diff --git a/tests/tools/test_delegate_subagent_timeout_diagnostic.py b/tests/tools/test_delegate_subagent_timeout_diagnostic.py
index a94f0a095e..05d8269626 100644
--- a/tests/tools/test_delegate_subagent_timeout_diagnostic.py
+++ b/tests/tools/test_delegate_subagent_timeout_diagnostic.py
@@ -112,6 +112,26 @@ class TestDumpSubagentTimeoutDiagnostic:
         # We assert no exception propagates — the return value is advisory.
         assert result is None or Path(result).exists()
 
+    def test_timeout_diagnostic_marks_long_goal_as_non_original(self, hermes_home):
+        # #121572: an elided goal must carry the non-imitable counted marker,
+        # never the bare "...[truncated]" the model could copy.
+        from agent.compression_marker import _COMPRESSION_MARKER_RE
+        from tools.delegate_tool import _dump_subagent_timeout_diagnostic
+        child = _StubChild()
+        path = _dump_subagent_timeout_diagnostic(
+            child=child,
+            task_index=0,
+            timeout_seconds=300.0,
+            duration_seconds=300.0,
+            worker_thread=None,
+            goal="g" * 1200,
+        )
+        child.interrupt()
+        text = Path(path).read_text(encoding="utf-8-sig")
+        assert _COMPRESSION_MARKER_RE.search(text)
+        assert "g" * 100 in text
+        assert "...[truncated]" not in text
+
 
 # ── _run_single_child timeout branch wiring ───────────────────────────