[v1.x] Deprecate the WebSocket transport and the experimental tasks entry points (#2828)

docs/experimental/index.md

@@ -1,12 +1,13 @@
 # Experimental Features
 
-!!! warning "Experimental APIs"
+!!! warning "Deprecated"
 
-    The features in this section are experimental and may change without notice.
+    The experimental tasks API is deprecated and will be removed in mcp 2.0.
-    They track the evolving MCP specification and are not yet stable.
+    Tasks (SEP-1686) were removed from the MCP specification and are expected
+    to return as a separate MCP extension in a future release.
 
 This section documents experimental features in the MCP Python SDK. These features
-implement draft specifications that are still being refined.
+are deprecated and remain available on the 1.x line only for existing users.
 
 ## Available Experimental Features
 
@@ -36,8 +37,10 @@ async def handle_get_task(request: GetTaskRequest) -> GetTaskResult:
 result = await session.experimental.call_tool_as_task("tool_name", {"arg": "value"})
 ```
 
+Accessing the `.experimental` properties emits a `DeprecationWarning`.
+
 ## Providing Feedback
 
-Since these features are experimental, feedback is especially valuable. If you encounter
+If you rely on these features and have feedback on their deprecation or the planned
-issues or have suggestions, please open an issue on the
+MCP extension, please open an issue on the
 [python-sdk repository](https://github.com/modelcontextprotocol/python-sdk/issues).

docs/experimental/tasks-client.md

@@ -1,8 +1,10 @@
 # Client Task Usage
 
-!!! warning "Experimental"
+!!! warning "Deprecated"
 
-    Tasks are an experimental feature. The API may change without notice.
+    The experimental tasks API is deprecated and will be removed in mcp 2.0.
+    Tasks (SEP-1686) were removed from the MCP specification and are expected
+    to return as a separate MCP extension in a future release.
 
 This guide covers calling task-augmented tools from clients, handling the `input_required` status, and advanced patterns like receiving task requests from servers.
 

docs/experimental/tasks-server.md

@@ -1,8 +1,10 @@
 # Server Task Implementation
 
-!!! warning "Experimental"
+!!! warning "Deprecated"
 
-    Tasks are an experimental feature. The API may change without notice.
+    The experimental tasks API is deprecated and will be removed in mcp 2.0.
+    Tasks (SEP-1686) were removed from the MCP specification and are expected
+    to return as a separate MCP extension in a future release.
 
 This guide covers implementing task support in MCP servers, from basic setup to advanced patterns like elicitation and sampling within tasks.
 

docs/experimental/tasks.md

@@ -1,9 +1,10 @@
 # Tasks
 
-!!! warning "Experimental"
+!!! warning "Deprecated"
 
-    Tasks are an experimental feature tracking the draft MCP specification.
+    The experimental tasks API is deprecated and will be removed in mcp 2.0.
-    The API may change without notice.
+    Tasks (SEP-1686) were removed from the MCP specification and are expected
+    to return as a separate MCP extension in a future release.
 
 Tasks enable asynchronous request handling in MCP. Instead of blocking until an operation completes, the receiver creates a task, returns immediately, and the requestor polls for the result.
 

pyproject.toml

@@ -107,7 +107,13 @@ venv = ".venv"
 # those private functions instead of testing the private functions directly. It makes it easier to maintain the code source
 # and refactor code that is not public.
 executionEnvironments = [
+    # The experimental tasks API and the WebSocket transport are deprecated; the suites and
+    # examples that cover them intentionally use the deprecated APIs.
