diff --git a/agent/shell_hooks.py b/agent/shell_hooks.py
index 2bc8abc7a6..0b93ede6d5 100644
--- a/agent/shell_hooks.py
+++ b/agent/shell_hooks.py
@@ -13,7 +13,6 @@ import os
 import re
 import subprocess
 import sys
-import tempfile
 import threading
 import time
 from contextlib import ExitStack, contextmanager, suppress
@@ -31,7 +30,7 @@ except ImportError:  # pragma: no cover
     fcntl = None  # type: ignore[assignment]
 
 from hermes_constants import get_hermes_home
-from utils import atomic_replace
+from utils import atomic_json_write
 
 logger = logging.getLogger(__name__)
 
@@ -469,16 +468,7 @@ def save_allowlist(data: Dict[str, Any]) -> None:
     """Atomic write; on OSError log and keep the in-process approval."""
     p = allowlist_path()
     try:
-        p.parent.mkdir(parents=True, exist_ok=True)
-        fd, tmp_path = tempfile.mkstemp(prefix=f"{p.name}.", suffix=".tmp", dir=str(p.parent))
-        try:
-            with os.fdopen(fd, "w", encoding="utf-8") as fh:
-                fh.write(json.dumps(data, indent=2, sort_keys=True))
-            atomic_replace(tmp_path, p)
-        except Exception:
-            with suppress(OSError):
-                os.unlink(tmp_path)
-            raise
+        atomic_json_write(p, data, sort_keys=True)
     except OSError as exc:
         logger.warning("Failed to persist shell hook allowlist to %s: %s. The approval is in-memory for this run, "
                        "but the next startup will re-prompt (or skip registration on non-TTY runs without "
diff --git a/cron/suggestions.py b/cron/suggestions.py
index 15e1602832..57c82e86b8 100644
--- a/cron/suggestions.py
+++ b/cron/suggestions.py
@@ -12,8 +12,6 @@ from __future__ import annotations
 
 import json
 import logging
-import os
-import tempfile
 import threading
 import uuid
 from pathlib import Path
@@ -21,7 +19,7 @@ from typing import Any, Dict, List, Optional
 
 from hermes_constants import get_hermes_home
 from hermes_time import now as _hermes_now
-from utils import atomic_replace
+from utils import atomic_json_write
 
 logger = logging.getLogger(__name__)
 
@@ -48,13 +46,6 @@ def _current_suggestions_file() -> Path:
     return SUGGESTIONS_FILE or (get_hermes_home().resolve() / "cron" / "suggestions.json")
 
 
-def _secure_file(path: Path) -> None:
-    try:
-        os.chmod(path, 0o600)
-    except OSError:
-        pass
-
-
 def _ensure_dir() -> None:
     from cron.jobs import _ensure_cron_dir
 
@@ -81,22 +72,8 @@ def _load_raw() -> Dict[str, Any]:
 
 def _save_raw(suggestions: List[Dict[str, Any]]) -> None:
     _ensure_dir()
-    suggestions_file = _current_suggestions_file()
-    fd, tmp_path = tempfile.mkstemp(dir=str(suggestions_file.parent), suffix=".tmp", prefix=".sugg_")
-    try:
-        with os.fdopen(fd, "w", encoding="utf-8") as f:
-            payload = {"suggestions": suggestions, "updated_at": _hermes_now().isoformat()}
-            json.dump(payload, f, indent=2)
-            f.flush()
-            os.fsync(f.fileno())
-        atomic_replace(tmp_path, suggestions_file)
-        _secure_file(suggestions_file)
-    except BaseException:
-        try:
-            os.unlink(tmp_path)
-        except OSError:
-            pass
-        raise
+    payload = {"suggestions": suggestions, "updated_at": _hermes_now().isoformat()}
+    atomic_json_write(_current_suggestions_file(), payload, mode=0o600)
 
 
 def load_suggestions() -> List[Dict[str, Any]]:
diff --git a/gateway/hosted_room_peer.py b/gateway/hosted_room_peer.py
index fdc8c7d36a..39195a8d7b 100644
--- a/gateway/hosted_room_peer.py
+++ b/gateway/hosted_room_peer.py
@@ -48,7 +48,7 @@ _ROOM_GRANT_SECRET_FILE = ".room-link-grant-secret"
 @lru_cache(maxsize=32)
 def _gateway_room_grant_secret_for_home(home_value: str) -> bytes:
     """Load one restart-scoped grant secret for an exact installation root."""
-    from hermes_cli.install_identity import _fsync_directory
+    from utils import fsync_directory
     (home := Path(home_value)).mkdir(parents=True, exist_ok=True)
     path = home / _ROOM_GRANT_SECRET_FILE
     def _read() -> bytes:
@@ -75,7 +75,7 @@ def _gateway_room_grant_secret_for_home(home_value: str) -> bytes:
             except FileExistsError:
                 material = _read()
             else:
-                _fsync_directory(home)
+                fsync_directory(home)
         finally:
             temporary.unlink(missing_ok=True)
     return hmac.new(material, b"hermes-hosted-room-installation-grant-v1", hashlib.sha256).digest()
diff --git a/gateway/rich_sent_store.py b/gateway/rich_sent_store.py
index 237977edeb..145e755b6a 100644
--- a/gateway/rich_sent_store.py
+++ b/gateway/rich_sent_store.py
@@ -15,6 +15,7 @@ import json
 import os
 import time
 from typing import Optional
+from utils import atomic_json_write
 
