diff --git a/hermes_cli/config.py b/hermes_cli/config.py
index c67dd2d167e5..04e4887e645f 100644
--- a/hermes_cli/config.py
+++ b/hermes_cli/config.py
@@ -8645,6 +8645,20 @@ def _validate_config_key(key: str) -> tuple[bool, Optional[str]]:
 
     segments = key.split(".")
     top = segments[0]
+
+    # ── Underscore-prefixed keys are internal/test markers ───────────
+    # A leading underscore on the top-level segment (e.g. ``_test.shim_marker``)
+    # signals an intentionally non-schema, internal key. Test harnesses and
+    # tooling use these to write a deterministic marker into config.yaml
+    # without polluting the user-facing schema (see the Docker privilege-drop
+    # shim test, which writes ``_test.shim_marker`` to probe file ownership).
+    # Python's own convention treats a leading underscore as "private"; we
+    # honour that here so schema validation never blocks deliberately-internal
+    # keys. This is narrow: only the FIRST segment is checked, so a real typo
+    # like ``agent._max_turns`` still gets caught at the sub-key level.
+    if top.startswith("_"):
+        return True, None
+
     known = _known_top_level_keys()
 
     # ── First-segment validation ─────────────────────────────────────
diff --git a/tests/hermes_cli/test_set_config_value.py b/tests/hermes_cli/test_set_config_value.py
index 9b8c3a90de71..86ba5aa3c921 100644
--- a/tests/hermes_cli/test_set_config_value.py
+++ b/tests/hermes_cli/test_set_config_value.py
@@ -587,3 +587,25 @@ class TestValidateConfigKey:
         if expected_in_suggestion is not None:
             assert suggestion is not None and expected_in_suggestion in suggestion, \
                 f"Expected suggestion to contain {expected_in_suggestion!r}, got {suggestion!r}"
+
+    @pytest.mark.parametrize("key", [
+        "_test.shim_marker",
+        "_internal",
+        "_test.nested.deep.marker",
+        "_x",
+    ])
+    def test_underscore_prefixed_keys_are_accepted(self, key):
+        """Underscore-prefixed top-level keys are internal/test markers and
+        bypass schema validation. The Docker privilege-drop shim test writes
+        ``_test.shim_marker`` to probe config.yaml ownership; that must not
+        be rejected. (Regression: #34250 schema validation broke this.)"""
+        from hermes_cli.config import _validate_config_key
+        is_known, _ = _validate_config_key(key)
+        assert is_known, f"Expected underscore-prefixed {key!r} to be accepted"
+
+    def test_underscore_only_first_segment_escapes(self):
+        """The underscore escape only applies to the FIRST segment. A real
+        typo in a sub-key (e.g. agent._max_turns) is still caught."""
+        from hermes_cli.config import _validate_config_key
+        is_known, suggestion = _validate_config_key("agent._max_turns")
+        assert not is_known, "Sub-key typo under a known top-level key must still be flagged"