Strict types on the client side

pyproject.toml

@@ -77,6 +77,7 @@ venvPath = "."
 venv = ".venv"
 strict = [
     "src/mcp/server/fastmcp/tools/base.py",
+    "src/mcp/client/*.py"
 ]
 
 [tool.ruff.lint]

src/mcp/client/__main__.py

@@ -5,10 +5,12 @@
 from urllib.parse import urlparse
 
 import anyio
+from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 
 from mcp.client.session import ClientSession
 from mcp.client.sse import sse_client
 from mcp.client.stdio import StdioServerParameters, stdio_client
+from mcp.types import JSONRPCMessage
 
 if not sys.warnoptions:
     import warnings
@@ -29,7 +31,10 @@ async def receive_loop(session: ClientSession):
         logger.info("Received message from server: %s", message)
 
 
-async def run_session(read_stream, write_stream):
+async def run_session(
+    read_stream: MemoryObjectReceiveStream[JSONRPCMessage | Exception],
+    write_stream: MemoryObjectSendStream[JSONRPCMessage],
+):
     async with (
         ClientSession(read_stream, write_stream) as session,
         anyio.create_task_group() as tg,

src/mcp/client/session.py

@@ -76,18 +76,12 @@ def __init__(
         self._list_roots_callback = list_roots_callback or _default_list_roots_callback
 
     async def initialize(self) -> types.InitializeResult:
-        sampling = (
-            types.SamplingCapability() if self._sampling_callback is not None else None
-        )
-        roots = (
-            types.RootsCapability(
-                # TODO: Should this be based on whether we
-                # _will_ send notifications, or only whether
-                # they're supported?
-                listChanged=True,
-            )
-            if self._list_roots_callback is not None
-            else None
+        sampling = types.SamplingCapability()
+        roots = types.RootsCapability(
+            # TODO: Should this be based on whether we
+            # _will_ send notifications, or only whether
+            # they're supported?
+            listChanged=True,
         )
 
         result = await self.send_request(

src/mcp/client/sse.py

@@ -98,6 +98,10 @@ async def sse_reader(
                                             continue
 
                                         await read_stream_writer.send(message)
+                                    case _:
+                                        logger.warning(
+                                            f"Unknown SSE event: {sse.event}"
+                                        )
                         except Exception as exc:
                             logger.error(f"Error in sse_reader: {exc}")
                             await read_stream_writer.send(exc)

src/mcp/client/websocket.py

@@ -39,6 +39,11 @@ async def websocket_client(
     # Create two in-memory streams:
     # - One for incoming messages (read_stream, written by ws_reader)
     # - One for outgoing messages (write_stream, read by ws_writer)
+    read_stream: MemoryObjectReceiveStream[types.JSONRPCMessage | Exception]
+    read_stream_writer: MemoryObjectSendStream[types.JSONRPCMessage | Exception]
+    write_stream: MemoryObjectSendStream[types.JSONRPCMessage]
+    write_stream_reader: MemoryObjectReceiveStream[types.JSONRPCMessage]
+
     read_stream_writer, read_stream = anyio.create_memory_object_stream(0)
     write_stream, write_stream_reader = anyio.create_memory_object_stream(0)
 

src/mcp/shared/version.py

@@ -1,3 +1,3 @@
 from mcp.types import LATEST_PROTOCOL_VERSION
 
-SUPPORTED_PROTOCOL_VERSIONS = [1, LATEST_PROTOCOL_VERSION]
+SUPPORTED_PROTOCOL_VERSIONS: tuple[int, str] = (1, LATEST_PROTOCOL_VERSION)