Fix resource docstring descriptions (Issue #488)

Author: Soki Fukuda

src/mcp/server/fastmcp/server.py

@@ -388,7 +388,7 @@ def decorator(fn: AnyFunction) -> AnyFunction:
                 resource = FunctionResource(
                     uri=AnyUrl(uri),
                     name=name,
-                    description=description,
+                    description=description or fn.__doc__ or None,
                     mime_type=mime_type or "text/plain",
                     fn=fn,
                 )

tests/server/fastmcp/resources/test_function_resources.py

@@ -136,3 +136,18 @@ async def get_data() -> str:
         content = await resource.read()
         assert content == "Hello, world!"
         assert resource.mime_type == "text/plain"
+        
+    def test_docstring_as_description(self):
+        """Test that function docstring is used as description when not provided."""
+
+        def my_func() -> str:
+            """This is a docstring description."""
+            return "test content"
+
+        resource = FunctionResource(
+            uri=AnyUrl("fn://test"),
+            name="test",
+            description=None,  # No explicit description
+            fn=my_func,
+        )
+        assert resource.description == "This is a docstring description."

tests/server/fastmcp/test_server.py

@@ -347,6 +347,20 @@ async def test_file_resource_binary(self, tmp_path: Path):
                 result.contents[0].blob
                 == base64.b64encode(b"Binary file data").decode()
             )
+            
+    @pytest.mark.anyio
+    async def test_resource_docstring_description(self):
+        """Test that resource decorator uses function docstring as description."""
+        mcp = FastMCP()
+
+        @mcp.resource("resource://docstring-test")
+        def get_data() -> str:
+            """This is a docstring description for a resource."""
+            return "Data"
+
+        resources = await mcp.list_resources()
+        assert len(resources) == 1
+        assert resources[0].description == "This is a docstring description for a resource."
 
 
 class TestServerResourceTemplates: