feat: allow lowlevel servers to return a list of resources

The resource/read message in MCP allows of multiple resources
to be returned. However, in the SDK we do not allow this. This
change is such that we allow returning multiple resource in
the lowlevel API if needed. However in FastMCP we stick to
one, since a FastMCP resource defines the mime_type in the decorator
and hence a resource cannot dynamically return different mime_typed resources.
It also is just the better default to only return one resource.
However in the lowlevel API we will allow this.

Strictly speaking this is not a BC break since the new return value
is additive, but if people subclassed server, it will break them.

Author: dsp-ant

Diff for: src/mcp/server/fastmcp/server.py

@@ -3,7 +3,7 @@
 import inspect
 import json
 import re
-from collections.abc import AsyncIterator
+from collections.abc import AsyncIterator, Iterable
 from contextlib import (
     AbstractAsyncContextManager,
     asynccontextmanager,
@@ -236,7 +236,7 @@ async def list_resource_templates(self) -> list[MCPResourceTemplate]:
             for template in templates
         ]
 
-    async def read_resource(self, uri: AnyUrl | str) -> ReadResourceContents:
+    async def read_resource(self, uri: AnyUrl | str) -> Iterable[ReadResourceContents]:
         """Read a resource by URI."""
 
         resource = await self._resource_manager.get_resource(uri)
@@ -245,7 +245,7 @@ async def read_resource(self, uri: AnyUrl | str) -> ReadResourceContents:
 
         try:
             content = await resource.read()
-            return ReadResourceContents(content=content, mime_type=resource.mime_type)
+            return [ReadResourceContents(content=content, mime_type=resource.mime_type)]
         except Exception as e:
             logger.error(f"Error reading resource {uri}: {e}")
             raise ResourceError(str(e))
@@ -649,7 +649,7 @@ async def report_progress(
             progress_token=progress_token, progress=progress, total=total
         )
 
-    async def read_resource(self, uri: str | AnyUrl) -> ReadResourceContents:
+    async def read_resource(self, uri: str | AnyUrl) -> Iterable[ReadResourceContents]:
         """Read a resource by URI.
 
         Args:

Diff for: src/mcp/server/lowlevel/server.py

@@ -67,7 +67,7 @@ async def main():
 import contextvars
 import logging
 import warnings
-from collections.abc import Awaitable, Callable
+from collections.abc import Awaitable, Callable, Iterable
 from contextlib import AbstractAsyncContextManager, AsyncExitStack, asynccontextmanager
 from typing import Any, AsyncIterator, Generic, Sequence, TypeVar
 
@@ -279,7 +279,9 @@ async def handler(_: Any):
 
     def read_resource(self):
         def decorator(
-            func: Callable[[AnyUrl], Awaitable[str | bytes | ReadResourceContents]],
+            func: Callable[
+                [AnyUrl], Awaitable[str | bytes | Iterable[ReadResourceContents]]
+            ],
         ):
             logger.debug("Registering handler for ReadResourceRequest")
 
@@ -307,13 +309,22 @@ def create_content(data: str | bytes, mime_type: str | None):
                     case str() | bytes() as data:
                         warnings.warn(
                             "Returning str or bytes from read_resource is deprecated. "
-                            "Use ReadResourceContents instead.",
+                            "Use Iterable[ReadResourceContents] instead.",
                             DeprecationWarning,
                             stacklevel=2,
                         )
                         content = create_content(data, None)
-                    case ReadResourceContents() as contents:
-                        content = create_content(contents.content, contents.mime_type)
+                    case Iterable() as contents:
+                        contents_list = [
+                            create_content(content_item.content, content_item.mime_type)
+                            for content_item in contents
+                            if isinstance(content_item, ReadResourceContents)
+                        ]
+                        return types.ServerResult(
+                            types.ReadResourceResult(
+                                contents=contents_list,
+                            )
+                        )
                     case _:
                         raise ValueError(
                             f"Unexpected return type from read_resource: {type(result)}"

Diff for: tests/issues/test_141_resource_templates.py

@@ -51,8 +51,10 @@ def get_user_profile_missing(user_id: str) -> str:
 
     # Verify valid template works
     result = await mcp.read_resource("resource://users/123/posts/456")
-    assert result.content == "Post 456 by user 123"
-    assert result.mime_type == "text/plain"
+    result_list = list(result)
+    assert len(result_list) == 1
+    assert result_list[0].content == "Post 456 by user 123"
+    assert result_list[0].mime_type == "text/plain"
 
