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Why synchronous @mcp.tool definition is not supported for parallelism? #1646
Description
Question
In mcp.server.fastmcp.utilities.func_metadata.FuncMetadata.call_fn_with_arg_validation
if fn_is_async:
return await fn(**arguments_parsed_dict)
else:
return fn(**arguments_parsed_dict)In FastMCP, if a tool is defined as a synchronous function, it will block the whole running loop instead of using to_thread().
May I know the consideration for it's different from FastAPI's concurrency model?
Additional Context
No response
Activity
Kludex commentedon Dec 3, 2025
This is a bug.
It's not "FastAPI concurrency model".
Don't block the event loop on sync resource and prompt functions
maxisbey commentedon May 18, 2026
Fixed on
main(v2): #1909 routes sync@mcp.toolfunctions throughanyio.to_thread.run_syncso they no longer block the event loop, and #2380 extends the same to sync resource and prompt handlers. Closing — if you need this backported to the v1.x line, please say so.AI Disclaimer