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Offload sync function calls to a thread #3010

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6 changes: 5 additions & 1 deletion src/mcp/server/fastmcp/utilities/func_metadata.py
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Original file line number Diff line number Diff line change
Expand Up @@ -93,7 +93,11 @@ async def call_fn_with_arg_validation(
if fn_is_async:
return await fn(**arguments_parsed_dict)
else:
return fn(**arguments_parsed_dict)
return await anyio.to_thread.run_sync(lambda: fn(**arguments_parsed_dict))

@cubic-dev-ai cubic-dev-ai Bot Jun 27, 2026 •
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P1: Add the missing anyio import before using anyio.to_thread.run_sync; otherwise every synchronous tool call fails at runtime.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At src/mcp/server/fastmcp/utilities/func_metadata.py, line 96:

<comment>Add the missing `anyio` import before using `anyio.to_thread.run_sync`; otherwise every synchronous tool call fails at runtime.</comment>

<file context>
@@ -93,7 +93,11 @@ async def call_fn_with_arg_validation(
             return await fn(**arguments_parsed_dict)
         else:
-            return fn(**arguments_parsed_dict)
+            return await anyio.to_thread.run_sync(lambda: fn(**arguments_parsed_dict))
+            # Sync functions are offloaded to a thread to avoid blocking the event loop.
+            # anyio.to_thread.run_sync() uses copy_context() internally, so contextvars
</file context>
Fix with cubic

# Sync functions are offloaded to a thread to avoid blocking the event loop.
# anyio.to_thread.run_sync() uses copy_context() internally, so contextvars
# propagate correctly. Context is passed explicitly via arguments_parsed_dict,
# not through contextvars, so it is unaffected.

def convert_result(self, result: Any) -> Any:
"""
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