     # Verify invalid parameters raise error
     with pytest.raises(ValueError, match="Unknown resource"):

Diff for: tests/issues/test_152_resource_mime_type.py

@@ -99,11 +99,11 @@ async def handle_list_resources():
     @server.read_resource()
     async def handle_read_resource(uri: AnyUrl):
         if str(uri) == "test://image":
-            return ReadResourceContents(content=base64_string, mime_type="image/png")
+            return [ReadResourceContents(content=base64_string, mime_type="image/png")]
         elif str(uri) == "test://image_bytes":
-            return ReadResourceContents(
-                content=bytes(image_bytes), mime_type="image/png"
-            )
+            return [
+                ReadResourceContents(content=bytes(image_bytes), mime_type="image/png")
+            ]
         raise Exception(f"Resource not found: {uri}")
 
     # Test that resources are listed with correct mime type

Diff for: tests/server/fastmcp/servers/test_file_server.py

@@ -88,7 +88,10 @@ async def test_list_resources(mcp: FastMCP):
 
 @pytest.mark.anyio
 async def test_read_resource_dir(mcp: FastMCP):
-    res = await mcp.read_resource("dir://test_dir")
+    res_iter = await mcp.read_resource("dir://test_dir")
+    res_list = list(res_iter)
+    assert len(res_list) == 1
+    res = res_list[0]
     assert res.mime_type == "text/plain"
 
     files = json.loads(res.content)
@@ -102,7 +105,10 @@ async def test_read_resource_dir(mcp: FastMCP):
 
 @pytest.mark.anyio
 async def test_read_resource_file(mcp: FastMCP):
-    res = await mcp.read_resource("file://test_dir/example.py")
+    res_iter = await mcp.read_resource("file://test_dir/example.py")
+    res_list = list(res_iter)
+    assert len(res_list) == 1
+    res = res_list[0]
     assert res.content == "print('hello world')"
 
 
@@ -119,5 +125,8 @@ async def test_delete_file_and_check_resources(mcp: FastMCP, test_dir: Path):
     await mcp.call_tool(
         "delete_file", arguments=dict(path=str(test_dir / "example.py"))
     )
-    res = await mcp.read_resource("file://test_dir/example.py")
+    res_iter = await mcp.read_resource("file://test_dir/example.py")
+    res_list = list(res_iter)
+    assert len(res_list) == 1
+    res = res_list[0]
     assert res.content == "File not found"

Diff for: tests/server/fastmcp/test_server.py

@@ -581,7 +581,10 @@ def test_resource() -> str:
 
         @mcp.tool()
         async def tool_with_resource(ctx: Context) -> str:
-            r = await ctx.read_resource("test://data")
+            r_iter = await ctx.read_resource("test://data")
+            r_list = list(r_iter)
+            assert len(r_list) == 1
+            r = r_list[0]
             return f"Read resource: {r.content} with mime type {r.mime_type}"
 
         async with client_session(mcp._mcp_server) as client:

Diff for: tests/server/test_read_resource.py

@@ -1,3 +1,4 @@
+from collections.abc import Iterable
 from pathlib import Path
 from tempfile import NamedTemporaryFile
 
@@ -26,8 +27,8 @@ async def test_read_resource_text(temp_file: Path):
     server = Server("test")
 
     @server.read_resource()
-    async def read_resource(uri: AnyUrl) -> ReadResourceContents:
-        return ReadResourceContents(content="Hello World", mime_type="text/plain")
+    async def read_resource(uri: AnyUrl) -> Iterable[ReadResourceContents]:
+        return [ReadResourceContents(content="Hello World", mime_type="text/plain")]
 
     # Get the handler directly from the server
     handler = server.request_handlers[types.ReadResourceRequest]
@@ -54,10 +55,12 @@ async def test_read_resource_binary(temp_file: Path):
     server = Server("test")
 
     @server.read_resource()
-    async def read_resource(uri: AnyUrl) -> ReadResourceContents:
-        return ReadResourceContents(
-            content=b"Hello World", mime_type="application/octet-stream"
-        )
+    async def read_resource(uri: AnyUrl) -> Iterable[ReadResourceContents]:
+        return [
+            ReadResourceContents(
+                content=b"Hello World", mime_type="application/octet-stream"
+            )
+        ]
 
     # Get the handler directly from the server
     handler = server.request_handlers[types.ReadResourceRequest]
@@ -83,11 +86,13 @@ async def test_read_resource_default_mime(temp_file: Path):
     server = Server("test")
 
     @server.read_resource()
-    async def read_resource(uri: AnyUrl) -> ReadResourceContents:
-        return ReadResourceContents(
-            content="Hello World",
-            # No mime_type specified, should default to text/plain
-        )
+    async def read_resource(uri: AnyUrl) -> Iterable[ReadResourceContents]:
+        return [
+            ReadResourceContents(
+                content="Hello World",
+                # No mime_type specified, should default to text/plain
+            )
+        ]
 
     # Get the handler directly from the server
     handler = server.request_handlers[types.ReadResourceRequest]