diff --git a/tests/tools/test_delegate.py b/tests/tools/test_delegate.py
index b37bb35d9..b9c0eeb6e 100644
--- a/tests/tools/test_delegate.py
+++ b/tests/tools/test_delegate.py
@@ -554,6 +554,37 @@ class TestDelegateObservability(unittest.TestCase):
             self.assertIn("result_bytes", entry["tool_trace"][0])
             self.assertEqual(entry["tool_trace"][0]["status"], "ok")
 
+    def test_tool_trace_handles_list_content_blocks(self):
+        """Tool-result content blocks should not crash observability metadata."""
+        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": "done",
+                "completed": True,
+                "interrupted": False,
+                "api_calls": 1,
+                "messages": [
+                    {"role": "assistant", "tool_calls": [
+                        {"id": "tc_1", "function": {"name": "image_generate", "arguments": '{"prompt": "x"}'}}
+                    ]},
+                    {"role": "tool", "tool_call_id": "tc_1", "content": [
+                        {"type": "text", "text": '{"success": true}'},
+                    ]},
+                ],
+            }
+            MockAgent.return_value = mock_child
+
+            result = json.loads(delegate_task(goal="Test list content", parent_agent=parent))
+            trace = result["results"][0]["tool_trace"]
+            self.assertEqual(trace[0]["tool"], "image_generate")
+            self.assertEqual(trace[0]["status"], "ok")
+            self.assertGreater(trace[0]["result_bytes"], 0)
+
     def test_tool_trace_detects_error(self):
         """Tool results containing 'error' should be marked as error status."""
         parent = _make_mock_parent(depth=0)
diff --git a/tools/delegate_tool.py b/tools/delegate_tool.py
index 40e19a0e4..57998dcd0 100644
--- a/tools/delegate_tool.py
+++ b/tools/delegate_tool.py
@@ -276,7 +276,35 @@ def _extract_output_tail(
     return tail
 
 
-def _looks_like_error_output(content: str) -> bool:
+def _stringify_tool_content(content: Any) -> str:
+    """Return a stable text representation for tool-result content.
+
+    Most providers store tool results as strings, but some OpenAI-compatible
+    paths can return content-block lists. Delegate observability must never
+    crash while summarising a child run just because the transport used blocks.
+    """
+    if content is None:
+        return ""
+    if isinstance(content, str):
+        return content
+    if isinstance(content, list):
+        parts = []
+        for item in content:
+            if isinstance(item, dict):
+                text = item.get("text")
+                if isinstance(text, str):
+                    parts.append(text)
+                else:
+                    parts.append(json.dumps(item, ensure_ascii=False, default=str))
+            else:
+                parts.append(str(item))
+        return "\n".join(parts)
+    if isinstance(content, dict):
+        return json.dumps(content, ensure_ascii=False, default=str)
+    return str(content)
+
+
+def _looks_like_error_output(content: Any) -> bool:
     """Conservative stderr/error detector for tool-result previews.
 
     The old heuristic flagged any preview containing the substring "error",
@@ -286,6 +314,7 @@ def _looks_like_error_output(content: str) -> bool:
       - structured JSON with ``status`` of error/failed
       - first line starts with a classic error marker
     """
+    content = _stringify_tool_content(content)
     if not content:
         return False
 
@@ -1672,7 +1701,7 @@ def _run_single_child(
                         if tc_id:
                             trace_by_id[tc_id] = entry_t
                 elif msg.get("role") == "tool":
-                    content = msg.get("content", "")
+                    content = _stringify_tool_content(msg.get("content", ""))
                     is_error = _looks_like_error_output(content)
                     result_meta = {
                         "result_bytes": len(content),