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
+            if inspect.iscoroutinefunction(self.fn):
-            result = self.fn(**call_args)
+                result = await self.fn(**call_args)
-            if inspect.iscoroutine(result):
+            else:
-                result = await result
+                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
+            if inspect.iscoroutinefunction(self.fn):
-            result = self.fn(**params)
+                result = await self.fn(**params)
-            if inspect.iscoroutine(result):
+            else:
-                result = await result
+                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
+            if inspect.iscoroutinefunction(self.fn):
-            result = self.fn()
+                result = await self.fn()
-            # If it's a coroutine, await it
+            else:
-            if inspect.iscoroutine(result):
+                result = await anyio.to_thread.run_sync(self.fn)
-                result = await result
 
             if isinstance(result, Resource):  # pragma: no cover
                 return await result.read()

tests/server/mcpserver/prompts/test_base.py

@@ -1,3 +1,4 @@
+import threading
 from typing import Any
 
 import pytest
@@ -190,3 +191,21 @@ class TestRenderPrompt:
                 )
             )
         ]
+
+
+@pytest.mark.anyio
+async def test_sync_fn_runs_in_worker_thread():
+    """Sync prompt functions must run in a worker thread, not the event loop."""
+
+    main_thread = threading.get_ident()
+    fn_thread: list[int] = []
+
+    def blocking_fn() -> str:
+        fn_thread.append(threading.get_ident())
+        return "hello"
+
+    prompt = Prompt.from_function(blocking_fn)
+    messages = await prompt.render(None, Context())
+
+    assert messages == [UserMessage(content=TextContent(type="text", text="hello"))]
+    assert fn_thread[0] != main_thread

tests/server/mcpserver/resources/test_function_resources.py

@@ -1,3 +1,7 @@
+import threading
+
+import anyio
+import anyio.from_thread
 import pytest
 from pydantic import BaseModel
 
@@ -190,3 +194,51 @@ class TestFunctionResourceMetadata:
         )
 
         assert resource.meta is None
+
+
+@pytest.mark.anyio
+async def test_sync_fn_runs_in_worker_thread():
+    """Sync resource functions must run in a worker thread, not the event loop."""
+
+    main_thread = threading.get_ident()
+    fn_thread: list[int] = []
+
+    def blocking_fn() -> str:
+        fn_thread.append(threading.get_ident())
+        return "data"
+
+    resource = FunctionResource(uri="resource://test", name="test", fn=blocking_fn)
+    result = await resource.read()
+
+    assert result == "data"
+    assert fn_thread[0] != main_thread
+
+
+@pytest.mark.anyio
+async def test_sync_fn_does_not_block_event_loop():
+    """A blocking sync resource function must not stall the event loop.
+
+    On regression (sync runs inline), anyio.from_thread.run_sync raises
+    RuntimeError because there is no worker-thread context, failing fast.
+    """
+    handler_entered = anyio.Event()
+    release = threading.Event()
+
+    def blocking_fn() -> str:
+        anyio.from_thread.run_sync(handler_entered.set)
+        release.wait()
+        return "done"
+
+    resource = FunctionResource(uri="resource://test", name="test", fn=blocking_fn)
+    result: list[str | bytes] = []
+
+    async def run() -> None:
+        result.append(await resource.read())
+
+    with anyio.fail_after(5):
+        async with anyio.create_task_group() as tg:
+            tg.start_soon(run)
+            await handler_entered.wait()
+            release.set()
+
+    assert result == ["done"]

tests/server/mcpserver/resources/test_resource_template.py

@@ -1,4 +1,5 @@
 import json
+import threading
 from typing import Any
 
 import pytest
@@ -310,3 +311,22 @@ class TestResourceTemplateMetadata:
         assert resource.meta == metadata
         assert resource.meta["category"] == "inventory"
         assert resource.meta["cacheable"] is True
+
+
+@pytest.mark.anyio
+async def test_sync_fn_runs_in_worker_thread():
+    """Sync template functions must run in a worker thread, not the event loop."""
+
+    main_thread = threading.get_ident()
+    fn_thread: list[int] = []
+
+    def blocking_fn(name: str) -> str:
+        fn_thread.append(threading.get_ident())
+        return f"hello {name}"
+
+    template = ResourceTemplate.from_function(fn=blocking_fn, uri_template="test://{name}")
+    resource = await template.create_resource("test://world", {"name": "world"}, Context())
+
+    assert isinstance(resource, FunctionResource)
+    assert await resource.read() == "hello world"
+    assert fn_thread[0] != main_thread