 _MAX_ENTRIES = 1000
 _MAX_TEXT_CHARS = 2000
@@ -47,10 +48,7 @@ def _update(chat_id, message_id, fields: dict) -> None:
         if len(data) > _MAX_ENTRIES:  # trim oldest by timestamp
             for k, _ in sorted(data.items(), key=lambda kv: kv[1].get("ts", 0))[: len(data) - _MAX_ENTRIES]:
                 data.pop(k, None)
-        tmp = f"{path}.tmp.{os.getpid()}"
-        with open(tmp, "w", encoding="utf-8") as fh:
-            json.dump(data, fh, ensure_ascii=False)
-        os.replace(tmp, path)  # atomic; tolerates concurrent writers racing
+        atomic_json_write(path, data, indent=None)  # atomic; tolerates concurrent writers racing
     except Exception:
         return
 
diff --git a/gateway/session_persistence.py b/gateway/session_persistence.py
index 1f81acee1c..e881c735f1 100644
--- a/gateway/session_persistence.py
+++ b/gateway/session_persistence.py
@@ -7,12 +7,10 @@ from __future__ import annotations
 import contextlib
 import logging
 import json
-import os
-import tempfile
 import threading
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Dict, Optional
-from utils import atomic_replace
+from utils import atomic_json_write
 
 if TYPE_CHECKING:
     from gateway.session import SessionEntry
@@ -475,22 +473,7 @@ class SessionPersistenceMixin:
 
     def _save_sessions_json(self, data: Dict[str, Any]) -> None:
         """Write the legacy sessions.json mirror of the routing index (atomic + fsync)."""
-        self.sessions_dir.mkdir(parents=True, exist_ok=True)
-        sessions_file = self.sessions_dir / "sessions.json"
-        data = {"_README": _SESSIONS_JSON_README, **data}
-        fd, tmp_path = tempfile.mkstemp(dir=str(self.sessions_dir), suffix=".tmp", prefix=".sessions_")
-        try:
-            with os.fdopen(fd, "w", encoding="utf-8") as f:
-                json.dump(data, f, indent=2)
-                f.flush()
-                os.fsync(f.fileno())
-            atomic_replace(tmp_path, sessions_file)
-        except BaseException:
-            try:
-                os.unlink(tmp_path)
-            except OSError as e:
-                logger.debug("Could not remove temp file %s: %s", tmp_path, e)
-            raise
+        atomic_json_write(self.sessions_dir / "sessions.json", {"_README": _SESSIONS_JSON_README, **data})
 
     def _save_entries(self) -> None:
         """Snapshot latest state under ``_lock`` and persist after releasing it."""
diff --git a/hermes_cli/active_sessions.py b/hermes_cli/active_sessions.py
index 253b633fe8..f159d64d09 100644
--- a/hermes_cli/active_sessions.py
+++ b/hermes_cli/active_sessions.py
@@ -20,6 +20,7 @@ from pathlib import Path
 from typing import Any, Iterator, Optional
 
 from hermes_constants import get_default_hermes_root, get_hermes_home
+from utils import atomic_json_write
 
 logger = logging.getLogger(__name__)
 
@@ -287,17 +288,7 @@ def _valid_process_start(v: Any) -> bool:
 
 
 def _write_entries(path: Path, entries: list[dict[str, Any]]) -> None:
-    path.parent.mkdir(parents=True, exist_ok=True)
-    tmp = path.with_name(f"{path.name}.{os.getpid()}.{uuid.uuid4().hex}.tmp")
-    try:
-        with open(tmp, "w", encoding="utf-8") as fh:
-            json.dump({"entries": entries}, fh, sort_keys=True)
-        os.replace(tmp, path)
-    finally:
-        try:
-            tmp.unlink(missing_ok=True)
-        except OSError:
-            pass
+    atomic_json_write(path, {"entries": entries}, indent=None, sort_keys=True)
 
 
 def _process_start_time(pid: int) -> Optional[float]:
diff --git a/hermes_cli/banner.py b/hermes_cli/banner.py
index 350104c951..20ac5b2278 100644
--- a/hermes_cli/banner.py
+++ b/hermes_cli/banner.py
@@ -681,13 +681,8 @@ def save_banner_snapshot(tools: List[dict], enabled_toolsets: List[str], availab
     }
 
     def _write():
-        import tempfile
-        path = _banner_snapshot_path()
-        path.parent.mkdir(parents=True, exist_ok=True)
-        fd, tmp = tempfile.mkstemp(dir=str(path.parent), prefix=".banner_snap.")
-        with os.fdopen(fd, "w", encoding="utf-8") as fh:
-            json.dump(payload, fh)
-        os.replace(tmp, path)
+        from utils import atomic_json_write
+        atomic_json_write(_banner_snapshot_path(), payload, indent=None)
     _quiet(_write)
 
 
diff --git a/hermes_cli/codex_runtime_plugin_migration.py b/hermes_cli/codex_runtime_plugin_migration.py
index 0b379b043c..ebacb391d1 100644
--- a/hermes_cli/codex_runtime_plugin_migration.py
+++ b/hermes_cli/codex_runtime_plugin_migration.py
@@ -374,21 +374,9 @@ def _build_hermes_tools_mcp_entry() -> dict:
 
 
 def _write_atomic(target: Path, text: str) -> None:
-    """Write via a same-directory temp file + rename (atomic on POSIX, ReplaceFile on Windows) so a
-    crash mid-write never leaves a half-written config.toml that codex would refuse to load."""
-    import tempfile
-    tmp_fd, tmp_path_str = tempfile.mkstemp(prefix=".config.toml.", dir=str(target.parent))
-    tmp_path = Path(tmp_path_str)
-    try:
-        with os.fdopen(tmp_fd, "w", encoding="utf-8") as fh:
-            fh.write(text)
-        tmp_path.replace(target)
-    except Exception:
-        try:
-            tmp_path.unlink(missing_ok=True)
-        except Exception:
-            pass
-        raise
+    """Atomic rewrite so a crash mid-write never leaves a half-written config.toml codex refuses to load."""
+    from utils import atomic_write_text
+    atomic_write_text(target, text, tmp_prefix=".config.toml.", preserve_mode=True)
 
 
 def migrate(
diff --git a/hermes_cli/debug.py b/hermes_cli/debug.py
index d7df489a56..c783168e95 100644
--- a/hermes_cli/debug.py
+++ b/hermes_cli/debug.py
@@ -16,7 +16,7 @@ from types import SimpleNamespace
 from typing import Optional
 
 from hermes_constants import get_hermes_home
-from utils import atomic_replace
+from utils import atomic_json_write
 
 logger = logging.getLogger(__name__)
 
