Create Client websocket.py

Author: calclavia

src/mcp/client/websocket.py

@@ -0,0 +1,100 @@
+import json
+import logging
+from contextlib import asynccontextmanager
+from typing import AsyncGenerator
+
+import anyio
+import websockets
+from anyio.streams.memory import (
+    MemoryObjectReceiveStream,
+    MemoryObjectSendStream,
+    create_memory_object_stream,
+)
+
+import mcp.types as types
+
+logger = logging.getLogger(__name__)
+
+@asynccontextmanager
+async def websocket_client(
+    url: str
+) -> AsyncGenerator[
+    tuple[
+        MemoryObjectReceiveStream[types.JSONRPCMessage | Exception],
+        MemoryObjectSendStream[types.JSONRPCMessage],
+    ],
+    None
+]:
+    """
+    WebSocket client transport for MCP, symmetrical to the server version.
+
+    Connects to 'url' using the 'mcp' subprotocol, then yields:
+        (read_stream, write_stream)
+
+    - read_stream: As you read from this stream, you'll receive either valid
+      JSONRPCMessage objects or Exception objects (when validation fails).
+    - write_stream: Write JSONRPCMessage objects to this stream to send them
+      over the WebSocket to the server.
+    """
+
+    # Create two in-memory streams:
+    # - One for incoming messages (read_stream_recv, written by ws_reader)
+    # - One for outgoing messages (write_stream_send, read by ws_writer)
+    read_stream_send, read_stream_recv = create_memory_object_stream(0)
+    write_stream_send, write_stream_recv = create_memory_object_stream(0)
+
+    # Connect using websockets, requesting the "mcp" subprotocol
+    async with websockets.connect(url, subprotocols=["mcp"]) as ws:
+        # Optional check to ensure the server actually accepted "mcp"
+        if ws.subprotocol != "mcp":
+            raise ValueError(
+                f"Server did not accept subprotocol 'mcp'. Actual subprotocol: {ws.subprotocol}"
+            )
+
+        async def ws_reader():
+            """
+            Reads text messages from the WebSocket, parses them as JSON-RPC messages,
+            and sends them into read_stream_send.
+            """
+            try:
+                async for raw_text in ws:
+                    try:
+                        data = json.loads(raw_text)
+                        message = types.JSONRPCMessage.model_validate(data)
+                        await read_stream_send.send(message)
+                    except Exception as exc:
+                        # If JSON parse or model validation fails, send the exception
+                        await read_stream_send.send(exc)
+            except (anyio.ClosedResourceError, websockets.ConnectionClosed):
+                pass
+            finally:
+                # Ensure our read stream is closed
+                await read_stream_send.aclose()
+
+        async def ws_writer():
+            """
+            Reads JSON-RPC messages from write_stream_recv and sends them to the server.
+            """
+            try:
+                async for message in write_stream_recv:
+                    # Convert to a dict, then to JSON
+                    msg_dict = message.model_dump(
+                        by_alias=True, mode="json", exclude_none=True
+                    )
+                    await ws.send(json.dumps(msg_dict))
+            except (anyio.ClosedResourceError, websockets.ConnectionClosed):
+                pass
+            finally:
+                # Ensure our write stream is closed
+                await write_stream_recv.aclose()
+
+        async with anyio.create_task_group() as tg:
+            # Start reader and writer tasks
+            tg.start_soon(ws_reader)
+            tg.start_soon(ws_writer)
+
+            # Yield the receive/send streams
+            yield (read_stream_recv, write_stream_send)
+
+            # Once the caller's 'async with' block exits, we shut down
+            tg.cancel_scope.cancel()