fix: Update @mcp.resource to use function documentation as default descrip… (#489)

Author: jingyugao

src/mcp/server/fastmcp/resources/types.py

@@ -11,7 +11,7 @@
 import httpx
 import pydantic
 import pydantic_core
-from pydantic import Field, ValidationInfo
+from pydantic import AnyUrl, Field, ValidationInfo, validate_call
 
 from mcp.server.fastmcp.resources.base import Resource
 
@@ -68,6 +68,31 @@ async def read(self) -> str | bytes:
         except Exception as e:
             raise ValueError(f"Error reading resource {self.uri}: {e}")
 
+    @classmethod
+    def from_function(
+        cls,
+        fn: Callable[..., Any],
+        uri: str,
+        name: str | None = None,
+        description: str | None = None,
+        mime_type: str | None = None,
+    ) -> "FunctionResource":
+        """Create a FunctionResource from a function."""
+        func_name = name or fn.__name__
+        if func_name == "<lambda>":
+            raise ValueError("You must provide a name for lambda functions")
+
+        # ensure the arguments are properly cast
+        fn = validate_call(fn)
+
+        return cls(
+            uri=AnyUrl(uri),
+            name=func_name,
+            description=description or fn.__doc__ or "",
+            mime_type=mime_type or "text/plain",
+            fn=fn,
+        )
+
 
 class FileResource(Resource):
     """A resource that reads from a file.

src/mcp/server/fastmcp/server.py

@@ -148,9 +148,11 @@ def __init__(
         self._mcp_server = MCPServer(
             name=name or "FastMCP",
             instructions=instructions,
-            lifespan=lifespan_wrapper(self, self.settings.lifespan)
-            if self.settings.lifespan
-            else default_lifespan,
+            lifespan=(
+                lifespan_wrapper(self, self.settings.lifespan)
+                if self.settings.lifespan
+                else default_lifespan
+            ),
         )
         self._tool_manager = ToolManager(
             warn_on_duplicate_tools=self.settings.warn_on_duplicate_tools
@@ -465,16 +467,16 @@ def decorator(fn: AnyFunction) -> AnyFunction:
                     uri_template=uri,
                     name=name,
                     description=description,
-                    mime_type=mime_type or "text/plain",
+                    mime_type=mime_type,
                 )
             else:
                 # Register as regular resource
-                resource = FunctionResource(
-                    uri=AnyUrl(uri),
+                resource = FunctionResource.from_function(
+                    fn=fn,
+                    uri=uri,
                     name=name,
                     description=description,
-                    mime_type=mime_type or "text/plain",
-                    fn=fn,
+                    mime_type=mime_type,
                 )
                 self.add_resource(resource)
             return fn

tests/server/fastmcp/resources/test_function_resources.py

@@ -136,3 +136,22 @@ async def get_data() -> str:
         content = await resource.read()
         assert content == "Hello, world!"
         assert resource.mime_type == "text/plain"
+
+    @pytest.mark.anyio
+    async def test_from_function(self):
+        """Test creating a FunctionResource from a function."""
+
+        async def get_data() -> str:
+            """get_data returns a string"""
+            return "Hello, world!"
+
+        resource = FunctionResource.from_function(
+            fn=get_data,
+            uri="function://test",
+            name="test",
+        )
+
+        assert resource.description == "get_data returns a string"
+        assert resource.mime_type == "text/plain"
+        assert resource.name == "test"
+        assert resource.uri == AnyUrl("function://test")

tests/server/fastmcp/test_server.py

@@ -441,6 +441,24 @@ async def test_file_resource_binary(self, tmp_path: Path):
                 == base64.b64encode(b"Binary file data").decode()
             )
 
+    @pytest.mark.anyio
+    async def test_function_resource(self):
+        mcp = FastMCP()
+
+        @mcp.resource("function://test", name="test_get_data")
+        def get_data() -> str:
+            """get_data returns a string"""
+            return "Hello, world!"
+
+        async with client_session(mcp._mcp_server) as client:
+            resources = await client.list_resources()
+            assert len(resources.resources) == 1
+            resource = resources.resources[0]
+            assert resource.description == "get_data returns a string"
+            assert resource.uri == AnyUrl("function://test")
+            assert resource.name == "test_get_data"
+            assert resource.mimeType == "text/plain"
+
 
 class TestServerResourceTemplates:
     @pytest.mark.anyio