source: codex_auth.CodexAuth reads ~/.codex/auth.json (access_token, refresh_token, id_token, account_id; both top-level and nested tokens layouts are accepted).
• Refresh: when the access token's JWT exp is near, it refreshes against https://auth.openai.com/oauth/token with the public Codex client id app_EMoamEEZ73f0CkXaXp7hrann and writes the new tokens back to auth.json.
• Wire shape: the Codex endpoint speaks the Responses API (not chat/completions) and streams SSE; codex_backend translates the router's chat-style request to input items and folds the response.output_text.delta / response.completed events back into text + usage.

Setup

# 1. Authenticate (opens a browser; "Sign in with ChatGPT")
codex login                       # writes ~/.codex/auth.json

# 2. Start the shim; it picks up auth.json lazily on the first codex call
python -m hosts.python_shim --config hosts/python_shim/config.live.lua \
    --codex-auth ~/.codex/auth.json

--codex-auth defaults to ~/.codex/auth.json. Treat that file like a password — it holds bearer tokens.

Caveats

• The backend endpoint is undocumented and can change without notice; if the Responses event names or the /responses path change, codex_backend needs updating.
• The live streaming call is not covered by tests (no subscription in CI). The token read/refresh, request translation, header building and SSE aggregation are unit-tested in hosts/python_shim/tests/test_codex.py.
• Tool calls are not yet translated to the Responses API tool schema; v1 sends text in / text out. Add tool translation in build_codex_body when needed.