feat: add async resources for FastMCP

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

@@ -1,5 +1,6 @@
 """Concrete resource implementations."""
 
+import inspect
 import json
 from collections.abc import Callable
 from pathlib import Path
@@ -53,7 +54,9 @@ class FunctionResource(Resource):
     async def read(self) -> str | bytes:
         """Read the resource by calling the wrapped function."""
         try:
-            result = self.fn()
+            result = (
+                await self.fn() if inspect.iscoroutinefunction(self.fn) else self.fn()
+            )
             if isinstance(result, Resource):
                 return await result.read()
             if isinstance(result, bytes):

src/mcp/server/fastmcp/server.py

@@ -1,6 +1,5 @@
 """FastMCP - A more ergonomic interface for MCP servers."""
 
-import functools
 import inspect
 import json
 import re
@@ -305,9 +304,19 @@ def resource(
             def get_data() -> str:
                 return "Hello, world!"
 
+            @server.resource("resource://my-resource")
+            async get_data() -> str:
+                data = await fetch_data()
+                return f"Hello, world! {data}"
+
             @server.resource("resource://{city}/weather")
             def get_weather(city: str) -> str:
                 return f"Weather for {city}"
+
+            @server.resource("resource://{city}/weather")
+            async def get_weather(city: str) -> str:
+                data = await fetch_weather(city)
+                return f"Weather for {city}: {data}"
         """
         # Check if user passed function directly instead of calling decorator
         if callable(uri):
@@ -317,10 +326,6 @@ def get_weather(city: str) -> str:
             )
 
         def decorator(fn: Callable) -> Callable:
-            @functools.wraps(fn)
-            def wrapper(*args: Any, **kwargs: Any) -> Any:
-                return fn(*args, **kwargs)
-
             # Check if this should be a template
             has_uri_params = "{" in uri and "}" in uri
             has_func_params = bool(inspect.signature(fn).parameters)
@@ -338,7 +343,7 @@ def wrapper(*args: Any, **kwargs: Any) -> Any:
 
                 # Register as template
                 self._resource_manager.add_template(
-                    wrapper,
+                    fn=fn,
                     uri_template=uri,
                     name=name,
                     description=description,
@@ -351,10 +356,10 @@ def wrapper(*args: Any, **kwargs: Any) -> Any:
                     name=name,
                     description=description,
                     mime_type=mime_type or "text/plain",
-                    fn=wrapper,
+                    fn=fn,
                 )
                 self.add_resource(resource)
-            return wrapper
+            return fn
 
         return decorator
 

tests/server/fastmcp/resources/test_function_resources.py

@@ -120,3 +120,19 @@ def get_data() -> CustomData:
         )
         content = await resource.read()
         assert isinstance(content, str)
+
+    @pytest.mark.anyio
+    async def test_async_read_text(self):
+        """Test reading text from async FunctionResource."""
+
+        async def get_data() -> str:
+            return "Hello, world!"
+
+        resource = FunctionResource(
+            uri=AnyUrl("function://test"),
+            name="test",
+            fn=get_data,
+        )
+        content = await resource.read()
+        assert content == "Hello, world!"
+        assert resource.mime_type == "text/plain"