Update lambda handling

src/fastmcp/tools.py

@@ -30,6 +30,10 @@ class Tool(BaseModel):
     ) -> "Tool":
         """Create a Tool from a function."""
         func_name = name or func.__name__
+
+        if func_name == "<lambda>":
+            raise ValueError("You must provide a name for lambda functions")
+
         func_doc = description or func.__doc__ or ""
         is_async = inspect.iscoroutinefunction(func)
 

tests/test_server.py

@@ -10,22 +10,27 @@ class TestServer:
         assert server.name == "FastMCPServer"
 
 
+def tool_fn(x: int, y: int) -> int:
+    return x + y
+
+
 class TestServerTools:
     async def test_add_tool(self):
         server = FastMCPServer()
-        server.add_tool(lambda x: x)
+        server.add_tool(tool_fn)
+        server.add_tool(tool_fn)
         assert len(server._tool_manager.list_tools()) == 1
 
     async def test_list_tools(self):
         server = FastMCPServer()
-        server.add_tool(lambda x: x)
+        server.add_tool(tool_fn)
         async with client_session(server._mcp_server) as client:
             tools = await client.list_tools()
             assert len(tools.tools) == 1
 
     async def test_call_tool(self):
         server = FastMCPServer()
-        server.add_tool(lambda x: x)
+        server.add_tool(tool_fn)
         async with client_session(server._mcp_server) as client:
             result = await client.call_tool("my_tool", {"arg1": "value"})
             assert "error" not in result

tests/test_tool_manager.py

@@ -73,8 +73,15 @@ class TestAddTools:
 
     def test_add_lambda(self):
         manager = ToolManager()
-        manager.add_tool(lambda x: x)
-        assert len(manager.list_tools()) == 1
+        tool = manager.add_tool(lambda x: x, name="my_tool")
+        assert tool.name == "my_tool"
+
+    def test_add_lambda_with_no_name(self):
+        manager = ToolManager()
+        with pytest.raises(
+            ValueError, match="You must provide a name for lambda functions"
+        ):
+            manager.add_tool(lambda x: x)
 
     def test_warn_on_duplicate_tools(self):
         """Test warning on duplicate tools."""