diff --git a/hermes_cli/oneshot.py b/hermes_cli/oneshot.py
index f13fe64029d5c..8aa6c9662d4ce 100644
--- a/hermes_cli/oneshot.py
+++ b/hermes_cli/oneshot.py
@@ -277,6 +277,15 @@ def run_oneshot(
 
     _write_usage_file(usage_file, result)
 
+    # Model text can contain lone UTF-16 surrogates (invalid in UTF-8). Writing
+    # those to a real stdout TextIO raises UnicodeEncodeError and aborts with
+    # exit 1 after the turn already completed — scrub to U+FFFD first.
+    # See #80366.
+    if response:
+        from agent.message_sanitization import _sanitize_surrogates
+
+        response = _sanitize_surrogates(response)
+
     if response:
         real_stdout.write(response)
         if not response.endswith("\n"):
diff --git a/tests/hermes_cli/test_oneshot_surrogate.py b/tests/hermes_cli/test_oneshot_surrogate.py
new file mode 100644
index 0000000000000..9039690a84063
--- /dev/null
+++ b/tests/hermes_cli/test_oneshot_surrogate.py
@@ -0,0 +1,39 @@
+"""Oneshot stdout must survive lone UTF-16 surrogates in model text (#80366)."""
+
+from __future__ import annotations
+
+import subprocess
+import sys
+import textwrap
+from pathlib import Path
+
+
+def test_oneshot_replaces_lone_surrogate_and_exits_zero():
+    """hermes -z must print U+FFFD and exit 0 when the model returns U+D800."""
+    program = textwrap.dedent(
+        """
+        import hermes_cli.oneshot as oneshot
+
+        dirty = "answer \\ud800 here"
+        oneshot._run_agent = lambda *args, **kwargs: (
+            dirty,
+            {"final_response": dirty, "failed": False, "partial": False, "completed": True},
+        )
+        raise SystemExit(oneshot.run_oneshot("hello"))
+        """
+    )
+
+    result = subprocess.run(
+        [sys.executable, "-c", program],
+        cwd=Path(__file__).resolve().parents[2],
+        capture_output=True,
+        timeout=30,
+        check=False,
+    )
+
+    assert result.returncode == 0, result.stderr.decode("utf-8", errors="replace")
+    # U+FFFD as UTF-8; no raw surrogate bytes
+    assert "\ufffd".encode("utf-8") in result.stdout
+    assert b"answer " in result.stdout
+    assert b" here\n" in result.stdout
+    assert b"Traceback" not in result.stderr