+    { root = "tests/experimental", extraPaths = ["."], reportUnusedFunction = false, reportPrivateUsage = false, reportDeprecated = false },
+    { root = "tests/shared/test_ws.py", extraPaths = ["."], reportUnusedFunction = false, reportPrivateUsage = false, reportDeprecated = false },
     { root = "tests", extraPaths = ["."], reportUnusedFunction = false, reportPrivateUsage = false },
+    { root = "examples/servers/simple-task", reportUnusedFunction = false, reportDeprecated = false },
+    { root = "examples/servers/simple-task-interactive", reportUnusedFunction = false, reportDeprecated = false },
     { root = "examples/servers", reportUnusedFunction = false },
     { root = "examples/snippets/clients/logging_client.py", reportUndefinedVariable = false, reportUnknownArgumentType = false },
     { root = "examples/snippets/clients/roots_example.py", reportUndefinedVariable = false, reportUnknownArgumentType = false, reportArgumentType = false },

src/mcp/client/session.py

@@ -1,4 +1,5 @@
 import logging
+import warnings
 from datetime import timedelta
 from typing import Any, Protocol, overload
 
@@ -17,6 +18,15 @@ from mcp.shared.version import SUPPORTED_PROTOCOL_VERSIONS
 
 DEFAULT_CLIENT_INFO = types.Implementation(name="mcp", version="0.1.0")
 
+# Type checkers only surface `@deprecated` messages given as string literals, so the same text is
+# repeated inline at each deprecated entry point (here, server/session.py, server/lowlevel/server.py).
+# Keep those copies, the filterwarnings marks in the tasks test suites, and the pytest.warns match in
+# tests/experimental/tasks/test_deprecations.py prefix-aligned when rewording.
+_EXPERIMENTAL_TASKS_DEPRECATION = (
+    "The experimental tasks API is deprecated and will be removed in mcp 2.0: tasks (SEP-1686) were removed"
+    " from the MCP specification and are expected to return as a separate MCP extension."
+)
+
 logger = logging.getLogger("client")
 
 
@@ -143,6 +153,8 @@ class ClientSession(
         self._experimental_features: ExperimentalClientFeatures | None = None
 
         # Experimental: Task handlers (use defaults if not provided)
+        if experimental_task_handlers is not None:
+            warnings.warn(_EXPERIMENTAL_TASKS_DEPRECATION, DeprecationWarning, stacklevel=2)
         self._task_handlers = experimental_task_handlers or ExperimentalTaskHandlers()
 
     async def initialize(self) -> types.InitializeResult:
@@ -204,10 +216,16 @@ class ClientSession(
         return self._server_capabilities
 
     @property
+    @deprecated(
+        "The experimental tasks API is deprecated and will be removed in mcp 2.0: tasks (SEP-1686) were removed"
+        " from the MCP specification and are expected to return as a separate MCP extension."
+    )
     def experimental(self) -> ExperimentalClientFeatures:
         """Experimental APIs for tasks and other features.
 
-        WARNING: These APIs are experimental and may change without notice.
+        Deprecated: the experimental tasks API will be removed in mcp 2.0. Tasks
+        (SEP-1686) were removed from the MCP specification and are expected to
+        return as a separate MCP extension.
 
         Example:
             status = await session.experimental.get_task(task_id)

src/mcp/client/websocket.py

@@ -6,6 +6,7 @@ from contextlib import asynccontextmanager
 import anyio
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 from pydantic import ValidationError
+from typing_extensions import deprecated
 from websockets.asyncio.client import connect as ws_connect
 from websockets.typing import Subprotocol
 
@@ -15,6 +16,10 @@ from mcp.shared.message import SessionMessage
 logger = logging.getLogger(__name__)
 
 
+@deprecated(
+    "The WebSocket client transport is deprecated and will be removed in mcp 2.0. WebSocket was never part of"
+    " the MCP specification; use the streamable HTTP transport (`streamable_http_client`) instead."
+)
 @asynccontextmanager
 async def websocket_client(
     url: str,
@@ -25,6 +30,9 @@ async def websocket_client(
     """
     WebSocket client transport for MCP, symmetrical to the server version.
 
+    Deprecated: this transport will be removed in mcp 2.0. WebSocket was never
+    part of the MCP specification; use the streamable HTTP transport instead.
