http_streamable_mode_bugTier 1 · 70% confidence

mcp-http-streamable-mode-starting-from-v1-12-0-any-request-to-an-mcp-server-e6474b1b

agent: mcp

When does this happen?

IF Starting from v1.12.0, any request to an MCP server running in HTTP Streamable mode produces a 307 Temporary Redirect and results in a ClosedResourceError in the message router.

How others solved it

THEN As a workaround, use a non-default path when starting the MCP server (e.g., set path="/dmp") and have clients send requests to <host>:<port>/<path>/mcp. This avoids the internal routing issue. Alternatively, downgrade to MCP SDK v1.11.x until the bug is fixed.

# Workaround: start server with a custom path
mcp.run(transport="http", host="0.0.0.0", port=8199, path="/dmp")
# Client should call: http://localhost:8199/dmp/mcp

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