Fix ElicitRequestParams usage in task_session.py

Use ElicitRequestFormParams instead of the TypeAlias union type
which cannot be instantiated directly.

Author: maxisbey

src/mcp/server/session.py

@@ -47,8 +47,8 @@ async def handle_list_prompts(ctx: RequestContext) -> list[types.Prompt]:
 
 import mcp.types as types
 from mcp.server.models import InitializationOptions
-from mcp.shared.experimental.tasks import TaskResultHandler
 from mcp.shared.exceptions import McpError
+from mcp.shared.experimental.tasks import TaskResultHandler
 from mcp.shared.message import ServerMessageMetadata, SessionMessage
 from mcp.shared.session import (
     BaseSession,

src/mcp/shared/experimental/tasks/task_session.py

@@ -26,7 +26,7 @@
     CreateMessageResult,
     ElicitationCapability,
     ElicitRequestedSchema,
-    ElicitRequestParams,
+    ElicitRequestFormParams,
     ElicitResult,
     ErrorData,
     IncludeContext,
@@ -161,7 +161,8 @@ async def elicit(
         request_id = self._next_request_id()
 
         # Build params with _meta containing related-task info
-        params = ElicitRequestParams(
+        # Use ElicitRequestFormParams (form mode) since we have message + requestedSchema
+        params = ElicitRequestFormParams(
             message=message,
             requestedSchema=requestedSchema,
         )

src/mcp/types.py

@@ -1721,6 +1721,7 @@ class CancelledNotification(Notification[CancelledNotificationParams, Literal["n
     method: Literal["notifications/cancelled"] = "notifications/cancelled"
     params: CancelledNotificationParams
 
+
 class ElicitCompleteNotificationParams(NotificationParams):
     """Parameters for elicitation completion notifications."""
 
@@ -1924,7 +1925,7 @@ class ServerRequest(RootModel[ServerRequestType]):
     | ToolListChangedNotification
     | PromptListChangedNotification
     | ElicitCompleteNotification
-        | TaskStatusNotification
+    | TaskStatusNotification
 )