Fix @mcp.tool() Failing to Detect Context Argument When Type Variables Are Used in Context

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

@@ -2,7 +2,7 @@
 
 import inspect
 from collections.abc import Callable
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, get_origin
 
 from pydantic import BaseModel, Field
 
@@ -53,7 +53,9 @@ def from_function(
         if context_kwarg is None:
             sig = inspect.signature(fn)
             for param_name, param in sig.parameters.items():
-                if param.annotation is Context:
+                if get_origin(param.annotation) is not None:
+                    continue
+                if issubclass(param.annotation, Context):
                     context_kwarg = param_name
                     break
 

tests/server/fastmcp/test_tool_manager.py

@@ -4,8 +4,11 @@
 import pytest
 from pydantic import BaseModel
 
+from mcp.server.fastmcp import Context, FastMCP
 from mcp.server.fastmcp.exceptions import ToolError
 from mcp.server.fastmcp.tools import ToolManager
+from mcp.server.session import ServerSessionT
+from mcp.shared.context import LifespanContextT
 
 
 class TestAddTools:
@@ -194,8 +197,6 @@ def concat_strs(vals: list[str] | str) -> str:
 
     @pytest.mark.anyio
     async def test_call_tool_with_complex_model(self):
-        from mcp.server.fastmcp import Context
-
         class MyShrimpTank(BaseModel):
             class Shrimp(BaseModel):
                 name: str
@@ -223,8 +224,6 @@ def name_shrimp(tank: MyShrimpTank, ctx: Context) -> list[str]:
 class TestToolSchema:
     @pytest.mark.anyio
     async def test_context_arg_excluded_from_schema(self):
-        from mcp.server.fastmcp import Context
-
         def something(a: int, ctx: Context) -> int:
             return a
 
@@ -241,7 +240,6 @@ class TestContextHandling:
     def test_context_parameter_detection(self):
         """Test that context parameters are properly detected in
         Tool.from_function()."""
-        from mcp.server.fastmcp import Context
 
         def tool_with_context(x: int, ctx: Context) -> str:
             return str(x)
@@ -256,10 +254,17 @@ def tool_without_context(x: int) -> str:
         tool = manager.add_tool(tool_without_context)
         assert tool.context_kwarg is None
 
+        def tool_with_parametrized_context(
+            x: int, ctx: Context[ServerSessionT, LifespanContextT]
+        ) -> str:
+            return str(x)
+
+        tool = manager.add_tool(tool_with_parametrized_context)
+        assert tool.context_kwarg == "ctx"
+
     @pytest.mark.anyio
     async def test_context_injection(self):
         """Test that context is properly injected during tool execution."""
-        from mcp.server.fastmcp import Context, FastMCP
 
         def tool_with_context(x: int, ctx: Context) -> str:
             assert isinstance(ctx, Context)
@@ -276,7 +281,6 @@ def tool_with_context(x: int, ctx: Context) -> str:
     @pytest.mark.anyio
     async def test_context_injection_async(self):
         """Test that context is properly injected in async tools."""
-        from mcp.server.fastmcp import Context, FastMCP
 
         async def async_tool(x: int, ctx: Context) -> str:
             assert isinstance(ctx, Context)
@@ -293,7 +297,6 @@ async def async_tool(x: int, ctx: Context) -> str:
     @pytest.mark.anyio
     async def test_context_optional(self):
         """Test that context is optional when calling tools."""
-        from mcp.server.fastmcp import Context
 
         def tool_with_context(x: int, ctx: Context | None = None) -> str:
             return str(x)
@@ -307,7 +310,6 @@ def tool_with_context(x: int, ctx: Context | None = None) -> str:
     @pytest.mark.anyio
     async def test_context_error_handling(self):
         """Test error handling when context injection fails."""
-        from mcp.server.fastmcp import Context, FastMCP
 
         def tool_with_context(x: int, ctx: Context) -> str:
             raise ValueError("Test error")