Address proxy review feedback

Author: Kludex

src/mcp/proxy.py

@@ -2,12 +2,15 @@
 
 from __future__ import annotations
 
-import inspect
 from collections.abc import AsyncGenerator, Awaitable, Callable
 from contextlib import asynccontextmanager
+from functools import partial
+from typing import cast
 
 import anyio
+from anyio import to_thread
 
+from mcp.shared._callable_inspection import is_async_callable
 from mcp.shared._stream_protocols import ReadStream, WriteStream
 from mcp.shared.message import SessionMessage
 
@@ -19,6 +22,7 @@
 async def mcp_proxy(
     transport_to_client: MessageStream,
     transport_to_server: MessageStream,
+    *,
     on_error: ErrorHandler | None = None,
 ) -> AsyncGenerator[None]:
     """Proxy messages bidirectionally between two MCP transports."""
@@ -60,8 +64,9 @@ async def _run_error_handler(error: Exception, on_error: ErrorHandler | None) ->
         return
 
     try:
-        result = on_error(error)
-        if inspect.isawaitable(result):
-            await result
+        if is_async_callable(on_error):
+            await cast(Awaitable[None], on_error(error))
+        else:
+            await to_thread.run_sync(partial(on_error, error))
     except Exception:
         return

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

@@ -1,7 +1,5 @@
 from __future__ import annotations
 
-import functools
-import inspect
 from collections.abc import Callable
 from functools import cached_property
 from typing import TYPE_CHECKING, Any
@@ -11,6 +9,7 @@
 from mcp.server.mcpserver.exceptions import ToolError
 from mcp.server.mcpserver.utilities.context_injection import find_context_parameter
 from mcp.server.mcpserver.utilities.func_metadata import FuncMetadata, func_metadata
+from mcp.shared._callable_inspection import is_async_callable
 from mcp.shared.exceptions import UrlElicitationRequiredError
 from mcp.shared.tool_name_validation import validate_and_warn_tool_name
 from mcp.types import Icon, ToolAnnotations
@@ -63,7 +62,7 @@ def from_function(
             raise ValueError("You must provide a name for lambda functions")
 
         func_doc = description or fn.__doc__ or ""
-        is_async = _is_async_callable(fn)
+        is_async = is_async_callable(fn)
 
         if context_kwarg is None:  # pragma: no branch
             context_kwarg = find_context_parameter(fn)
@@ -118,12 +117,3 @@ async def run(
             raise
         except Exception as e:
             raise ToolError(f"Error executing tool {self.name}: {e}") from e
-
-
-def _is_async_callable(obj: Any) -> bool:
-    while isinstance(obj, functools.partial):  # pragma: lax no cover
-        obj = obj.func
-
-    return inspect.iscoroutinefunction(obj) or (
-        callable(obj) and inspect.iscoroutinefunction(getattr(obj, "__call__", None))
-    )

src/mcp/shared/_callable_inspection.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+import functools
+import inspect
+from typing import Any
+
+
+def is_async_callable(obj: Any) -> bool:
+    while isinstance(obj, functools.partial):  # pragma: lax no cover
+        obj = obj.func
+
+    return inspect.iscoroutinefunction(obj) or (
+        callable(obj) and inspect.iscoroutinefunction(getattr(obj, "__call__", None))
+    )