+
     Connects to 'url' using the 'mcp' subprotocol, then yields:
         (read_stream, write_stream)
 

src/mcp/server/experimental/request_context.py

@@ -252,7 +252,8 @@ class Experimental:
         task_group = support.task_group
 
         if task_id is None:
-            session_scope = self._session.experimental.task_session_scope
+            features = self._session._experimental  # pyright: ignore[reportPrivateUsage]
+            session_scope = features.task_session_scope
             if session_scope is not None:
                 task_id = scoped_task_id(session_scope)
 

src/mcp/server/experimental/task_context.py

@@ -488,12 +488,13 @@ class ServerTaskContext:
             create_result = CreateTaskResult.model_validate(response_data)
             client_task_id = create_result.task.taskId
 
-            # Poll the client's task using session.experimental
+            # Poll the client's task using the session's experimental features
-            async for _ in self._session.experimental.poll_task(client_task_id):
+            features = self._session._experimental  # pyright: ignore[reportPrivateUsage]
+            async for _ in features.poll_task(client_task_id):
                 pass
 
             # Get final result from client
-            result = await self._session.experimental.get_task_result(
+            result = await features.get_task_result(
                 client_task_id,
                 ElicitResult,
             )
@@ -594,12 +595,13 @@ class ServerTaskContext:
             create_result = CreateTaskResult.model_validate(response_data)
             client_task_id = create_result.task.taskId
 
-            # Poll the client's task using session.experimental
+            # Poll the client's task using the session's experimental features
-            async for _ in self._session.experimental.poll_task(client_task_id):
+            features = self._session._experimental  # pyright: ignore[reportPrivateUsage]
+            async for _ in features.poll_task(client_task_id):
                 pass
 
             # Get final result from client
-            result = await self._session.experimental.get_task_result(
+            result = await features.get_task_result(
                 client_task_id,
                 CreateMessageResult,
             )

src/mcp/server/experimental/task_support.py

@@ -106,8 +106,10 @@ class TaskSupport:
             stateless: Whether the session belongs to a stateless server run
         """
         session.add_response_router(self.handler)
-        if not stateless and session.experimental.task_session_scope is None:
+        if not stateless:
-            session.experimental.task_session_scope = new_session_scope()
+            features = session._experimental  # pyright: ignore[reportPrivateUsage]
+            if features.task_session_scope is None:
+                features.task_session_scope = new_session_scope()
 
     @classmethod
     def in_memory(cls) -> "TaskSupport":

src/mcp/server/lowlevel/experimental.py

@@ -129,7 +129,8 @@ class ExperimentalHandlers:
 
     def _requestor_session_scope(self) -> str | None:
         """Return the task session scope of the session making the current request."""
-        return self._server.request_context.session.experimental.task_session_scope
+        session = self._server.request_context.session
+        return session._experimental.task_session_scope  # pyright: ignore[reportPrivateUsage]
 
     def _require_task_in_requestor_scope(self, task_id: str) -> None:
         """Reject task IDs that belong to a different session.

src/mcp/server/lowlevel/server.py

@@ -80,7 +80,7 @@ import anyio
 import jsonschema
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 from pydantic import AnyUrl
-from typing_extensions import TypeVar
+from typing_extensions import TypeVar, deprecated
 
 import mcp.types as types
 from mcp.server.experimental.request_context import Experimental
@@ -244,10 +244,16 @@ class Server(Generic[LifespanResultT, RequestT]):
         return request_ctx.get()
 
     @property
+    @deprecated(
+        "The experimental tasks API is deprecated and will be removed in mcp 2.0: tasks (SEP-1686) were removed"
+        " from the MCP specification and are expected to return as a separate MCP extension."
+    )
     def experimental(self) -> ExperimentalHandlers:
         """Experimental APIs for tasks and other features.
 
-        WARNING: These APIs are experimental and may change without notice.
+        Deprecated: the experimental tasks API will be removed in mcp 2.0. Tasks
+        (SEP-1686) were removed from the MCP specification and are expected to
+        return as a separate MCP extension.
         """
 
         # We create this inline so we only add these capabilities _if_ they're actually used

src/mcp/server/session.py

@@ -44,6 +44,7 @@ import anyio
 import anyio.lowlevel
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 from pydantic import AnyUrl
+from typing_extensions import deprecated
 
 import mcp.types as types
 from mcp.server.experimental.session_features import ExperimentalServerSessionFeatures
@@ -108,15 +109,26 @@ class ServerSession(
         return self._client_params  # pragma: no cover
 
     @property
-    def experimental(self) -> ExperimentalServerSessionFeatures:
+    def _experimental(self) -> ExperimentalServerSessionFeatures:
-        """Experimental APIs for server→client task operations.
