From 33bfe60fb0ca7a96b17de2d96de2f17f1305acb2 Mon Sep 17 00:00:00 2001
From: Rifa Ferzana <78103780+Rifa-111@users.noreply.github.com>
Date: Sat, 27 Jun 2026 18:06:09 +0100
Subject: [PATCH] Offload sync function calls to a thread

Offload synchronous function calls to a thread to prevent blocking the event loop. This change ensures that context is correctly propagated using anyio's run_sync.
---
 src/mcp/server/fastmcp/utilities/func_metadata.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/mcp/server/fastmcp/utilities/func_metadata.py b/src/mcp/server/fastmcp/utilities/func_metadata.py
index 241100d317..7352b7fe52 100644
--- a/src/mcp/server/fastmcp/utilities/func_metadata.py
+++ b/src/mcp/server/fastmcp/utilities/func_metadata.py
@@ -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))
+            # 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:
         """