diff --git a/mempalace/entity_registry.py b/mempalace/entity_registry.py
index 78d8a8b..c8ac517 100644
--- a/mempalace/entity_registry.py
+++ b/mempalace/entity_registry.py
@@ -16,6 +16,7 @@ Usage:
 """
 
 import json
+import os
 import re
 import urllib.request
 import urllib.parse
@@ -320,11 +321,35 @@ class EntityRegistry:
             self._path.parent.chmod(0o700)
         except (OSError, NotImplementedError):
             pass
-        self._path.write_text(json.dumps(self._data, indent=2), encoding="utf-8")
+        # Atomic write: serialize to a sibling temp file in the same dir
+        # (so os.replace stays on one filesystem), fsync, then rename over
+        # the target. A crash mid-write leaves the previous registry intact
+        # instead of a half-written file or an empty file from the truncate.
+        payload = json.dumps(self._data, indent=2)
+        tmp_path = self._path.with_name(self._path.name + ".tmp")
+        with open(tmp_path, "w", encoding="utf-8") as f:
+            f.write(payload)
+            f.flush()
+            os.fsync(f.fileno())
         try:
-            self._path.chmod(0o600)
+            tmp_path.chmod(0o600)
         except (OSError, NotImplementedError):
             pass
+        os.replace(tmp_path, self._path)
+        # On ext4 (and similar) the rename's durability across power loss
+        # requires an additional fsync on the parent directory. Without it,
+        # the kernel can ack the rename and a crash reverts to the state
+        # where the temp file is present and the target is at the old version.
+        try:
+            dir_fd = os.open(str(self._path.parent), os.O_RDONLY)
+            try:
+                os.fsync(dir_fd)
+            finally:
+                os.close(dir_fd)
+        except OSError:
+            # Windows and some special filesystems reject directory fds — they
+            # have different durability semantics on rename anyway.
+            pass
 
     @staticmethod
     def _empty() -> dict:
diff --git a/tests/test_entity_registry.py b/tests/test_entity_registry.py
index c857a07..a5f237c 100644
--- a/tests/test_entity_registry.py
+++ b/tests/test_entity_registry.py
@@ -2,6 +2,8 @@
 
 from unittest.mock import patch
 
+import pytest
+
 from mempalace.entity_registry import (
     COMMON_ENGLISH_WORDS,
     PERSON_CONTEXT_PATTERNS,
@@ -71,6 +73,50 @@ def test_save_creates_file(tmp_path):
     assert (tmp_path / "entity_registry.json").exists()
 
 
+def test_save_is_atomic_does_not_leave_tmp(tmp_path):
+    # Atomic write must not leave the .tmp sidecar file after a successful save.
+    registry = EntityRegistry.load(config_dir=tmp_path)
+    registry.save()
+    leftover = list(tmp_path.glob("entity_registry.json.tmp*"))
+    assert leftover == [], f"atomic write leaked tmp file(s): {leftover}"
+
+
+def test_save_preserves_previous_on_serialization_failure(tmp_path, monkeypatch):
+    # If serialization fails mid-write, the previous registry must remain
+    # intact — this is the whole point of atomic write vs truncating in place.
+    registry = EntityRegistry.load(config_dir=tmp_path)
+    registry.seed(
+        mode="personal",
+        people=[{"name": "Alice", "relationship": "friend", "context": "personal"}],
+        projects=[],
+    )
+    registry.save()
+    target = tmp_path / "entity_registry.json"
+    original = target.read_text(encoding="utf-8")
+
+    # Force os.replace to raise — simulates filesystem full / permission flip
+    # AFTER the temp file is written but BEFORE the rename completes.
+    import os as _os
+
+    real_replace = _os.replace
+
+    def boom(src, dst):
+        raise OSError("simulated rename failure")
+
+    monkeypatch.setattr(_os, "replace", boom)
+    with pytest.raises(OSError):
+        registry.seed(
+            mode="personal",
+            people=[{"name": "Bob", "relationship": "friend", "context": "personal"}],
+            projects=[],
+        )
+        registry.save()
+
+    # Restore os.replace before reading so the assertion can rely on it.
+    monkeypatch.setattr(_os, "replace", real_replace)
+    assert target.read_text(encoding="utf-8") == original
+
+
 # ── seed ────────────────────────────────────────────────────────────────