+        """Internal accessor for experimental features that skips the deprecation warning."""
-
-        WARNING: These APIs are experimental and may change without notice.
-        """
         if self._experimental_features is None:
             self._experimental_features = ExperimentalServerSessionFeatures(self)
         return self._experimental_features
 
+    @property
+    @deprecated(
+        "The experimental tasks API is deprecated and will be removed in mcp 2.0: tasks (SEP-1686) were removed"
+        " from the MCP specification and are expected to return as a separate MCP extension."
+    )
+    def experimental(self) -> ExperimentalServerSessionFeatures:
+        """Experimental APIs for server→client task operations.
+
+        Deprecated: the experimental tasks API will be removed in mcp 2.0. Tasks
+        (SEP-1686) were removed from the MCP specification and are expected to
+        return as a separate MCP extension.
+        """
+        return self._experimental
+
     def check_client_capability(self, capability: types.ClientCapabilities) -> bool:  # pragma: no cover
         """Check if the client supports a specific capability."""
         if self._client_params is None:

src/mcp/server/websocket.py

@@ -6,6 +6,7 @@ from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStre
 from pydantic_core import ValidationError
 from starlette.types import Receive, Scope, Send
 from starlette.websockets import WebSocket
+from typing_extensions import deprecated
 
 import mcp.types as types
 from mcp.shared.message import SessionMessage
@@ -13,11 +14,18 @@ from mcp.shared.message import SessionMessage
 logger = logging.getLogger(__name__)
 
 
-@asynccontextmanager  # pragma: no cover
+@deprecated(  # pragma: no cover
+    "The WebSocket server transport is deprecated and will be removed in mcp 2.0. WebSocket was never part of"
+    " the MCP specification; use the streamable HTTP transport instead."
+)
+@asynccontextmanager
 async def websocket_server(scope: Scope, receive: Receive, send: Send):
     """
     WebSocket server transport for MCP. This is an ASGI application, suitable to be
     used with a framework like Starlette and a server like Hypercorn.
+
+    Deprecated: this transport will be removed in mcp 2.0. WebSocket was never
+    part of the MCP specification; use the streamable HTTP transport instead.
     """
 
     websocket = WebSocket(scope, receive, send)

tests/experimental/tasks/conftest.py

@@ -0,0 +1,18 @@
+"""Shared configuration for the experimental tasks suite."""
+
+from pathlib import Path
+
+import pytest
+
+_HERE = Path(__file__).parent
+
+# The tasks suite intentionally exercises the deprecated experimental tasks API.
+_TASKS_DEPRECATION_IGNORE = pytest.mark.filterwarnings(
+    "ignore:The experimental tasks API is deprecated:DeprecationWarning"
+)
+
+
+def pytest_collection_modifyitems(items: list[pytest.Item]) -> None:
+    for item in items:
+        if _HERE in item.path.parents:
+            item.add_marker(_TASKS_DEPRECATION_IGNORE)

tests/experimental/tasks/test_deprecations.py

@@ -0,0 +1,72 @@
+"""Tests for the deprecation warnings on the experimental tasks entry points."""
+
+import warnings
+
+import pytest
+
+import mcp.types as types
+from mcp.client.experimental.task_handlers import ExperimentalTaskHandlers
+from mcp.client.session import ClientSession
+from mcp.server.lowlevel import Server
+from mcp.server.models import InitializationOptions
+from mcp.server.session import ServerSession
+from mcp.shared.memory import create_client_server_memory_streams, create_connected_server_and_client_session
+
+_DEPRECATION_MATCH = "The experimental tasks API is deprecated"
+
+
+@pytest.mark.anyio
+async def test_client_session_experimental_property_is_deprecated() -> None:
+    async with create_client_server_memory_streams() as (client_streams, _):
+        read_stream, write_stream = client_streams
+        session = ClientSession(read_stream, write_stream)
+        with pytest.warns(DeprecationWarning, match=_DEPRECATION_MATCH):
+            features = session.experimental
+        # The cached path warns as well
+        with pytest.warns(DeprecationWarning, match=_DEPRECATION_MATCH):
+            assert session.experimental is features
+
+
+@pytest.mark.anyio
+async def test_client_session_experimental_task_handlers_kwarg_is_deprecated() -> None:
+    async with create_client_server_memory_streams() as (client_streams, _):
+        read_stream, write_stream = client_streams
+        with pytest.warns(DeprecationWarning, match=_DEPRECATION_MATCH):
+            ClientSession(read_stream, write_stream, experimental_task_handlers=ExperimentalTaskHandlers())
+
+
+@pytest.mark.anyio
+async def test_server_session_experimental_property_is_deprecated() -> None:
+    init_options = InitializationOptions(
+        server_name="test-server",
+        server_version="0.1.0",
+        capabilities=types.ServerCapabilities(),
+    )
+    async with create_client_server_memory_streams() as (_, server_streams):
+        read_stream, write_stream = server_streams
+        async with ServerSession(read_stream, write_stream, init_options) as session:
+            with pytest.warns(DeprecationWarning, match=_DEPRECATION_MATCH):
+                features = session.experimental
+            # The cached path warns as well. coverage.py misreports the branch arcs of the
+            # last statement in a nested `async with` body on Python 3.11+.