@@ -54,12 +54,8 @@ def _load_pending() -> list[dict]:
 
 
 def _save_pending(entries: list[dict]) -> None:
-    path = _pending_file()
     try:
-        path.parent.mkdir(parents=True, exist_ok=True)
-        tmp = path.with_suffix(".json.tmp")
-        tmp.write_text(json.dumps(entries, indent=2), encoding="utf-8")
-        atomic_replace(tmp, path)
+        atomic_json_write(_pending_file(), entries)
     except OSError:
         pass  # non-fatal — worst case the user runs ``hermes debug delete`` manually
 
diff --git a/hermes_cli/install_identity.py b/hermes_cli/install_identity.py
index 32669039ab..21fc6fd4a3 100644
--- a/hermes_cli/install_identity.py
+++ b/hermes_cli/install_identity.py
@@ -6,12 +6,12 @@ import contextlib
 import os
 from pathlib import Path
 import re
-import tempfile
 import threading
 from typing import Optional
 import uuid
 
 from hermes_constants import get_default_hermes_root
+from utils import atomic_write_text
 
 _INSTALL_ID_FILENAME = "install_id"
 _INSTALL_ID_RE = re.compile(r"^[0-9a-f]{32}$")
@@ -47,22 +47,6 @@ def _install_id_file_lock(root: Path):
             os.close(fd)
 
 
-def _fsync_directory(path: Path) -> None:
-    """Best-effort durability for the directory entry after replace."""
-    if os.name == "nt":
-        return
-    try:
-        fd = os.open(path, os.O_RDONLY | getattr(os, "O_DIRECTORY", 0))
-    except OSError:
-        return
-    try:
-        os.fsync(fd)
-    except OSError:
-        pass
-    finally:
-        os.close(fd)
-
-
 def _read_existing(path: Path) -> tuple[Optional[str], bool]:
     """``(valid id or None, mint?)`` — mint on a missing or malformed file, never on a read failure."""
     try:
@@ -92,18 +76,7 @@ def read_or_create_install_id(root: Path | None = None) -> Optional[str]:
             existing, mint = _read_existing(path)
             if not mint:
                 return existing
-            fd, tmp_name = tempfile.mkstemp(dir=str(root), prefix=".install_id-")
-            try:
-                with os.fdopen(fd, "w", encoding="utf-8") as handle:
-                    handle.write(uuid.uuid4().hex + "\n")
-                    handle.flush()
-                    os.fsync(handle.fileno())
-                os.replace(tmp_name, path)
-                _fsync_directory(root)
-            except BaseException:
-                with contextlib.suppress(OSError):
-                    os.unlink(tmp_name)
-                raise
+            atomic_write_text(path, uuid.uuid4().hex + "\n", tmp_prefix=".install_id-", fsync_dir=True)
             committed = path.read_text(encoding="utf-8").strip().lower()
             return committed if _INSTALL_ID_RE.fullmatch(committed) else None
     except OSError:
diff --git a/hermes_cli/local_runtime/presets.py b/hermes_cli/local_runtime/presets.py
index f10dca7c32..217957036a 100644
--- a/hermes_cli/local_runtime/presets.py
+++ b/hermes_cli/local_runtime/presets.py
@@ -155,17 +155,8 @@ def generate_presets(models_dir: Path, budget: HardwareBudget, preset_path: Path
             body = "\n".join(f"{k} = {v}" for k, v in entry.keys.items())
             sections.append(f"[{entry.model_id}]\n{body}\n")
 
-    preset_path.parent.mkdir(parents=True, exist_ok=True)
-    import os
-    import tempfile
-
-    fd, tmp = tempfile.mkstemp(prefix=preset_path.name, suffix=".tmp", dir=preset_path.parent)
-    try:
-        with os.fdopen(fd, "w", encoding="utf-8") as stream:
-            stream.write("\n".join(sections))
-        os.replace(tmp, preset_path)
-    finally:
-        Path(tmp).unlink(missing_ok=True)
+    from utils import atomic_write_text
+    atomic_write_text(preset_path, "\n".join(sections), tmp_prefix=f".{preset_path.name}_")
     logger.info("wrote %d preset sections to %s", sum(e.keys is not None for e in entries), preset_path)
     return entries
 
diff --git a/hermes_cli/model_catalog.py b/hermes_cli/model_catalog.py
index d938789212..d2f332e609 100644
--- a/hermes_cli/model_catalog.py
+++ b/hermes_cli/model_catalog.py
@@ -18,7 +18,7 @@ from pathlib import Path
 from typing import Any
 
 from hermes_cli import __version__ as _HERMES_VERSION
-from utils import atomic_replace
+from utils import atomic_json_write
 
 logger = logging.getLogger(__name__)
 
@@ -154,14 +154,8 @@ def _read_disk_cache() -> tuple[dict[str, Any] | None, float]:
 
 
 def _write_disk_cache(data: dict[str, Any]) -> None:
-    path = _cache_path()
     try:
-        path.parent.mkdir(parents=True, exist_ok=True)
-        tmp = path.with_suffix(path.suffix + ".tmp")
-        with open(tmp, "w", encoding="utf-8") as fh:
-            json.dump(data, fh, indent=2)
-            fh.write("\n")
-        atomic_replace(tmp, path)
+        atomic_json_write(_cache_path(), data)
     except OSError as exc:
         logger.info("model catalog cache write failed: %s", exc)
 
diff --git a/hermes_cli/plugin_compat.py b/hermes_cli/plugin_compat.py
index 310b077f1f..023a7bacad 100644
--- a/hermes_cli/plugin_compat.py
+++ b/hermes_cli/plugin_compat.py
@@ -27,6 +27,7 @@ import warnings
 from dataclasses import dataclass
 from pathlib import Path
 from typing import Dict, Iterable, List, Optional, Tuple
+from utils import atomic_json_write
 
