feat: add resource annotations support to FastMCP (#1468)

Author: fennb

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

@@ -66,6 +66,14 @@ async def read(self, context: Any | None = None) -> str | bytes:
             else:
                 result = self.fn(**args)
 
+            # Support cases where a sync function returns a coroutine
+            if inspect.iscoroutine(result):
+                result = await result
+
+            # Support returning a Resource instance (recursive read)
+            if isinstance(result, Resource):
+                return await result.read(context)
+
             if isinstance(result, str | bytes):
                 return result
             if isinstance(result, pydantic.BaseModel):
@@ -208,4 +216,4 @@ async def read(self, context: Any | None = None) -> str:  # Always returns JSON
             file_list = [str(f.relative_to(self.path)) for f in files if f.is_file()]
             return json.dumps({"files": file_list}, indent=2)
         except Exception as e:
-            raise ValueError(f"Error reading directory {self.path}: {e}")
+            raise ValueError(f"Error reading directory {self.path}: {e}")

tests/server/fastmcp/resources/test_resource_template.py

@@ -257,4 +257,4 @@ def get_item(item_id: str) -> str:  # pragma: no cover
 
         # Verify the resource works correctly
         content = await resource.read()
-        assert content == "Item 123"
+        assert content == "Item 123"

tests/server/fastmcp/resources/test_resources.py

@@ -200,4 +200,4 @@ def test_audience_validation(self):
 
         # Invalid roles should raise validation error
         with pytest.raises(Exception):  # Pydantic validation error
-            Annotations(audience=["invalid_role"])  # type: ignore
+            Annotations(audience=["invalid_role"])  # type: ignore