Don't block the event loop on sync resource and prompt functions (#2380)

src/mcp/server/mcpserver/prompts/base.py

@@ -2,10 +2,12 @@
 
 from __future__ import annotations
 
+import functools
 import inspect
 from collections.abc import Awaitable, Callable, Sequence
 from typing import TYPE_CHECKING, Any, Literal
 
+import anyio.to_thread
 import pydantic_core
 from pydantic import BaseModel, Field, TypeAdapter, validate_call
 
@@ -155,10 +157,10 @@ class Prompt(BaseModel):
             # Add context to arguments if needed
             call_args = inject_context(self.fn, arguments or {}, context, self.context_kwarg)
 
-            # Call function and check if result is a coroutine
-            result = self.fn(**call_args)
-            if inspect.iscoroutine(result):
-                result = await result
+            if inspect.iscoroutinefunction(self.fn):
+                result = await self.fn(**call_args)
+            else:
+                result = await anyio.to_thread.run_sync(functools.partial(self.fn, **call_args))
 
             # Validate messages
             if not isinstance(result, list | tuple):

src/mcp/server/mcpserver/resources/templates.py

@@ -2,12 +2,14 @@
 
 from __future__ import annotations
 
+import functools
 import inspect
 import re
 from collections.abc import Callable
 from typing import TYPE_CHECKING, Any
 from urllib.parse import unquote
 
+import anyio.to_thread
 from pydantic import BaseModel, Field, validate_call
 
 from mcp.server.mcpserver.resources.types import FunctionResource, Resource
@@ -110,10 +112,10 @@ class ResourceTemplate(BaseModel):
             # Add context to params if needed
             params = inject_context(self.fn, params, context, self.context_kwarg)
 
-            # Call function and check if result is a coroutine
-            result = self.fn(**params)
-            if inspect.iscoroutine(result):
-                result = await result
+            if inspect.iscoroutinefunction(self.fn):
+                result = await self.fn(**params)
+            else:
+                result = await anyio.to_thread.run_sync(functools.partial(self.fn, **params))
 
             return FunctionResource(
                 uri=uri,  # type: ignore

src/mcp/server/mcpserver/resources/types.py

@@ -55,11 +55,10 @@ class FunctionResource(Resource):
     async def read(self) -> str | bytes:
         """Read the resource by calling the wrapped function."""
         try:
-            # Call the function first to see if it returns a coroutine
-            result = self.fn()
-            # If it's a coroutine, await it
-            if inspect.iscoroutine(result):
-                result = await result
+            if inspect.iscoroutinefunction(self.fn):
+                result = await self.fn()
+            else:
+                result = await anyio.to_thread.run_sync(self.fn)
 
             if isinstance(result, Resource):  # pragma: no cover
                 return await result.read()