 COMPAT_REMOVAL_DATE = _dt.date(2026, 9, 14)
 COMPAT_REMOVAL = COMPAT_REMOVAL_DATE.isoformat()
@@ -247,10 +248,7 @@ def _write_report_file(report: Dict[str, List[Hit]]) -> None:
                    "written_at": _dt.datetime.now(_dt.timezone.utc).isoformat(timespec="seconds"),
                    "plugins": {k: [h.__dict__ for h in v] for k, v in report.items()},
                    "lines": summary_lines(report)}
-        p.parent.mkdir(parents=True, exist_ok=True)
-        tmp = p.with_suffix(".tmp")
-        tmp.write_text(json.dumps(payload, indent=1), encoding="utf-8")
-        os.replace(tmp, p)
+        atomic_json_write(p, payload, indent=1)
     except Exception:
         pass
 
diff --git a/hermes_cli/plugins.py b/hermes_cli/plugins.py
index fef322e691..facd240a58 100644
--- a/hermes_cli/plugins.py
+++ b/hermes_cli/plugins.py
@@ -1621,18 +1621,13 @@ def _plugin_toolset_keys_cache_path() -> Path:
 def _persist_plugin_toolset_keys() -> None:
     """Persist discovered plugin toolset keys + portable MCP names (best-effort)."""
     try:
-        import tempfile
+        from utils import atomic_json_write
         keys = sorted({ts_key for ts_key, _, _ in get_plugin_toolsets()})
         try:
             portable = sorted(get_plugin_manager().get_portable_mcp_servers())
         except Exception:
             portable = []
-        path = _plugin_toolset_keys_cache_path()
-        path.parent.mkdir(parents=True, exist_ok=True)
-        fd, tmp = tempfile.mkstemp(dir=str(path.parent), prefix=".pt_keys.")
-        with os.fdopen(fd, "w", encoding="utf-8") as fh:
-            json.dump({"toolset_keys": keys, "portable_mcp": portable}, fh)
-        os.replace(tmp, path)
+        atomic_json_write(_plugin_toolset_keys_cache_path(), {"toolset_keys": keys, "portable_mcp": portable}, indent=None)
     except Exception:
         logger.debug("plugin toolset key persist failed", exc_info=True)
 
diff --git a/hermes_cli/profiles.py b/hermes_cli/profiles.py
index 12c98d1048..e0204201d4 100644
--- a/hermes_cli/profiles.py
+++ b/hermes_cli/profiles.py
@@ -1591,15 +1591,12 @@ def import_profile(archive_path: str, name: Optional[str] = None) -> Path:
 # Rename
 
 def _atomic_write_json(path: Path, data: dict) -> bool:
-    """Write *data* to *path* via a sibling ``.tmp`` + rename. Returns False (tmp cleaned) on OSError."""
-    tmp = path.with_suffix(path.suffix + ".tmp")
+    """Atomic rewrite of a third-party JSON config; False on OSError (nothing partially written)."""
+    from utils import atomic_json_write
     try:
-        tmp.write_text(json.dumps(data, indent=2, ensure_ascii=False) + "\n", encoding="utf-8")
-        tmp.replace(path)
+        atomic_json_write(path, data)
         return True
     except OSError:
-        with contextlib.suppress(OSError):
-            tmp.unlink(missing_ok=True)
         return False
 
 
diff --git a/hermes_cli/terminal_breadcrumbs.py b/hermes_cli/terminal_breadcrumbs.py
index ed05eb4dda..5808f8160d 100644
--- a/hermes_cli/terminal_breadcrumbs.py
+++ b/hermes_cli/terminal_breadcrumbs.py
@@ -11,6 +11,7 @@ import sys
 import time
 from pathlib import Path
 from typing import Optional
+from utils import atomic_json_write
 
 # Multiplexer / terminal-emulator identity env vars, checked in order when no real tty path is
 # available (e.g. stdin piped but stdout still a pty owned by a known terminal).
@@ -86,9 +87,7 @@ def write_breadcrumb(session_id: str, cwd: Optional[str] = None) -> None:
         directory.mkdir(parents=True, exist_ok=True)
         now = time.time()
         payload = {"session_id": session_id, "cwd": cwd or os.getcwd(), "ts": now}
-        tmp = directory / f".{terminal_id}.tmp"
-        tmp.write_text(json.dumps(payload), encoding="utf-8")
-        os.replace(tmp, directory / terminal_id)
+        atomic_json_write(directory / terminal_id, payload, indent=None)
         _prune_stale(directory, now)
     except Exception:
         pass
diff --git a/hermes_cli/web_server_lifecycle.py b/hermes_cli/web_server_lifecycle.py
index 4a6ba17cc0..6bea65b426 100644
--- a/hermes_cli/web_server_lifecycle.py
+++ b/hermes_cli/web_server_lifecycle.py
@@ -4,15 +4,14 @@
 import asyncio
 import logging
 import ipaddress
-import json
 import os
 import subprocess
 import sys
-import tempfile
 import threading
 import time
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Optional
+from utils import atomic_json_write
 
 if TYPE_CHECKING:  # pragma: no cover - annotation only
     import uvicorn
@@ -212,25 +211,9 @@ def _write_dashboard_ready_file(actual_port: int) -> None:
     if not target:
         return
 
-    tmp_name = ""
     try:
-        path = Path(target)
-        path.parent.mkdir(parents=True, exist_ok=True)
-        payload = json.dumps({"port": int(actual_port)}, separators=(",", ":"))
-        with tempfile.NamedTemporaryFile(
-            "w", encoding="utf-8", dir=str(path.parent), prefix=f"{path.name}.", suffix=".tmp", delete=False
-        ) as fh:
-            fh.write(payload)
-            fh.flush()
-            os.fsync(fh.fileno())
-            tmp_name = fh.name
-        os.replace(tmp_name, path)
+        atomic_json_write(Path(target), {"port": int(actual_port)}, indent=None, separators=(",", ":"))
     except Exception as exc:
-        if tmp_name:
-            try:
-                Path(tmp_name).unlink(missing_ok=True)
-            except Exception:
-                pass
         _log.warning("Failed to write dashboard ready file %r: %s", target, exc)
 
 
diff --git a/hermes_cli/worktree_ops.py b/hermes_cli/worktree_ops.py
index e9e6965033..72efdd1174 100644
--- a/hermes_cli/worktree_ops.py
+++ b/hermes_cli/worktree_ops.py
@@ -19,6 +19,7 @@ from pathlib import Path
 from typing import Dict, Optional
 
 from hermes_constants import get_hermes_home
+from utils import atomic_json_write
 
 logger = logging.getLogger("cli")
 
