fix: preserve parent agent's tool list after subagent delegation (#1778)

Save and restore the process-global _last_resolved_tool_names in
_run_single_child() so the parent's execute_code sandbox generates
correct tool imports after delegation completes.

The global was already mostly mitigated (run_agent.py passes
enabled_tools via self.valid_tool_names), but the global itself
remained corrupted — a footgun for any code that reads it directly.

Co-authored-by: shane9coy <shane9coy@users.noreply.github.com>

AGENTS.md

@@ -364,7 +364,7 @@ Rendering bugs in tmux/iTerm2 — ghosting on scroll. Use `curses` (stdlib) inst
 Leaks as literal `?[K` text under `prompt_toolkit`'s `patch_stdout`. Use space-padding: `f"\r{line}{' ' * pad}"`.
 
 ### `_last_resolved_tool_names` is a process-global in `model_tools.py`
-When subagents overwrite this global, `execute_code` calls after delegation may fail with missing tool imports. Known bug.
+`_run_single_child()` in `delegate_tool.py` saves and restores this global around subagent execution. If you add new code that reads this global, be aware it may be temporarily stale during child agent runs.
 
 ### Tests must not write to `~/.hermes/`
 The `_isolate_hermes_home` autouse fixture in `tests/conftest.py` redirects `HERMES_HOME` to a temp dir. Never hardcode `~/.hermes/` paths in tests.

tests/tools/test_delegate.py

@@ -249,6 +249,49 @@ class TestDelegateTask(unittest.TestCase):
             self.assertEqual(kwargs["api_mode"], parent.api_mode)
 
 
+class TestToolNamePreservation(unittest.TestCase):
+    """Verify _last_resolved_tool_names is restored after subagent runs."""
+
+    def test_global_tool_names_restored_after_delegation(self):
+        """The process-global _last_resolved_tool_names must be restored
+        after a subagent completes so the parent's execute_code sandbox
+        generates correct imports."""
+        import model_tools
+
+        parent = _make_mock_parent(depth=0)
+        original_tools = ["terminal", "read_file", "web_search", "execute_code", "delegate_task"]
+        model_tools._last_resolved_tool_names = list(original_tools)
+
+        with patch("run_agent.AIAgent") as MockAgent:
+            mock_child = MagicMock()
+            mock_child.run_conversation.return_value = {
+                "final_response": "done", "completed": True, "api_calls": 1,
+            }
+            MockAgent.return_value = mock_child
+
+            delegate_task(goal="Test tool preservation", parent_agent=parent)
+
+        self.assertEqual(model_tools._last_resolved_tool_names, original_tools)
+
+    def test_global_tool_names_restored_after_child_failure(self):
+        """Even when the child agent raises, the global must be restored."""
+        import model_tools
+
+        parent = _make_mock_parent(depth=0)
+        original_tools = ["terminal", "read_file", "web_search"]
+        model_tools._last_resolved_tool_names = list(original_tools)
+
+        with patch("run_agent.AIAgent") as MockAgent:
+            mock_child = MagicMock()
+            mock_child.run_conversation.side_effect = RuntimeError("boom")
+            MockAgent.return_value = mock_child
+
+            result = json.loads(delegate_task(goal="Crash test", parent_agent=parent))
+            self.assertEqual(result["results"][0]["status"], "error")
+
+        self.assertEqual(model_tools._last_resolved_tool_names, original_tools)
+
+
 class TestDelegateObservability(unittest.TestCase):
     """Tests for enriched metadata returned by _run_single_child."""
 

tools/delegate_tool.py

@@ -171,6 +171,11 @@ def _build_child_agent(
     model on OpenRouter while the parent runs on Nous Portal).
     """
     from run_agent import AIAgent
+    import model_tools
+
+    # Save the parent's resolved tool names before the child agent can
+    # overwrite the process-global via get_tool_definitions().
+    _saved_tool_names = list(model_tools._last_resolved_tool_names)
 
     # When no explicit toolsets given, inherit from parent's enabled toolsets
     # so disabled tools (e.g. web) don't leak to subagents.
@@ -365,6 +370,10 @@ def _run_single_child(
         }
 
     finally:
+        # Restore the parent's tool names so the process-global is correct
+        # for any subsequent execute_code calls or other consumers.
+        model_tools._last_resolved_tool_names = _saved_tool_names
+
         # Unregister child from interrupt propagation
         if hasattr(parent_agent, '_active_children'):
             try: