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
SessionMessage objects or Exception objects (when validation fails).
- write_stream: Write SessionMessage objects to this stream to send them
over the WebSocket to the server.
Source code in src/mcp/client/websocket.py
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async def websocket_client(
url: str,
) -> AsyncGenerator[
tuple[MemoryObjectReceiveStream[SessionMessage | Exception], MemoryObjectSendStream[SessionMessage]],
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
SessionMessage objects or Exception objects (when validation fails).
- write_stream: Write SessionMessage objects to this stream to send them
over the WebSocket to the server.
"""
# 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[SessionMessage | Exception]
read_stream_writer: MemoryObjectSendStream[SessionMessage | Exception]
write_stream: MemoryObjectSendStream[SessionMessage]
write_stream_reader: MemoryObjectReceiveStream[SessionMessage]
# Connect using websockets, requesting the "mcp" subprotocol
async with ws_connect(url, subprotocols=[Subprotocol("mcp")]) as ws:
read_stream_writer, read_stream = anyio.create_memory_object_stream(0)
write_stream, write_stream_reader = anyio.create_memory_object_stream(0)
async def ws_reader():
"""Reads text messages from the WebSocket, parses them as JSON-RPC messages,
and sends them into read_stream_writer.
"""
async with read_stream_writer:
async for raw_text in ws:
try:
message = types.jsonrpc_message_adapter.validate_json(raw_text, by_name=False)
session_message = SessionMessage(message)
await read_stream_writer.send(session_message)
except ValidationError as exc: # pragma: no cover
# If JSON parse or model validation fails, send the exception
await read_stream_writer.send(exc)
async def ws_writer():
"""Reads JSON-RPC messages from write_stream_reader and
sends them to the server.
"""
async with write_stream_reader:
async for session_message in write_stream_reader:
# Convert to a dict, then to JSON
msg_dict = session_message.message.model_dump(by_alias=True, mode="json", exclude_unset=True)
await ws.send(json.dumps(msg_dict))
async with (
read_stream_writer,
read_stream,
write_stream,
write_stream_reader,
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, write_stream)
# Once the caller's 'async with' block exits, we shut down
tg.cancel_scope.cancel()
|