fix(delegation): report failed children as failed, not completed

A subagent whose loop gave up on a structured failure (e.g. "API call
failed after 3 retries: HTTP 524") returns that error message as
final_response together with completed=False / failed=True /
failure_reason. _run_single_child derived the batch-entry status from
the summary alone (`elif summary and not _empty_sentinel: status =
"completed"`), so the non-empty error text made the batch report show
the task as "✓ status=completed" — the `failed` flag was never
consulted anywhere in delegate_tool.py. Only the "(empty)" sentinel was
mapped to failed.

Fix, at the single status-determination choke point both the single-task
and batch paths share:

- `failed=True` on the child result now wins over a non-empty summary:
  status = "failed".
- The child's classified failure_reason (rate_limit / billing /
  server_error / ...) is propagated onto the batch entry so the parent
  can tell a quota wall from a real task error without parsing prose.
- exit_reason for a structured failure is "error" instead of falling
  through to "max_iterations" (which also wrongly set truncated=True).

Successful children (completed=True, no failed flag) are untouched —
covered by an explicit control test alongside the regression test,
which is red on the old code and green with the fix.

tests/tools/test_delegate.py

@@ -675,6 +675,75 @@ class TestDelegateObservability(unittest.TestCase):
             result = json.loads(delegate_task(goal="Test empty sentinel", parent_agent=parent))
             self.assertEqual(result["results"][0]["status"], "failed")
 
+    def test_failed_child_with_error_summary_marks_status_failed(self):
+        """Regression: a child whose loop gave up on a structured failure
+        (``failed=True``, ``completed=False``, e.g. "API call failed after 3
+        retries: HTTP 524") returns that error message as final_response.
+        Status was derived from summary alone, so the non-empty error text
+        made the batch report show the task as ✓ status=completed. The
+        ``failed`` flag must win over a non-empty summary."""
+        parent = _make_mock_parent(depth=0)
+
+        with patch("run_agent.AIAgent") as MockAgent:
+            mock_child = MagicMock()
+            mock_child.model = "claude-sonnet-4-6"
+            mock_child.session_prompt_tokens = 0
+            mock_child.session_completion_tokens = 0
+            mock_child.run_conversation.return_value = {
+                "final_response": (
+                    "API call failed after 3 retries: HTTP 524 — origin timeout"
+                ),
+                "completed": False,
+                "failed": True,
+                "error": "HTTP 524 — origin timeout",
+                "failure_reason": "server_error",
+                "interrupted": False,
+                "api_calls": 3,
+                "messages": [],
+            }
+            MockAgent.return_value = mock_child
+
+            result = json.loads(
+                delegate_task(goal="Test failed child", parent_agent=parent)
+            )
+            entry = result["results"][0]
+            self.assertEqual(entry["status"], "failed")
+            # The classified reason must survive into the batch entry so the
+            # parent can tell a quota wall from a real task error.
+            self.assertEqual(entry["failure_reason"], "server_error")
+            self.assertEqual(entry["error"], "HTTP 524 — origin timeout")
+            # A structured failure is not budget truncation.
+            self.assertEqual(entry["exit_reason"], "error")
+            self.assertFalse(entry["truncated"])
+
+    def test_successful_child_still_completed(self):
+        """Control for the failed-flag check: a child that succeeds
+        (``completed=True``, no ``failed`` flag) must keep reporting
+        status=completed — the fix must not change success behavior."""
+        parent = _make_mock_parent(depth=0)
+
+        with patch("run_agent.AIAgent") as MockAgent:
+            mock_child = MagicMock()
+            mock_child.model = "claude-sonnet-4-6"
+            mock_child.session_prompt_tokens = 0
+            mock_child.session_completion_tokens = 0
+            mock_child.run_conversation.return_value = {
+                "final_response": "All done.",
+                "completed": True,
+                "interrupted": False,
+                "api_calls": 2,
+                "messages": [],
+            }
+            MockAgent.return_value = mock_child
+
+            result = json.loads(
+                delegate_task(goal="Test success control", parent_agent=parent)
+            )
+            entry = result["results"][0]
+            self.assertEqual(entry["status"], "completed")
+            self.assertEqual(entry["exit_reason"], "completed")
+            self.assertNotIn("failure_reason", entry)
+
 
 class TestDelegateFailedChildStatus(unittest.TestCase):
     """Honest status / exit_reason for failed subagents (issue #97655).

tools/delegate_tool.py

@@ -3317,6 +3317,12 @@ def _run_single_child(
         )
         if status == "failed":
             entry["error"] = result.get("error", "Subagent did not produce a response.")
+            # Classified reason from the child loop (e.g. "rate_limit",
+            # "billing", "server_error") — lets the parent distinguish a
+            # quota wall from a real task error without parsing prose.
+            _failure_reason = result.get("failure_reason")
+            if isinstance(_failure_reason, str) and _failure_reason:
+                entry["failure_reason"] = _failure_reason
 
         # T1-24: schema-validation outcome — emitted ONLY when a schema was
         # requested, so legacy (schema-less) payloads keep their exact shape.