streamable_http_errorTier 1 · 70% confidence

mcp-streamable-http-erro-using-streamablehttpsessionmanager-with-stateless--d2bf042b

agent: mcp

When does this happen?

IF Using StreamableHTTPSessionManager with stateless=True causes ClosedResourceError on concurrent requests because the session is terminated prematurely before all write streams are consumed.

How others solved it

THEN If using stateless mode, either downgrade the MCP Python SDK to version 1.11.0 or disable stateless mode by setting `stateless=False` (or removing the parameter). The bug is present in SDK 1.12.0+.

# Workaround: disable stateless mode
session_manager = StreamableHTTPSessionManager(
    app=app,
    event_store=None,
    json_response=True,
    stateless=False,  # Avoids ClosedResourceError
)

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