diff --git a/hermes_cli/process_identity.py b/hermes_cli/process_identity.py
index 731ba061a6..d2ad83aad6 100644
--- a/hermes_cli/process_identity.py
+++ b/hermes_cli/process_identity.py
@@ -267,7 +267,9 @@ def _append_entry(entry: LedgerEntry) -> bool:
         pruned.append(asdict(entry))
         try:
             path.parent.mkdir(parents=True, exist_ok=True)
-            atomic_json_write(path, pruned, mode=0o600)
+            # argv may carry surrogate-escaped bytes (non-UTF-8 paths); ensure_ascii keeps the
+            # utf-8 text handle from raising UnicodeEncodeError (a ValueError, not an OSError).
+            atomic_json_write(path, pruned, mode=0o600, ensure_ascii=True)
             return True
         except OSError:
             logger.debug("spawn ledger write failed", exc_info=True)
diff --git a/tests/hermes_cli/test_process_identity.py b/tests/hermes_cli/test_process_identity.py
index 5f9a85dfbe..dc627ab9ee 100644
--- a/tests/hermes_cli/test_process_identity.py
+++ b/tests/hermes_cli/test_process_identity.py
@@ -134,6 +134,19 @@ def test_register_self_writes_and_prunes_dead(tmp_path):
     assert me["create_time"] == pytest.approx(50.0, abs=0.01)
 
 
+def test_register_self_survives_non_utf8_argv(tmp_path):
+    ledger = tmp_path / "spawn-ledger.json"
+    fake = _fake_psutil({999: 50.0})
+    bad_argv = ["hermes", "serve", os.fsdecode(b"/tmp/project-\xff")]  # surrogate-escaped path
+    with patch.dict(sys.modules, {"psutil": fake}), \
+         patch.object(pi, "_ledger_path", return_value=ledger), \
+         patch.object(pi.os, "getpid", return_value=999), \
+         patch.object(sys, "argv", bad_argv):
+        assert pi.register_self("serve", project_root=Path("/x/install")) is True
+    me = next(e for e in json.loads(ledger.read_text(encoding="utf-8")) if e["pid"] == 999)
+    assert me["argv"] == " ".join(bad_argv)
+
+
 @pytest.mark.skipif(os.name == "nt", reason="POSIX mode bits are platform-specific")
 def test_register_self_writes_ledger_with_0600(tmp_path):
     ledger = tmp_path / "spawn-ledger.json"
diff --git a/utils.py b/utils.py
index ed9e35f37a..bbc2da7d81 100644
--- a/utils.py
+++ b/utils.py
@@ -217,10 +217,17 @@ def atomic_write_text(path: Union[str, Path], content: str, *, encoding: str = "
                   mode=_mode_for_write(path, create_mode, preserve=preserve_mode), preserve_owner=preserve_mode)
 
 
-def atomic_json_write(path: Union[str, Path], data: Any, *, indent: int = 2, mode: int | None = None, **dump_kwargs: Any) -> None:
-    """Write JSON to *path* atomically (temp file + fsync + replace)."""
+def atomic_json_write(
+    path: Union[str, Path], data: Any, *, indent: int = 2, mode: int | None = None,
+    ensure_ascii: bool = False, **dump_kwargs: Any,
+) -> None:
+    """Write JSON to *path* atomically (temp file + fsync + replace).
+
+    ``ensure_ascii=True`` lets callers persist surrogate-escaped strings (non-UTF-8 argv/paths)
+    that a utf-8 text handle would otherwise reject with ``UnicodeEncodeError``.
+    """
     path = Path(path)
-    _atomic_write(path, lambda f: json.dump(data, f, indent=indent, ensure_ascii=False, **dump_kwargs),
+    _atomic_write(path, lambda f: json.dump(data, f, indent=indent, ensure_ascii=ensure_ascii, **dump_kwargs),
                   prefix=f".{path.stem}_", mode=mode if mode is not None else _preserve_file_mode(path))