Remove legacy _task_capable_initialize() workaround (#2612)

The function set experimental={"tasks": {}} but per the MCP spec:
1. Tasks belong in capabilities.tasks, not capabilities.experimental
2. Clients only need to declare task capabilities if receiving task-augmented
   requests from the server (bi-directional support)

For client→server task requests (tools/call with task=True), only the server
needs to declare task capabilities. The SDK's native session.initialize()
handles this correctly.

src/fastmcp/client/client.py

@@ -56,7 +56,6 @@ from fastmcp.client.tasks import (
     ResourceTask,
     TaskNotificationHandler,
     ToolTask,
-    _task_capable_initialize,
 )
 from fastmcp.exceptions import ToolError
 from fastmcp.mcp_config import MCPConfig
@@ -458,10 +457,7 @@ class Client(Generic[ClientTransportT]):
 
         try:
             with anyio.fail_after(timeout):
-                self._session_state.initialize_result = await _task_capable_initialize(
-                    self.session
-                )
-
+                self._session_state.initialize_result = await self.session.initialize()
                 return self._session_state.initialize_result
         except TimeoutError as e:
             raise RuntimeError("Failed to initialize server session") from e

src/fastmcp/client/tasks.py

@@ -12,13 +12,6 @@ from datetime import datetime, timezone
 from typing import TYPE_CHECKING, Generic, TypeVar
 
 import mcp.types
-from mcp import ClientSession
-from mcp.client.session import (
-    SUPPORTED_PROTOCOL_VERSIONS,
-    _default_elicitation_callback,
-    _default_list_roots_callback,
-    _default_sampling_callback,
-)
 from mcp.types import GetTaskResult, TaskStatusNotification
 
 from fastmcp.client.messages import Message, MessageHandler
@@ -30,62 +23,6 @@ if TYPE_CHECKING:
     from fastmcp.client.client import CallToolResult, Client
 
 
-# TODO(SEP-1686): Remove this function when the MCP SDK adds an
-# `experimental_capabilities` parameter to ClientSession (the server side
-# already has this via `create_initialization_options(experimental_capabilities={})`).
-# The SDK currently hardcodes `experimental=None` in ClientSession.initialize().
-async def _task_capable_initialize(
-    session: ClientSession,
-) -> mcp.types.InitializeResult:
-    """Initialize a session with task capabilities declared."""
-    sampling = (
-        mcp.types.SamplingCapability()
-        if session._sampling_callback != _default_sampling_callback
-        else None
-    )
-    elicitation = (
-        mcp.types.ElicitationCapability()
-        if session._elicitation_callback != _default_elicitation_callback
-        else None
-    )
-    roots = (
-        mcp.types.RootsCapability(listChanged=True)
-        if session._list_roots_callback != _default_list_roots_callback
-        else None
-    )
-
-    result = await session.send_request(
-        mcp.types.ClientRequest(
-            mcp.types.InitializeRequest(
-                params=mcp.types.InitializeRequestParams(
-                    protocolVersion=mcp.types.LATEST_PROTOCOL_VERSION,
-                    capabilities=mcp.types.ClientCapabilities(
-                        sampling=sampling,
-                        elicitation=elicitation,
-                        experimental={"tasks": {}},
-                        roots=roots,
-                    ),
-                    clientInfo=session._client_info,
-                ),
-            )
-        ),
-        mcp.types.InitializeResult,
-    )
-
-    if result.protocolVersion not in SUPPORTED_PROTOCOL_VERSIONS:
-        raise RuntimeError(
-            f"Unsupported protocol version from the server: {result.protocolVersion}"
-        )
-
-    session._server_capabilities = result.capabilities
-
-    await session.send_notification(
-        mcp.types.ClientNotification(mcp.types.InitializedNotification())
-    )
-
-    return result
-
-
 class TaskNotificationHandler(MessageHandler):
     """MessageHandler that routes task status notifications to Task objects."""