@@ -458,21 +459,11 @@ def _load_worktree_merge_cache() -> Dict[str, bool]:
 
 def _save_worktree_merge_cache(verdicts: Dict[str, bool]) -> None:
     """Atomically persist the newest ``_WORKTREE_MERGE_CACHE_MAX`` verdicts. Never raises."""
-    path = _worktree_merge_cache_path()
-    tmp = None
     try:
         items = list(verdicts.items())[-_WORKTREE_MERGE_CACHE_MAX:]
-        path.parent.mkdir(parents=True, exist_ok=True)
-        tmp = path.with_suffix(f".{os.getpid()}.tmp")
-        tmp.write_text(json.dumps({"version": 1, "verdicts": dict(items)}), encoding="utf-8")
-        os.replace(str(tmp), str(path))
+        atomic_json_write(_worktree_merge_cache_path(), {"version": 1, "verdicts": dict(items)}, indent=None)
     except Exception as e:
         logger.debug("Could not persist worktree merge cache: %s", e)
-        if tmp is not None:
-            try:
-                tmp.unlink()
-            except Exception:
-                pass
 
 
 def _worktree_commits_all_merged_upstream(
diff --git a/plugins/google_meet/_jsonfile.py b/plugins/google_meet/_jsonfile.py
index dd3a5549b3..2231df9cee 100644
--- a/plugins/google_meet/_jsonfile.py
+++ b/plugins/google_meet/_jsonfile.py
@@ -1,8 +1,7 @@
-"""Tiny JSON file helpers shared by the bot, process manager, node registry and node server."""
+"""Tiny JSON read helper shared by the bot, process manager, node registry and node server."""
 
 from __future__ import annotations
 
-import contextlib
 import json
 from pathlib import Path
 from typing import Any, Optional
@@ -16,15 +15,3 @@ def read_json(path: Path) -> Optional[Any]:
         return json.loads(path.read_text(encoding="utf-8"))
     except (OSError, ValueError):
         return None
-
-
-def write_json_atomic(path: Path, data: Any, mode: Optional[int] = None) -> None:
-    """Write ``json.dumps(data, indent=2)`` via a ``.json.tmp`` sibling + rename; *mode* (e.g.
-    ``0o600``) is applied to the temp file so the final file never exists with looser perms."""
-    path.parent.mkdir(parents=True, exist_ok=True)
-    tmp = path.with_suffix(".json.tmp")
-    tmp.write_text(json.dumps(data, indent=2), encoding="utf-8")
-    if mode is not None:
-        with contextlib.suppress(OSError, NotImplementedError):  # best-effort on non-POSIX filesystems
-            tmp.chmod(mode)
-    tmp.replace(path)
diff --git a/plugins/google_meet/meet_bot.py b/plugins/google_meet/meet_bot.py
index eddc2cc020..5abdbfdd13 100644
--- a/plugins/google_meet/meet_bot.py
+++ b/plugins/google_meet/meet_bot.py
@@ -22,7 +22,7 @@ from pathlib import Path
 from types import SimpleNamespace
 from typing import Optional
 
-from plugins.google_meet._jsonfile import write_json_atomic
+from utils import atomic_json_write
 
 # Short three-segment code, a lookup URL, or /new. Anything else is rejected.
 MEET_URL_RE = re.compile(
@@ -93,7 +93,7 @@ class _BotState:
     def _flush(self) -> None:
         data = {key: getattr(self, attr) if attr else None for key, attr, _ in _STATUS_FIELDS}
         data.update(transcriptPath=str(self.transcript_path), pid=os.getpid())  # keeps table key order
-        write_json_atomic(self.status_path, data)
+        atomic_json_write(self.status_path, data)
 
     def set(self, **kwargs) -> None:
         self.__dict__.update(kwargs)
diff --git a/plugins/google_meet/node/registry.py b/plugins/google_meet/node/registry.py
index 13015098f3..4eb7a91c06 100644
--- a/plugins/google_meet/node/registry.py
+++ b/plugins/google_meet/node/registry.py
@@ -13,7 +13,8 @@ from typing import Any, Dict, List, Optional
 
 from hermes_constants import get_hermes_home
 
-from plugins.google_meet._jsonfile import read_json, write_json_atomic
+from plugins.google_meet._jsonfile import read_json
+from utils import atomic_json_write
 
 
 def _default_path() -> Path:
@@ -33,7 +34,7 @@ class NodeRegistry:
         return nodes if isinstance(nodes, dict) else {}
 
     def _save(self, nodes: Dict[str, Dict[str, Any]]) -> None:
-        write_json_atomic(self.path, {"nodes": nodes})
+        atomic_json_write(self.path, {"nodes": nodes})
 
     def get(self, name: str) -> Optional[Dict[str, Any]]:
         entry = self._load().get(name)
diff --git a/plugins/google_meet/node/server.py b/plugins/google_meet/node/server.py
index 97ecb912a2..aaaac39a51 100644
--- a/plugins/google_meet/node/server.py
+++ b/plugins/google_meet/node/server.py
@@ -17,7 +17,8 @@ from pathlib import Path
 from typing import Any, Dict, Optional
 
 from hermes_constants import get_hermes_home
-from plugins.google_meet._jsonfile import read_json, write_json_atomic
+from plugins.google_meet._jsonfile import read_json
+from utils import atomic_json_write
 from plugins.google_meet.node import protocol as _proto
 
 _START_BOT_KEYS = ("url", "guest_name", "duration", "headed", "auth_state", "session_id", "out_dir")
@@ -80,7 +81,7 @@ class NodeServer:
         if not (isinstance(tok, str) and tok):
             tok = secrets.token_hex(16)  # 32 hex chars
             # Owner-only: the token grants full RPC access to the meet bot.
-            write_json_atomic(self.token_path, {"token": tok, "generated_at": time.time()}, mode=0o600)
+            atomic_json_write(self.token_path, {"token": tok, "generated_at": time.time()}, mode=0o600)
         self._token = tok
         return tok
 
diff --git a/plugins/google_meet/process_manager.py b/plugins/google_meet/process_manager.py
index 92e2c04831..572db16ae8 100644
--- a/plugins/google_meet/process_manager.py
+++ b/plugins/google_meet/process_manager.py
@@ -21,7 +21,8 @@ from typing import Any, Dict, Optional
 
 from hermes_constants import get_hermes_home
 
-from plugins.google_meet._jsonfile import read_json, write_json_atomic
+from plugins.google_meet._jsonfile import read_json
+from utils import atomic_json_write
 
 
 def _root() -> Path:
@@ -33,7 +34,7 @@ def _read_active() -> Optional[Dict[str, Any]]:
 
 
 def _write_active(data: Dict[str, Any]) -> None:
-    write_json_atomic(_root() / ".active.json", data)
+    atomic_json_write(_root() / ".active.json", data)
 
 
 def _pid_alive(pid: int) -> bool:
diff --git a/tests/agent/test_shell_hooks.py b/tests/agent/test_shell_hooks.py
index 71546e07e6..1cadf3062d 100644
--- a/tests/agent/test_shell_hooks.py
+++ b/tests/agent/test_shell_hooks.py
@@ -452,14 +452,15 @@ class TestAllowlistConcurrency:
         p.parent.mkdir(parents=True, exist_ok=True)
 
         tmp_paths_seen: list = []
-        real_mkstemp = shell_hooks.tempfile.mkstemp
+        import utils
+        real_mkstemp = utils.tempfile.mkstemp
 
         def spying_mkstemp(*args, **kwargs):
             fd, path = real_mkstemp(*args, **kwargs)
             tmp_paths_seen.append(path)
             return fd, path
 
-        monkeypatch.setattr(shell_hooks.tempfile, "mkstemp", spying_mkstemp)
+        monkeypatch.setattr(utils.tempfile, "mkstemp", spying_mkstemp)
 
         shell_hooks.save_allowlist({"approvals": [{"event": "a", "command": "x"}]})
         shell_hooks.save_allowlist({"approvals": [{"event": "b", "command": "y"}]})
diff --git a/tests/hermes_cli/test_codex_runtime_plugin_migration.py b/tests/hermes_cli/test_codex_runtime_plugin_migration.py
index 84b2b73961..62284fbd8d 100644
--- a/tests/hermes_cli/test_codex_runtime_plugin_migration.py
+++ b/tests/hermes_cli/test_codex_runtime_plugin_migration.py
@@ -79,13 +79,12 @@ class TestTomlValueFormatter:
         """If rename fails partway through (out of disk, permissions,
         crash), the temp file must be cleaned up. Otherwise repeated
         failed migrations would pile up .config.toml.* files."""
-        from pathlib import Path as _Path
-        original_replace = _Path.replace
+        import utils
 
-        def failing_replace(self, target):
+        def failing_replace(tmp, target):
             raise OSError("simulated disk full")
 
-        monkeypatch.setattr(_Path, "replace", failing_replace)
+        monkeypatch.setattr(utils, "atomic_replace", failing_replace)
         report = migrate(
             {"mcp_servers": {"x": {"command": "y"}}},
             codex_home=tmp_path,
diff --git a/tests/hermes_cli/test_install_identity.py b/tests/hermes_cli/test_install_identity.py
index 218b5156ce..6e8e2a402b 100644
--- a/tests/hermes_cli/test_install_identity.py
+++ b/tests/hermes_cli/test_install_identity.py
@@ -21,14 +21,15 @@ def _race_first_install_id(
     if start_barrier is not None:
         start_barrier.wait(timeout=10)
     if writer_entered is not None:
-        original_mkstemp = install_identity.tempfile.mkstemp
+        import utils
+        original_mkstemp = utils.tempfile.mkstemp
 
         def held_mkstemp(*args, **kwargs):
             writer_entered.set()
             assert release_writer.wait(timeout=10)
             return original_mkstemp(*args, **kwargs)
 
-        install_identity.tempfile.mkstemp = held_mkstemp
+        utils.tempfile.mkstemp = held_mkstemp
     results.put(read_or_create_install_id(root))
 
 
diff --git a/tests/test_atomic_json_writers_unified.py b/tests/test_atomic_json_writers_unified.py
new file mode 100644
index 0000000000..aa4fb9b43d
--- /dev/null
+++ b/tests/test_atomic_json_writers_unified.py
@@ -0,0 +1,69 @@
+"""Invariant for the non-secret atomic JSON writers that used to inline ``utils._atomic_write``.
+
+Contract under test for ``gateway/session_persistence``, ``cron/suggestions`` and
+``agent/shell_hooks``: a failed replace leaves the previous file byte-identical AND leaves no temp
+file behind (the interrupt-safe cleanup only the canonical helper guarantees). A hand-rolled copy
+that skips the cleanup, or writes through the target instead of a sibling temp, fails this.
+"""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+
+def _failing_replace(tmp, target):
+    raise OSError("simulated disk full")
+
+
+def _leftovers(directory: Path, keep: str) -> list[str]:
+    return sorted(p.name for p in directory.iterdir() if p.name != keep)
+
+
+@pytest.fixture
+def broken_replace(monkeypatch):
+    import utils
+
+    monkeypatch.setattr(utils, "atomic_replace", _failing_replace)
+
+
+def test_sessions_json_failed_replace_keeps_old_bytes_and_no_temp(tmp_path, monkeypatch, broken_replace):
+    from gateway.session_persistence import SessionPersistenceMixin
+
+    sessions_dir = tmp_path / "sessions"
+    sessions_dir.mkdir()
+    target = sessions_dir / "sessions.json"
+    target.write_text('{"old": true}', encoding="utf-8")
+    store = SessionPersistenceMixin()
+    store.sessions_dir = sessions_dir
+    with pytest.raises(OSError):
+        store._save_sessions_json({"k": "v"})
+    assert target.read_text(encoding="utf-8") == '{"old": true}'
+    assert _leftovers(sessions_dir, "sessions.json") == []
+
+
+def test_suggestions_failed_replace_keeps_old_bytes_and_no_temp(tmp_path, monkeypatch, broken_replace):
+    from cron import suggestions
+
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path))
+    target = suggestions._current_suggestions_file()
+    target.parent.mkdir(parents=True, exist_ok=True)
+    target.write_text('{"old": true}', encoding="utf-8")
+    with pytest.raises(OSError):
+        suggestions._save_raw([{"id": 1}])
+    assert target.read_text(encoding="utf-8") == '{"old": true}'
+    assert _leftovers(target.parent, target.name) == []
+
+
+def test_shell_hooks_allowlist_survives_failed_replace_without_temp(tmp_path, monkeypatch, broken_replace):
+    from agent import shell_hooks
+
+    monkeypatch.setenv("HERMES_HOME", str(tmp_path / "home"))
+    target = shell_hooks.allowlist_path()
+    target.parent.mkdir(parents=True, exist_ok=True)
+    target.write_text(json.dumps({"approvals": []}), encoding="utf-8")
+    shell_hooks.save_allowlist({"approvals": [{"event": "a", "command": "x"}]})  # logs, never raises
+    assert json.loads(target.read_text(encoding="utf-8")) == {"approvals": []}
+    assert _leftovers(target.parent, target.name) == []
diff --git a/tools/bot_live_delivery.py b/tools/bot_live_delivery.py
index 981346c779..2197bdbdb8 100644
--- a/tools/bot_live_delivery.py
+++ b/tools/bot_live_delivery.py
@@ -10,10 +10,11 @@ from __future__ import annotations
 import json
 import os
 import re
-import tempfile
 import time
 import uuid
 from contextlib import contextmanager
+
+from utils import atomic_json_write, fsync_directory
 from pathlib import Path
 from typing import Any
 
