fix(tasks): don't fake-clear MCP session with empty refresh on no-config transition

An empty mcpServers list is a notification-only no-op on the agent-server (it
returns without rebuilding the session), so sending it on an actor transition
that resolves no configs neither clears the previous actor's session nor
rebinds — yet it reported success and falsely marked the new actor. Stop
sending it and leave the binding on the previous actor, who still holds the
live session. Only reachable on deployments without a resolvable MCP URL.

Author: VojtechBartos

products/tasks/backend/temporal/process_task/activities/send_followup_to_sandbox.py

@@ -317,16 +317,24 @@ def _refresh_sandbox_mcp(
     if imported_mcp_configs:
         mcp_configs = mcp_configs + imported_mcp_configs
 
-    if not mcp_configs and not is_transition:
+    if not mcp_configs:
+        if is_transition:
+            # The new actor resolves no MCP configs, and an empty-list refresh is
+            # a notification-only no-op on the agent-server (see
+            # send_refresh_session) — it cannot tear down the previous actor's
+            # live session. So we neither send nor rebind: leave the binding on
+            # the previous actor, which accurately describes the live session,
+            # and re-attempt on their next message. Only reachable on deployments
+            # without a resolvable MCP URL (get_sandbox_ph_mcp_configs is
+            # otherwise never empty), so best-effort delivery is acceptable.
+            logger.info("refresh_mcp_no_configs_on_transition", run_id=run_id, previous_user_id=bound_user_id)
+            return True
         # First bind for this sandbox and the actor has no MCP configs: there is
         # no prior session to tear down, so just record the binding.
         mark_sandbox_mcp_session(scope, actor_user.id)
         logger.info("refresh_mcp_skipped_no_configs", run_id=run_id)
         return True
 
-    # An actor transition where the new actor resolves no configs still has to
-    # clear the previous actor's live session — an empty server list replaces it
-    # wholesale. Binding stays gated on a successful send below.
     mcp_servers = [config.to_dict() for config in mcp_configs]
 
     result = send_refresh_session(

products/tasks/backend/temporal/process_task/tests/test_send_followup_to_sandbox.py

@@ -308,24 +308,25 @@ def test_replacement_sandbox_starts_unmarked(
         assert get_sandbox_mcp_session_user("sb-2") == 42
         assert cache.get(_sandbox_mcp_session_cache_key("run-1")) == 42  # untouched
 
-    def test_transition_with_no_configs_still_clears_previous_session(
+    def test_transition_with_no_configs_leaves_previous_binding(
         self, mock_oauth, mock_ph_configs, mock_user_configs, mock_send_refresh
     ):
         # The prior actor holds the live session, but the new actor resolves no
-        # MCP configs. Rebinding without a refresh would leave the previous
-        # actor's session live under the new actor's binding, so we still send a
-        # refresh — an empty server list clears it wholesale.
+        # MCP configs. An empty-list refresh is a no-op on the agent-server, so
+        # we neither send it nor rebind: the binding stays on the previous actor
+        # (who still holds the live session) rather than falsely flipping to the
+        # new one.
         mock_oauth.return_value = "fresh-token"
         mock_ph_configs.return_value = []
         mock_user_configs.return_value = []
-        mock_send_refresh.return_value = CommandResult(success=True, status_code=200)
         mark_sandbox_mcp_session("run-1", 99)
 
-        _refresh(_make_task_run_mock(), actor_id=42)
+        actor = MagicMock(id=42)
+        safe = _refresh_sandbox_mcp(_make_task_run_mock(), "read_only", None, actor_user=actor, state=None)
 
-        mock_send_refresh.assert_called_once()
-        assert mock_send_refresh.call_args.args[1] == []
-        assert get_sandbox_mcp_session_user("run-1") == 42
+        assert safe is True  # best-effort: delivery proceeds
+        mock_send_refresh.assert_not_called()
+        assert get_sandbox_mcp_session_user("run-1") == 99  # binding unchanged
 
 
 class TestSendFollowupActivityRefreshOrdering: