fix: propagate child activity to parent during delegate_task (#7295)

When delegate_task runs, the parent agent's activity tracker freezes
because child.run_conversation() blocks and the child's own
_touch_activity() never propagates back to the parent. The gateway
inactivity timeout then fires a spurious 'No activity' warning and
eventually kills the agent, even though the subagent is actively working.

Fix: add a heartbeat thread in _run_single_child that calls
parent._touch_activity() every 30 seconds with detail from the child's
activity summary (current tool, iteration count). The thread is a daemon
that starts before child.run_conversation() and is cleaned up in the
finally block.

This also improves the gateway 'Still working...' status messages —
instead of just 'running: delegate_task', users now see what the
subagent is actually doing (e.g., 'delegate_task: subagent running
terminal (iteration 5/50)').

tests/tools/test_delegate.py

@@ -13,6 +13,7 @@ import json
 import os
 import sys
 import threading
+import time
 import unittest
 from unittest.mock import MagicMock, patch
 
@@ -1052,5 +1053,159 @@ class TestChildCredentialLeasing(unittest.TestCase):
         child._credential_pool.release_lease.assert_called_once_with("cred-a")
 
 
+class TestDelegateHeartbeat(unittest.TestCase):
+    """Heartbeat propagates child activity to parent during delegation.
+
+    Without the heartbeat, the gateway inactivity timeout fires because the
+    parent's _last_activity_ts freezes when delegate_task starts.
+    """
+
+    def test_heartbeat_touches_parent_activity_during_child_run(self):
+        """Parent's _touch_activity is called while child.run_conversation blocks."""
+        from tools.delegate_tool import _run_single_child
+
+        parent = _make_mock_parent()
+        touch_calls = []
+        parent._touch_activity = lambda desc: touch_calls.append(desc)
+
+        child = MagicMock()
+        child.get_activity_summary.return_value = {
+            "current_tool": "terminal",
+            "api_call_count": 3,
+            "max_iterations": 50,
+            "last_activity_desc": "executing tool: terminal",
+        }
+
+        # Make run_conversation block long enough for heartbeats to fire
+        def slow_run(**kwargs):
+            time.sleep(0.25)
+            return {"final_response": "done", "completed": True, "api_calls": 3}
+
+        child.run_conversation.side_effect = slow_run
+
+        # Patch the heartbeat interval to fire quickly
+        with patch("tools.delegate_tool._HEARTBEAT_INTERVAL", 0.05):
+            _run_single_child(
+                task_index=0,
+                goal="Test heartbeat",
+                child=child,
+                parent_agent=parent,
+            )
+
+        # Heartbeat should have fired at least once during the 0.25s sleep
+        self.assertGreater(len(touch_calls), 0,
+                           "Heartbeat did not propagate activity to parent")
+        # Verify the description includes child's current tool detail
+        self.assertTrue(
+            any("terminal" in desc for desc in touch_calls),
+            f"Heartbeat descriptions should include child tool info: {touch_calls}")
+
+    def test_heartbeat_stops_after_child_completes(self):
+        """Heartbeat thread is cleaned up when the child finishes."""
+        from tools.delegate_tool import _run_single_child
+
+        parent = _make_mock_parent()
+        touch_calls = []
+        parent._touch_activity = lambda desc: touch_calls.append(desc)
+
+        child = MagicMock()
+        child.get_activity_summary.return_value = {
+            "current_tool": None,
+            "api_call_count": 1,
+            "max_iterations": 50,
+            "last_activity_desc": "done",
+        }
+        child.run_conversation.return_value = {
+            "final_response": "done", "completed": True, "api_calls": 1,
+        }
+
+        with patch("tools.delegate_tool._HEARTBEAT_INTERVAL", 0.05):
+            _run_single_child(
+                task_index=0,
+                goal="Test cleanup",
+                child=child,
+                parent_agent=parent,
+            )
+
+        # Record count after completion, wait, and verify no more calls
+        count_after = len(touch_calls)
+        time.sleep(0.15)
+        self.assertEqual(len(touch_calls), count_after,
+                         "Heartbeat continued firing after child completed")
+
+    def test_heartbeat_stops_after_child_error(self):
+        """Heartbeat thread is cleaned up even when the child raises."""
+        from tools.delegate_tool import _run_single_child
+
+        parent = _make_mock_parent()
+        touch_calls = []
+        parent._touch_activity = lambda desc: touch_calls.append(desc)
+
+        child = MagicMock()
+        child.get_activity_summary.return_value = {
+            "current_tool": "web_search",
+            "api_call_count": 2,
+            "max_iterations": 50,
+            "last_activity_desc": "executing tool: web_search",
+        }
+
+        def slow_fail(**kwargs):
+            time.sleep(0.15)
+            raise RuntimeError("network timeout")
+
+        child.run_conversation.side_effect = slow_fail
+
+        with patch("tools.delegate_tool._HEARTBEAT_INTERVAL", 0.05):
+            result = _run_single_child(
+                task_index=0,
+                goal="Test error cleanup",
+                child=child,
+                parent_agent=parent,
+            )
+
+        self.assertEqual(result["status"], "error")
+
+        # Verify heartbeat stopped
+        count_after = len(touch_calls)
+        time.sleep(0.15)
+        self.assertEqual(len(touch_calls), count_after,
+                         "Heartbeat continued firing after child error")
+
+    def test_heartbeat_includes_child_activity_desc_when_no_tool(self):
+        """When child has no current_tool, heartbeat uses last_activity_desc."""
+        from tools.delegate_tool import _run_single_child
+
+        parent = _make_mock_parent()
+        touch_calls = []
+        parent._touch_activity = lambda desc: touch_calls.append(desc)
+
+        child = MagicMock()
+        child.get_activity_summary.return_value = {
+            "current_tool": None,
+            "api_call_count": 5,
+            "max_iterations": 90,
+            "last_activity_desc": "API call #5 completed",
+        }
+
+        def slow_run(**kwargs):
+            time.sleep(0.15)
+            return {"final_response": "done", "completed": True, "api_calls": 5}
+
+        child.run_conversation.side_effect = slow_run
+
+        with patch("tools.delegate_tool._HEARTBEAT_INTERVAL", 0.05):
+            _run_single_child(
+                task_index=0,
+                goal="Test desc fallback",
+                child=child,
+                parent_agent=parent,
+            )
+
+        self.assertGreater(len(touch_calls), 0)
+        self.assertTrue(
+            any("API call #5 completed" in desc for desc in touch_calls),
+            f"Heartbeat should include last_activity_desc: {touch_calls}")
+
+
 if __name__ == "__main__":
     unittest.main()

tools/delegate_tool.py

@@ -20,6 +20,7 @@ import json
 import logging
 logger = logging.getLogger(__name__)
 import os
+import threading
 import time
 from concurrent.futures import ThreadPoolExecutor, as_completed
 from typing import Any, Dict, List, Optional
@@ -37,6 +38,7 @@ DELEGATE_BLOCKED_TOOLS = frozenset([
 MAX_CONCURRENT_CHILDREN = 3
 MAX_DEPTH = 2  # parent (0) -> child (1) -> grandchild rejected (2)
 DEFAULT_MAX_ITERATIONS = 50
+_HEARTBEAT_INTERVAL = 30  # seconds between parent activity heartbeats during delegation
 DEFAULT_TOOLSETS = ["terminal", "file", "web"]
 
 
@@ -369,6 +371,44 @@ def _run_single_child(
             except Exception as exc:
                 logger.debug("Failed to bind child to leased credential: %s", exc)
 
+    # Heartbeat: periodically propagate child activity to the parent so the
+    # gateway inactivity timeout doesn't fire while the subagent is working.
+    # Without this, the parent's _last_activity_ts freezes when delegate_task
+    # starts and the gateway eventually kills the agent for "no activity".
+    _heartbeat_stop = threading.Event()
+
+    def _heartbeat_loop():
+        while not _heartbeat_stop.wait(_HEARTBEAT_INTERVAL):
+            if parent_agent is None:
+                continue
+            touch = getattr(parent_agent, '_touch_activity', None)
+            if not touch:
+                continue
+            # Pull detail from the child's own activity tracker
+            desc = f"delegate_task: subagent {task_index} working"
+            try:
+                child_summary = child.get_activity_summary()
+                child_tool = child_summary.get("current_tool")
+                child_iter = child_summary.get("api_call_count", 0)
+                child_max = child_summary.get("max_iterations", 0)
+                if child_tool:
+                    desc = (f"delegate_task: subagent running {child_tool} "
+                            f"(iteration {child_iter}/{child_max})")
+                else:
+                    child_desc = child_summary.get("last_activity_desc", "")
+                    if child_desc:
+                        desc = (f"delegate_task: subagent {child_desc} "
+                                f"(iteration {child_iter}/{child_max})")
+            except Exception:
+                pass
+            try:
+                touch(desc)
+            except Exception:
+                pass
+
+    _heartbeat_thread = threading.Thread(target=_heartbeat_loop, daemon=True)
+    _heartbeat_thread.start()
+
     try:
         result = child.run_conversation(user_message=goal)
 
@@ -479,6 +519,11 @@ def _run_single_child(
         }
 
     finally:
+        # Stop the heartbeat thread so it doesn't keep touching parent activity
+        # after the child has finished (or failed).
+        _heartbeat_stop.set()
+        _heartbeat_thread.join(timeout=5)
+
         if child_pool is not None and leased_cred_id is not None:
             try:
                 child_pool.release_lease(leased_cred_id)