fastmcp: allow passing Tool directly to FastMCP constructor (#699)

Author: vbarda

src/mcp/server/fastmcp/server.py

@@ -37,7 +37,7 @@
 from mcp.server.fastmcp.exceptions import ResourceError
 from mcp.server.fastmcp.prompts import Prompt, PromptManager
 from mcp.server.fastmcp.resources import FunctionResource, Resource, ResourceManager
-from mcp.server.fastmcp.tools import ToolManager
+from mcp.server.fastmcp.tools import Tool, ToolManager
 from mcp.server.fastmcp.utilities.logging import configure_logging, get_logger
 from mcp.server.fastmcp.utilities.types import Image
 from mcp.server.lowlevel.helper_types import ReadResourceContents
@@ -141,6 +141,8 @@ def __init__(
         auth_server_provider: OAuthAuthorizationServerProvider[Any, Any, Any]
         | None = None,
         event_store: EventStore | None = None,
+        *,
+        tools: list[Tool] | None = None,
         **settings: Any,
     ):
         self.settings = Settings(**settings)
@@ -155,7 +157,7 @@ def __init__(
             ),
         )
         self._tool_manager = ToolManager(
-            warn_on_duplicate_tools=self.settings.warn_on_duplicate_tools
+            tools=tools, warn_on_duplicate_tools=self.settings.warn_on_duplicate_tools
         )
         self._resource_manager = ResourceManager(
             warn_on_duplicate_resources=self.settings.warn_on_duplicate_resources

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

@@ -19,8 +19,19 @@
 class ToolManager:
     """Manages FastMCP tools."""
 
-    def __init__(self, warn_on_duplicate_tools: bool = True):
+    def __init__(
+        self,
+        warn_on_duplicate_tools: bool = True,
+        *,
+        tools: list[Tool] | None = None,
+    ):
         self._tools: dict[str, Tool] = {}
+        if tools is not None:
+            for tool in tools:
+                if warn_on_duplicate_tools and tool.name in self._tools:
+                    logger.warning(f"Tool already exists: {tool.name}")
+                self._tools[tool.name] = tool
+
         self.warn_on_duplicate_tools = warn_on_duplicate_tools
 
     def get_tool(self, name: str) -> Tool | None:

tests/server/fastmcp/test_tool_manager.py

@@ -6,7 +6,8 @@
 
 from mcp.server.fastmcp import Context, FastMCP
 from mcp.server.fastmcp.exceptions import ToolError
-from mcp.server.fastmcp.tools import ToolManager
+from mcp.server.fastmcp.tools import Tool, ToolManager
+from mcp.server.fastmcp.utilities.func_metadata import ArgModelBase, FuncMetadata
 from mcp.server.session import ServerSessionT
 from mcp.shared.context import LifespanContextT
 from mcp.types import ToolAnnotations
@@ -31,6 +32,35 @@ def add(a: int, b: int) -> int:
         assert tool.parameters["properties"]["a"]["type"] == "integer"
         assert tool.parameters["properties"]["b"]["type"] == "integer"
 
+    def test_init_with_tools(self, caplog):
+        def add(a: int, b: int) -> int:
+            return a + b
+
+        class AddArguments(ArgModelBase):
+            a: int
+            b: int
+
+        fn_metadata = FuncMetadata(arg_model=AddArguments)
+
+        original_tool = Tool(
+            name="add",
+            description="Add two numbers.",
+            fn=add,
+            fn_metadata=fn_metadata,
+            is_async=False,
+            parameters=AddArguments.model_json_schema(),
+            context_kwarg=None,
+            annotations=None,
+        )
+        manager = ToolManager(tools=[original_tool])
+        saved_tool = manager.get_tool("add")
+        assert saved_tool == original_tool
+
+        # warn on duplicate tools
+        with caplog.at_level(logging.WARNING):
+            manager = ToolManager(True, tools=[original_tool, original_tool])
+            assert "Tool already exists: add" in caplog.text
+
     @pytest.mark.anyio
     async def test_async_function(self):
         """Test registering and running an async function."""