MCP connection errors explained: 404 on /sse, 406 Not Acceptable, session 400s, 402
This story is from 2026-10-08. It is preserved in the archive; the latest stories are on the live feed.
MCP Streamable HTTP is one URL. The client POSTs JSON-RPC to it and may open a GET stream on the same URL; nothing else is part of the address. Most connection failures we see in our own server log are a client talking to a different address than the one it was given, or sending the wrong headers.…
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- 2026-10-08 23:01 · DEV Community — AI
MCP connection errors explained: 404 on /sse, 406 Not Acceptable, session 400s, 402