diff --git a/tests/rest/client/sliding_sync/test_rooms_required_state.py b/tests/rest/client/sliding_sync/test_rooms_required_state.py
index 586b127f8a..a989598738 100644
--- a/tests/rest/client/sliding_sync/test_rooms_required_state.py
+++ b/tests/rest/client/sliding_sync/test_rooms_required_state.py
@@ -2245,3 +2245,69 @@ class SlidingSyncRoomsRequiredStateTestCase(SlidingSyncBase):
             response_body["rooms"][room_id]["required_state"][0]["event_id"],
             first_event_id,
         )
+
+    def test_changing_required_state_returns_immediately(self) -> None:
+        """Test that if we change the required state, then we return immediately
+        with the new required state."""
+
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        room_id1 = self.helper.create_room_as(user1_id, tok=user1_tok)
+
+        # Make an initial sync request with no required state
+        sync_body = {
+            "lists": {
+                "foo-list": {
+                    "ranges": [[0, 1]],
+                    "required_state": [],
+                    "timeline_limit": 0,
+                }
+            }
+        }
+        response_body, from_token = self.do_sync(sync_body, tok=user1_tok)
+
+        # We should see no required state
+        self.assertIsNone(response_body["rooms"][room_id1].get("required_state"))
+
+        # Get the state_map before we change the state as this is the final state we
+        # expect to see when we update the required state.
+        state_map = self.get_success(
+            self.storage_controllers.state.get_current_state(room_id1)
+        )
+
+        # There is no new data, and so making another sync request will block.
+        channel = self.make_sync_request(
+            sync_body,
+            since=from_token,
+            tok=user1_tok,
+            timeout_ms=10_000,
+            await_result=False,
+        )
+        self.reactor.advance(0.1)  # Allow the request to start processing
+        self.reactor.advance(9.5)
+        self.assertFalse(channel.is_finished())
+
+        # Advance past the timeout to make sure the request finishes. (We do this
+        # to ensure log contexts don't leak between tests).
+        self.reactor.advance(1)
+        self.assertTrue(channel.is_finished())
+
+        # Now update the sliding sync requests to include a required state
+        # event, and make another sync request.
+        sync_body["lists"]["foo-list"]["required_state"] = [
+            [EventTypes.Create, ""],
+        ]
+
+        response_body, _ = self.do_sync(
+            sync_body, since=from_token, tok=user1_tok, timeout_ms=10_000
+        )
+
+        # We should see the new required state immediately without waiting.
+        self._assertRequiredStateIncludes(
+            response_body["rooms"][room_id1]["required_state"],
+            {
+                state_map[(EventTypes.Create, "")],
+            },
+            exact=True,
+        )
diff --git a/tests/rest/client/sliding_sync/test_sliding_sync.py b/tests/rest/client/sliding_sync/test_sliding_sync.py
index ac8dfd37d8..2b258ecb06 100644
--- a/tests/rest/client/sliding_sync/test_sliding_sync.py
+++ b/tests/rest/client/sliding_sync/test_sliding_sync.py
@@ -12,6 +12,7 @@
 # <https://www.gnu.org/licenses/agpl-3.0.html>.
 #
 import logging
+import urllib.parse
 from typing import Any, Iterable, Literal
 from unittest.mock import AsyncMock
 
@@ -81,7 +82,13 @@ class SlidingSyncBase(unittest.HomeserverTestCase):
         return config
 
     def make_sync_request(
-        self, sync_body: JsonDict, *, since: str | None = None, tok: str
+        self,
+        sync_body: JsonDict,
+        *,
+        since: str | None = None,
+        tok: str,
+        timeout_ms: int | None = None,
+        await_result: bool = True,
     ) -> FakeChannel:
         """Make a sliding sync request with given body.
 
@@ -89,25 +96,40 @@ class SlidingSyncBase(unittest.HomeserverTestCase):
             sync_body: The full request body to use
             since: Optional since token
             tok: Access token to use
-
+            timeout_ms: Optional timeout in milliseconds to use for the request.
+            await_result: Whether to block and wait for the result before returning.
         Returns:
             A tuple of the response body and the `pos` field.
         """
 
         sync_path = self.sync_endpoint
+
+        query_params: dict[str, Any] = {}
         if since:
-            sync_path += f"?pos={since}"
+            query_params["pos"] = since
+        if timeout_ms is not None:
+            query_params["timeout"] = timeout_ms
+
+        if query_params:
+            query_str = urllib.parse.urlencode(query_params)
+            sync_path += f"?{query_str}"
 
         channel = self.make_request(
             method="POST",
             path=sync_path,
             content=sync_body,
             access_token=tok,
+            await_result=await_result,
         )
         return channel
 
     def do_sync(
-        self, sync_body: JsonDict, *, since: str | None = None, tok: str
+        self,
+        sync_body: JsonDict,
+        *,
+        since: str | None = None,
+        tok: str,
+        timeout_ms: int | None = None,
     ) -> tuple[JsonDict, str]:
         """Do a sliding sync request with given body.
 
@@ -117,11 +139,14 @@ class SlidingSyncBase(unittest.HomeserverTestCase):
             sync_body: The full request body to use
             since: Optional since token
             tok: Access token to use
+            timeout_ms: Optional timeout in milliseconds to use for the request.
 
         Returns:
             A tuple of the response body and the `pos` field.
         """
-        channel = self.make_sync_request(sync_body, since=since, tok=tok)
+        channel = self.make_sync_request(
+            sync_body, since=since, tok=tok, timeout_ms=timeout_ms
+        )
         self.assertEqual(channel.code, 200, channel.json_body)
 
         return channel.json_body, channel.json_body["pos"]