@@ -73,25 +74,14 @@ def _root(home: Path | str) -> Path:
     return Path(home).resolve() / "runtime" / DELIVERY_DIR_NAME
 
 
-def _fsync_dir(path: Path) -> None:
-    # Windows cannot open directories with os.open; file fsync still applies.
-    if os.name == "nt":
-        return
-    fd = os.open(path, os.O_RDONLY)
-    try:
-        os.fsync(fd)
-    finally:
-        os.close(fd)
-
-
 @contextmanager
 def _locked(home: Path | str):
     root = _root(home)
     root.parent.mkdir(parents=True, exist_ok=True)
     root.mkdir(mode=0o700, exist_ok=True)
     root.chmod(0o700)
-    _fsync_dir(root.parent)
-    _fsync_dir(root.parent.parent)
+    fsync_directory(root.parent)
+    fsync_directory(root.parent.parent)
     lock = root / ".lock"
     fd = os.open(lock, os.O_CREAT | os.O_WRONLY, 0o600)
     os.close(fd)
@@ -107,16 +97,7 @@ def _read(path: Path) -> dict[str, Any] | None:
 
 
 def _write(path: Path, record: dict[str, Any]) -> None:
-    fd, temporary = tempfile.mkstemp(dir=path.parent, prefix=".delivery-")
-    try:
-        with os.fdopen(fd, "w", encoding="utf-8") as stream:
-            json.dump(record, stream, ensure_ascii=False, sort_keys=True)
-            stream.flush()
-            os.fsync(stream.fileno())
-        os.replace(temporary, path)
-        _fsync_dir(path.parent)
-    finally:
-        Path(temporary).unlink(missing_ok=True)
+    atomic_json_write(path, record, indent=None, sort_keys=True, fsync_dir=True)
 
 
 def deliver_to_live_owner(
diff --git a/tools/bot_mode_dm.py b/tools/bot_mode_dm.py
index 8366a8a4fa..a933d107d3 100644
--- a/tools/bot_mode_dm.py
+++ b/tools/bot_mode_dm.py
@@ -407,9 +407,8 @@ def _run_local_turn(argv: list[str], dm_file: str, *, env: Optional[dict[str, st
 
 def _admit_live_dm(profile_home: Path | None, dm_file: str, author: Optional[dict] = None) -> dict | None:
     """Pin intent before admission; retries may inspect, never change transport."""
-    from tools.bot_live_delivery import (
-        _fsync_dir, deliver_to_live_owner, find_canonical_live_owner, read_delivery_result,
-    )
+    from tools.bot_live_delivery import deliver_to_live_owner, find_canonical_live_owner, read_delivery_result
+    from utils import fsync_directory
 
     intent: dict[str, Any]
     intent_path = Path(dm_file + ".live.json")
@@ -432,7 +431,7 @@ def _admit_live_dm(profile_home: Path | None, dm_file: str, author: Optional[dic
                 json.dump(intent, stream)
                 stream.flush()
                 os.fsync(stream.fileno())
-            _fsync_dir(intent_path.parent)
+            fsync_directory(intent_path.parent)
     home = intent["owner"]["profile_home"]
     record = read_delivery_result(home, intent["delivery_id"])
     if record is None:
diff --git a/tools/bot_relay.py b/tools/bot_relay.py
index db9551a6e1..887daefac6 100644
--- a/tools/bot_relay.py
+++ b/tools/bot_relay.py
@@ -21,13 +21,13 @@ import re
 import shlex
 import shutil
 import sys
-import tempfile
 import time
 import uuid
 from pathlib import Path
 from typing import Any, Iterator, Optional
 
 from tools.bot_mode_probe import _default_home, _hermes_root
+from utils import atomic_json_write
 
 logger = logging.getLogger(__name__)
 
@@ -90,17 +90,8 @@ def _ensure_dirs(root: Path | str) -> Path:
     return base
 
 
-def _atomic_write_json(target: Path, payload: Any, *, prefix: str, sort_keys: bool = False) -> None:
-    """tempfile + os.replace so readers never see a partial file; tempfile removed on failure."""
-    fd, tmp = tempfile.mkstemp(dir=str(target.parent), prefix=prefix, suffix=".tmp")
-    try:
-        with os.fdopen(fd, "w", encoding="utf-8") as f:
-            json.dump(payload, f, ensure_ascii=False, sort_keys=sort_keys)
-        os.replace(tmp, target)
-    except Exception:
-        with contextlib.suppress(OSError):
-            os.unlink(tmp)
-        raise
+def _atomic_write_json(target: Path, payload: Any, *, sort_keys: bool = False) -> None:
+    atomic_json_write(target, payload, indent=None, sort_keys=sort_keys)
 
 
 def _bot_mode_cfg(key: str, *, loader: str) -> Any:
@@ -145,8 +136,7 @@ def write_remote_roster(root: Path | str, rows: Any) -> int:
     for norm in filter(None, map(_normalize_roster_row, rows if isinstance(rows, list) else [])):
         by_key.setdefault((norm["connection_id"], norm["profile"]), norm)
     cleaned = [by_key[k] for k in sorted(by_key)]
-    _atomic_write_json(base / ROSTER_FILE, {"updated_at": int(time.time()), "agents": cleaned},
-                       prefix=".roster-", sort_keys=True)
+    _atomic_write_json(base / ROSTER_FILE, {"updated_at": int(time.time()), "agents": cleaned}, sort_keys=True)
     return len(cleaned)
 
