Sign in with ChatGPT lets Plus and Pro users cover an app's AI requests from their own plan

Underneath, this is a standard Authorization Code grant with PKCE and OpenID Connect, according to a dev.to guide that works from OpenAI's documentation ^[7]. Teams that have integrated another OIDC provider will recognise each step. The provider metadata sits at https://auth.openai.com/.well-known/openid-configuration, and the issuer is https://auth.openai.com ^[7]. The backend generates a fresh state, a PKCE verifier with an S256 challenge, and a nonce. On the callback it validates state, exchanges the code for tokens, and checks the ID token's signature, iss, aud, exp and nonce ^[8].

The app receives a stable account ID plus the user's name, email and profile picture ^[1]. Local accounts should key on the verified combination of iss, aud and sub ^[5]. OpenAI warns that an email match alone is not proof of account ownership, so existing users have to confirm account linking ^[5].

The identity scopes, openid profile email, return an ID token and do not grant access to ChatGPT conversations or OpenAI API resources ^[4]. Billing needs one more scope. When the user approves it, the token response carries an access token for eligible Responses API requests ^[6]. That scope is how, since OpenAI's DevDay on 29 September 2026, Plus and Pro users let participating apps run requests on their plan instead of the developer's key ^[2]. The guide does not define which requests count as eligible. The app never sees the user's conversations, memory or API keys ^[3].

The registration path for open-source tools is the most carefully designed piece. A tool sends client_id=dynamic_agent_client, an agent_name_hint with its name, and an ext_agent_host_id that persists per host ^[10]. The callback returns an issued client ID of the form oaiapp_..., which the tool stores and reuses on later logins, and the redirect goes to loopback 127.0.0.1 with no client secret ^[10]. A secret shipped in public source code is readable by anyone, and public clients are barred from sending one anyway ^[9].

The billing rules need the most care. Because the plan's Work and Codex quota is the pool ^[11], an app draws from the same allowance as the user's own Codex sessions. The per-app cap limits what the app can take and reserves nothing for it; heavy use in other apps or services can exhaust the plan first ^[13]. Revoking access stops future usage and does not reverse usage already spent ^[16].

I think the right setup for a builder is to treat the user's plan as a discount on part of the traffic and keep the API key as the path every request can fall back to ^[2]. The fallback has to work mid-session, since a Plus user's shared five-hour window can close because of another app ^[15]. Plan billing also covers only Plus and Pro subscribers, so everyone else's requests stay on the builder's key ^[1].