SEP-2663: Disallow progress/logging notifications on tasks

Author: LucaButBoring

seps/2663-tasks-extension.md

@@ -508,6 +508,8 @@ Each notification carries a complete `DetailedTask` for the current status, iden
 
 The notification includes the full task object, allowing clients to access the complete task state and final results without polling the `tasks/get` method. Clients **MAY** continue polling `tasks/get` in addition to subscribing to task status notifications, but need not do so.
 
+`notifications/progress` and `notifications/message` notifications **MUST NOT** be sent on the `subscriptions/listen` stream for a task, and are not supported on tasks in general in this specification.
+
 ### Streamable HTTP: Routing Headers
 
 When `tasks/get`, `tasks/update`, or `tasks/cancel` is sent over the Streamable HTTP transport, the client **MUST** set the `Mcp-Name` header (defined by [SEP-2243](./2243-http-standardization.md)) to the value of `params.taskId`. This allows transport intermediaries and load balancers to route subsequent requests for the same task to the server instance holding its state, which is typically required for correctness. The `Mcp-Method` header is set to the JSON-RPC method name per [SEP-2243](./2243-http-standardization.md).