+            with pytest.warns(DeprecationWarning, match=_DEPRECATION_MATCH):  # pragma: no branch
+                assert session.experimental is features
+
+
+def test_lowlevel_server_experimental_property_is_deprecated() -> None:
+    server: Server = Server("test-server")
+    with pytest.warns(DeprecationWarning, match=_DEPRECATION_MATCH):
+        handlers = server.experimental
+    # The cached path warns as well
+    with pytest.warns(DeprecationWarning, match=_DEPRECATION_MATCH):
+        assert server.experimental is handlers
+
+
+@pytest.mark.anyio
+async def test_plain_session_usage_does_not_warn() -> None:
+    """Clients and servers that don't touch the tasks API must not see deprecation warnings."""
+    server: Server = Server("test-server")
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        async with create_connected_server_and_client_session(server) as session:
+            await session.send_ping()

tests/experimental/tasks/test_request_context.py

@@ -181,7 +181,7 @@ async def test_run_task_with_an_explicit_task_id_emits_a_deprecation_warning() -
         # deprecated argument has been reported.
         with pytest.raises(RuntimeError, match="Task support not enabled"):
             # The deliberate use of the deprecated overload is the point of this test.
-            await exp.run_task(work, task_id="explicitly-chosen")  # pyright: ignore[reportDeprecated]
+            await exp.run_task(work, task_id="explicitly-chosen")
 
 
 @pytest.mark.anyio

tests/shared/test_ws.py

@@ -10,6 +10,7 @@ import uvicorn
 from pydantic import AnyUrl
 from starlette.applications import Starlette
 from starlette.routing import WebSocketRoute
+from starlette.types import Message
 from starlette.websockets import WebSocket
 
 from mcp.client.session import ClientSession
@@ -30,6 +31,11 @@ from tests.test_helpers import wait_for_server
 
 SERVER_NAME = "test_server_for_WS"
 
+# This suite intentionally exercises the deprecated WebSocket transport.
+pytestmark = pytest.mark.filterwarnings(
+    "ignore:The WebSocket (client|server) transport is deprecated:DeprecationWarning"
+)
+
 
 @pytest.fixture
 def server_port() -> int:
@@ -198,3 +204,22 @@ async def test_ws_client_timeout(
         assert len(result.contents) > 0
         assert isinstance(result.contents[0], TextResourceContents)
         assert result.contents[0].text == "Read example"
+
+
+def test_websocket_client_is_deprecated() -> None:
+    """Creating the websocket_client context manager emits a DeprecationWarning."""
+    with pytest.warns(DeprecationWarning, match="The WebSocket client transport is deprecated"):
+        websocket_client("ws://127.0.0.1:1/ws")
+
+
+def test_websocket_server_is_deprecated() -> None:
+    """Creating the websocket_server context manager emits a DeprecationWarning."""
+
+    async def receive() -> Message:
+        raise NotImplementedError
+
+    async def send(message: Message) -> None:
+        raise NotImplementedError
+
+    with pytest.warns(DeprecationWarning, match="The WebSocket server transport is deprecated"):
+        websocket_server({"type": "websocket"}, receive, send)