fix: detect Context parameter in callable class instances

find_context_parameter() uses typing.get_type_hints() which raises
TypeError on callable class instances.  Fall back to inspecting the
__call__ method so that the Context parameter is properly detected
and excluded from the tool's JSON schema.

Fixes #1974

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: g97iulio1609

src/mcp/server/mcpserver/utilities/context_injection.py

@@ -22,9 +22,15 @@ def find_context_parameter(fn: Callable[..., Any]) -> str | None:
     """
     from mcp.server.mcpserver.server import Context
 
+    # Handle callable class instances by inspecting __call__ method
+    target = fn
+    if not (inspect.isfunction(fn) or inspect.ismethod(fn)):
+        if callable(fn) and hasattr(fn, "__call__"):
+            target = fn.__call__
+
     # Get type hints to properly resolve string annotations
     try:
-        hints = typing.get_type_hints(fn)
+        hints = typing.get_type_hints(target)
     except Exception:  # pragma: lax no cover
         # If we can't resolve type hints, we can't find the context parameter
         return None

tests/server/mcpserver/test_tool_manager.py

@@ -385,6 +385,25 @@ async def async_tool(x: int, ctx: Context[ServerSessionT, None]) -> str:
         result = await manager.call_tool("async_tool", {"x": 42}, context=ctx)
         assert result == "42"
 
+    @pytest.mark.anyio
+    async def test_context_injection_callable_class(self):
+        """Test that context is properly injected for callable class instances."""
+
+        class MyTool:
+            async def __call__(
+                self, x: int, ctx: Context[ServerSessionT, None]
+            ) -> str:
+                assert isinstance(ctx, Context)
+                return str(x)
+
+        manager = ToolManager()
+        manager.add_tool(MyTool(), name="callable_tool")
+
+        mcp = MCPServer()
+        ctx = mcp.get_context()
+        result = await manager.call_tool("callable_tool", {"x": 42}, context=ctx)
+        assert result == "42"
+
     @pytest.mark.anyio
     async def test_context_optional(self):
         """Test that context is optional when calling tools."""