 
@@ -229,7 +219,7 @@ def enqueue_envelope(root: Path | str, *, target: dict, message: str, sender_pro
         "target_connection": target["connection_id"], "target_profile": target["profile"],
         "target_handle": target["handle"], "message": message,
     }
-    _atomic_write_json(base / OUTBOX_DIR / f"{envelope['id']}.json", envelope, prefix=".env-")
+    _atomic_write_json(base / OUTBOX_DIR / f"{envelope['id']}.json", envelope)
     return envelope
 
 
@@ -289,8 +279,7 @@ def write_reply(root: Path | str, envelope_id: str, *, reply: str = "", error: s
 
         code = classify_agent_error(err)
     path = base / REPLIES_DIR / f"{safe}.json"
-    _atomic_write_json(path, {"id": safe, "at": int(time.time()), "reply": str(reply or ""), "error": err, "reason": code},
-                       prefix=".rep-")
+    _atomic_write_json(path, {"id": safe, "at": int(time.time()), "reply": str(reply or ""), "error": err, "reason": code})
     return path
 
 
diff --git a/tools/web_result_cache.py b/tools/web_result_cache.py
index 1ed9ce9172..dd4fa350c0 100644
--- a/tools/web_result_cache.py
+++ b/tools/web_result_cache.py
@@ -10,7 +10,6 @@ Lives here, not in tool dispatch, so hits sit *after* every safety check and ski
 import hashlib
 import json
 import logging
-import os
 import re
 import threading
 import time
@@ -18,6 +17,7 @@ from contextlib import suppress
 from pathlib import Path
 from typing import Dict, Optional, Tuple
 from urllib.parse import urlparse
+from utils import atomic_json_write
 
 logger = logging.getLogger(__name__)
 
@@ -183,11 +183,9 @@ def _save_index(index: dict) -> None:
         if len(index) > _INDEX_MAX_ENTRIES:
             newest = sorted(index.items(), key=lambda kv: kv[1].get("fetched_at", 0), reverse=True)
             index = dict(newest[:_INDEX_MAX_ENTRIES])
-        # Per-process tmp name: CLI, gateway, cron, and subagents all write this index; a shared tmp name
-        # would let concurrent writers truncate each other. os.replace is atomic: worst case is a lost insert.
-        tmp = path.with_suffix(f".tmp.{os.getpid()}")
-        tmp.write_text(json.dumps(index), encoding="utf-8")
-        tmp.replace(path)
+        # CLI, gateway, cron, and subagents all write this index; the replace is atomic, so the worst case
+        # under concurrent writers is a lost insert, never a truncated index.
+        atomic_json_write(path, index, indent=None)
     except Exception as exc:  # noqa: BLE001
         logger.debug("Failed to save web extract cache index: %s", exc)
 
diff --git a/tools/write_approval.py b/tools/write_approval.py
index f1e99b46e9..39c9a4cda8 100644
--- a/tools/write_approval.py
+++ b/tools/write_approval.py
@@ -14,7 +14,6 @@ from __future__ import annotations
 import difflib
 import json
 import logging
-import os
 import re
 import time
 import uuid
@@ -24,6 +23,7 @@ from pathlib import Path
 from typing import Any, Dict, List, Optional
 
 from hermes_constants import get_hermes_home
+from utils import atomic_json_write
 
 logger = logging.getLogger(__name__)
 
@@ -82,11 +82,7 @@ def stage_write(subsystem: str, payload: Dict[str, Any], *, summary: str, origin
         "created_at": time.time(), "payload": payload,
     }
     try:
-        path = _pending_path(subsystem, pid)
-        path.parent.mkdir(parents=True, exist_ok=True)
-        tmp = path.with_suffix(".json.tmp")
-        tmp.write_text(json.dumps(record, ensure_ascii=False, indent=2), encoding="utf-8")
-        os.replace(tmp, path)
+        atomic_json_write(_pending_path(subsystem, pid), record)
     except Exception as e:  # pragma: no cover - disk failure path
         logger.error("Failed to stage pending %s write: %s", subsystem, e, exc_info=True)
     return record
diff --git a/tui_gateway/turn_marker.py b/tui_gateway/turn_marker.py
index e17891feed..eefdc5de2d 100644
--- a/tui_gateway/turn_marker.py
+++ b/tui_gateway/turn_marker.py
@@ -8,15 +8,13 @@ marker" instead of raising."""
 
 from __future__ import annotations
 
-import contextlib
 import json
 import logging
-import os
-import tempfile
 import threading
 import time
 from pathlib import Path
 from typing import Any
+from utils import atomic_json_write
 
 logger = logging.getLogger(__name__)
 
@@ -59,16 +57,7 @@ def _store(path: Path, entries: dict[str, dict]) -> None:
     if not entries:
         path.unlink(missing_ok=True)
         return
-    path.parent.mkdir(parents=True, exist_ok=True)
-    fd, tmp = tempfile.mkstemp(dir=path.parent, prefix=".turn-marker-")
-    try:
-        with os.fdopen(fd, "w", encoding="utf-8") as f:
-            json.dump(entries, f)
-        os.replace(tmp, path)
-    except Exception:
-        with contextlib.suppress(OSError):
-            os.unlink(tmp)
-        raise
+    atomic_json_write(path, entries, indent=None)
 
 
 def _update(home: Path | str, session_key: str, mutate, what: str) -> None: