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MCP error: "Error POSTing to endpoint: <!DOCTYPE html>"

Error: Streamable HTTP error: Error POSTing to endpoint: <!DOCTYPE html>

What causes it

The hosting layer returned an HTML error page instead of a JSON-RPC response, usually from a timeout or a cold start. On grep.app our first code search failed this way after about 15 s, and the retry worked in under 1 s.

How to fix it

1. Retry once before treating the server as down.
2. Check our Connector Weather board for a live handshake reading.

Where we hit it

• grep.app MCP test report (tested Oct 16, 2026). Setup: Claude, Claude Code, Cursor, VS Code, ChatGPT

Related fixes

• MCP tool call times out on slow servers

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