GitHub OAuth connector fails with GitHub's HTML error page on Streamable HTTP POST to https://github.com/api/mcp

Claude Code version: 2.1.217

Description:

Attempting to connect the built-in "github" MCP connector (via /mcp in Claude Code CLI, connector authorized through claude.ai/customize/connectors) consistently fails.

Error shown in /mcp:

Status: ✘ failed
Issue: Streamable HTTP error: Error POSTing to endpoint: <GitHub's "What‽ / Oh no" HTML error page>
Auth: ✘ not authenticated
URL: https://github.com/api/mcp

The response body is GitHub's generic web-app error page ("Your browser did something unexpected... try disabling all browser extensions"), returned as the response to a server-side POST request — not a browser navigation — which suggests the endpoint is returning an HTML error page instead of a proper JSON API response.

Steps tried, all unsuccessful:

1. Retried the initial browser OAuth flow — got the same GitHub HTML error page directly in-browser.
2. Retried in an Incognito window with extensions disabled — same result, ruling out browser extensions.
3. Disconnected and reconnected the connector via claude.ai/customize/connectors.
4. Revoked the Claude/Anthropic OAuth App authorization directly on GitHub (github.com/settings/applications) and re-authorized from scratch.
5. All attempts produce the identical error; /mcp still shows Auth: not authenticated.

Environment:

• Claude Code CLI, /mcp connector flow
• GitHub account under a corporate org with SAML SSO enabled