[posthog] mcp servers: "OAuth discovery failed" when OAuth endpoints span domains

## Observed symptom mcp servers: "OAuth discovery failed" when OAuth endpoints span domains Adding an MCP server with OAuth authentication fails with "OAuth discovery failed." when the server's OAuth provider publishes a sign-in, token, or registration URL on a different registered domain from its issuer URL. The sign-in window never opens, and entering an OAuth client ID and secret under **Advanced OAuth settings** does not change the result. Google's remote MCP servers (Gmail, Drive, Calendar, Docs, Chat, and BigQuery) all hit this: Google's issuer is `https://accounts.google.com`, and its token URL is `https://oauth2.googleapis.com/token`. Expected: PostHog opens the provider's sign-in page. 1. In PostHog Cloud, open **MCP servers** and click **Add server**. 2. Set **Server URL** to `https://gmailmcp.googleapis.com/mcp/v1` and **Authentication** to **OAuth**. 3. Click **Add server**. 4. The request returns `400 {"detail": "OAuth discovery failed."}` and the Google sign-in w ## Literal error text ``` 400 {"detail": "OAuth discovery failed."} ``` ## Environment GitHub PostHog/posthog #109181 ## Trigger / context Public GitHub report in PostHog/posthog (FRESH_7D). ## Source evidence - https://github.com/PostHog/posthog/issues/109181 (2026-09-30T14:10:35Z) — evidence_class: PRIMARY_ISSUE_REPORT — #109181 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T10:47:55.298020Z - scout_id: C2c8cde6517 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.