Remove the unsupported WebSocket transport

WebSocket was never part of the MCP specification. Delete the client
and server transports (mcp.client.websocket, mcp.server.websocket),
the ws optional-dependency extra, the websockets filterwarnings, and
the transport's tests. tests/test_helpers.py goes too: the WebSocket
test was the last remaining user of run_uvicorn_in_thread after the
other transport tests moved in process.

Documented in docs/migration.md; use streamable HTTP instead.

Author: maxisbey

docs/migration.md

@@ -105,6 +105,10 @@ The `headers`, `timeout`, `sse_read_timeout`, and `auth` parameters have been re
 
 Note: `sse_client` retains its `headers`, `timeout`, `sse_read_timeout`, and `auth` parameters — only the streamable HTTP transport changed.
 
+### WebSocket transport removed
+
+The WebSocket transport has been removed: `mcp.client.websocket.websocket_client`, `mcp.server.websocket.websocket_server`, and the `ws` optional dependency extra (`mcp[ws]`) no longer exist. WebSocket was never part of the MCP specification. Use the streamable HTTP transport instead (`mcp.client.streamable_http.streamable_http_client` on the client, `streamable_http_app()` on the server), which supports bidirectional communication with server-to-client streaming over standard HTTP.
+
 ### Removed type aliases and classes
 
 The following deprecated type aliases and classes have been removed from `mcp.types`:

pyproject.toml

@@ -46,7 +46,6 @@ dependencies = [
 [project.optional-dependencies]
 rich = ["rich>=13.9.4"]
 cli = ["typer>=0.16.0", "python-dotenv>=1.0.0"]
-ws = ["websockets>=15.0.1"]
 
 [project.scripts]
 mcp = "mcp.cli:app [cli]"
@@ -75,8 +74,8 @@ build-constraint-dependencies = [
 
 [dependency-groups]
 dev = [
-    # We add mcp[cli,ws] so `uv sync` considers the extras.
-    "mcp[cli,ws]",
+    # We add mcp[cli] so `uv sync` considers the extras.
+    "mcp[cli]",
     "pyright>=1.1.400",
     "pytest>=8.4.0",
     "ruff>=0.8.5",
@@ -204,9 +203,6 @@ addopts = """
 """
 filterwarnings = [
     "error",
-    # This should be fixed on Uvicorn's side.
-    "ignore::DeprecationWarning:websockets",
-    "ignore:websockets.server.WebSocketServerProtocol is deprecated:DeprecationWarning",
     # pywin32 internal deprecation warning
     "ignore:getargs.*The 'u' format is deprecated:DeprecationWarning",
 ]

src/mcp/client/websocket.py

@@ -21,7 +21,6 @@
 
 from mcp.client.sse import sse_client
 from mcp.client.streamable_http import streamable_http_client
-from mcp.client.websocket import websocket_client
 
 
 @contextmanager
@@ -104,16 +103,3 @@ async def test_streamable_http_client_closes_all_streams_on_exit() -> None:
     with _assert_no_memory_stream_leak():
         async with streamable_http_client("http://127.0.0.1:1/mcp"):
             pass
-
-
-@pytest.mark.anyio
-async def test_websocket_client_closes_all_streams_on_connection_error(free_tcp_port: int) -> None:
-    """websocket_client must close all 4 stream ends when ws_connect fails.
-
-    Before the fix, there was no try/finally at all — if ws_connect raised,
-    all 4 streams were leaked.
-    """
-    with _assert_no_memory_stream_leak():
-        with pytest.raises(OSError):
-            async with websocket_client(f"ws://127.0.0.1:{free_tcp_port}/ws"):
-                pytest.fail("should not reach here")  # pragma: no cover