diff --git a/hermes_cli/stderr_timestamp.py b/hermes_cli/stderr_timestamp.py
index 83e8a20ac5..c9250e6214 100644
--- a/hermes_cli/stderr_timestamp.py
+++ b/hermes_cli/stderr_timestamp.py
@@ -3,14 +3,18 @@
 from __future__ import annotations
 
 import argparse
+import os
 import re
 import signal
 import subprocess
 import sys
+from collections.abc import Mapping
 from datetime import datetime
 from pathlib import Path
 from typing import BinaryIO, Sequence, TextIO
 
+from gateway.restart import EXTERNAL_GATEWAY_SUPERVISOR_ENV
+
 
 _TIMESTAMP_PREFIX = re.compile(
     r"^\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2},\d{3}(?:\s|$)"
@@ -66,6 +70,28 @@ def _restore_signal_handlers(previous: dict[int, object]) -> None:
         signal.signal(signum, handler)
 
 
+def _child_env_for_command(
+    environ: Mapping[str, str] | None = None,
+) -> dict[str, str] | None:
+    """Preserve launchd supervision across this one-hop wrapper.
+
+    launchd stamps ``XPC_SERVICE_NAME=<job label>`` only on its *direct*
+    child. This module is that child when the generated plist wraps
+    ``gateway run`` for timestamped stderr. The grandchild then sees
+    ``XPC_SERVICE_NAME=0`` and ``_guard_supervised_gateway_conflict``
+    treats the service's own spawn as a foreign supervised gateway
+    (#86893). Forward the existing opt-in marker so the grandchild
+    still recognizes itself as the supervised process.
+    """
+    env = os.environ if environ is None else environ
+    xpc_service = str(env.get("XPC_SERVICE_NAME", "")).strip()
+    if not xpc_service or xpc_service == "0":
+        return None
+    child_env = dict(env)
+    child_env[EXTERNAL_GATEWAY_SUPERVISOR_ENV] = "1"
+    return child_env
+
+
 def _parse_args(argv: Sequence[str] | None) -> argparse.Namespace:
     parser = argparse.ArgumentParser(
         description="Run a command and timestamp each stderr line into a log file."
@@ -85,7 +111,11 @@ def main(argv: Sequence[str] | None = None) -> int:
     log_path: Path = args.error_log
 
     try:
-        proc = subprocess.Popen(args.command, stderr=subprocess.PIPE)
+        proc = subprocess.Popen(
+            args.command,
+            stderr=subprocess.PIPE,
+            env=_child_env_for_command(),
+        )
     except OSError as exc:
         log_path.parent.mkdir(parents=True, exist_ok=True)
         with log_path.open("a", encoding="utf-8", buffering=1) as log_file:
diff --git a/tests/hermes_cli/test_stderr_timestamp.py b/tests/hermes_cli/test_stderr_timestamp.py
index 6bda125b8f..c73a871df9 100644
--- a/tests/hermes_cli/test_stderr_timestamp.py
+++ b/tests/hermes_cli/test_stderr_timestamp.py
@@ -3,6 +3,7 @@
 import re
 import sys
 
+from gateway.restart import EXTERNAL_GATEWAY_SUPERVISOR_ENV, is_gateway_supervisor_process
 from hermes_cli import stderr_timestamp
 
 
@@ -34,3 +35,85 @@ def test_main_timestamps_each_stderr_line(tmp_path):
     assert re.fullmatch(f"{timestamp} first failure", lines[0])
     assert re.fullmatch(f"{timestamp} second failure without newline", lines[1])
     assert lines[2] == "2026-07-15 12:34:56,789 already timestamped"
+
+
+def test_child_env_forwards_supervisor_marker_under_launchd():
+    """launchd's XPC label on the wrapper must survive into the grandchild."""
+    child_env = stderr_timestamp._child_env_for_command(
+        {
+            "PATH": "/usr/bin",
+            "XPC_SERVICE_NAME": "ai.hermes.gateway-butler",
+        }
+    )
+    assert child_env is not None
+    assert child_env["PATH"] == "/usr/bin"
+    assert child_env[EXTERNAL_GATEWAY_SUPERVISOR_ENV] == "1"
+    assert is_gateway_supervisor_process(child_env) is True
+
+
+def test_child_env_skips_interactive_xpc_zero():
+    """Interactive macOS shells inherit XPC_SERVICE_NAME=0 — do not mark them."""
+    assert (
+        stderr_timestamp._child_env_for_command(
+            {"PATH": "/usr/bin", "XPC_SERVICE_NAME": "0"}
+        )
+        is None
+    )
+    assert stderr_timestamp._child_env_for_command({"PATH": "/usr/bin"}) is None
+    assert is_gateway_supervisor_process({"XPC_SERVICE_NAME": "0"}) is False
+
+
+def test_main_forwards_supervisor_marker_to_child(tmp_path, monkeypatch):
+    """The wrapper hop must set HERMES_GATEWAY_EXTERNAL_SUPERVISOR in the child."""
+    monkeypatch.setenv("XPC_SERVICE_NAME", "ai.hermes.gateway-butler")
+    monkeypatch.delenv(EXTERNAL_GATEWAY_SUPERVISOR_ENV, raising=False)
+    log_path = tmp_path / "gateway.error.log"
+    marker_path = tmp_path / "marker.txt"
+    code = (
+        "import os\n"
+        f"from pathlib import Path\n"
+        f"Path({str(marker_path)!r}).write_text("
+        f"os.environ.get({EXTERNAL_GATEWAY_SUPERVISOR_ENV!r}, ''), encoding='utf-8')\n"
+    )
+
+    rc = stderr_timestamp.main(
+        [
+            "--error-log",
+            str(log_path),
+            "--",
+            sys.executable,
+            "-c",
+            code,
+        ]
+    )
+
+    assert rc == 0
+    assert marker_path.read_text(encoding="utf-8") == "1"
+
+
+def test_main_does_not_mark_unsupervised_child(tmp_path, monkeypatch):
+    """Foreground/unsupervised starts must not inherit a fabricated marker."""
+    monkeypatch.setenv("XPC_SERVICE_NAME", "0")
+    monkeypatch.delenv(EXTERNAL_GATEWAY_SUPERVISOR_ENV, raising=False)
+    log_path = tmp_path / "gateway.error.log"
+    marker_path = tmp_path / "marker.txt"
+    code = (
+        "import os\n"
+        f"from pathlib import Path\n"
+        f"Path({str(marker_path)!r}).write_text("
+        f"os.environ.get({EXTERNAL_GATEWAY_SUPERVISOR_ENV!r}, 'unset'), encoding='utf-8')\n"
+    )
+
+    rc = stderr_timestamp.main(
+        [
+            "--error-log",
+            str(log_path),
+            "--",
+            sys.executable,
+            "-c",
+            code,
+        ]
+    )
+
+    assert rc == 0
+    assert marker_path.read_text(encoding="utf-8") == "unset"