diff --git a/clawmetry/sync.py b/clawmetry/sync.py
index 7fe5470..d42c7f4 100644
--- a/clawmetry/sync.py
+++ b/clawmetry/sync.py
@@ -234,7 +234,9 @@ def load_state() -> dict:
 
 def save_state(state: dict) -> None:
     CONFIG_DIR.mkdir(parents=True, exist_ok=True)
-    STATE_FILE.write_text(json.dumps(state, indent=2))
+    tmp_path = STATE_FILE.with_suffix(".tmp")
+    tmp_path.write_text(json.dumps(state, indent=2))
+    tmp_path.rename(STATE_FILE)
 
 
 # ── HTTP ──────────────────────────────────────────────────────────────────────
diff --git a/tests/test_atomic_state.py b/tests/test_atomic_state.py
new file mode 100644
index 0000000..6a1fc02
--- /dev/null
+++ b/tests/test_atomic_state.py
@@ -0,0 +1,76 @@
+"""Tests for atomic state writes in sync.py."""
+
+import os
+import sys
+import json
+import tempfile
+import unittest
+from unittest.mock import patch
+
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+
+class TestAtomicStateWrite(unittest.TestCase):
+    """Test that save_state uses atomic write (temp file + rename)."""
+
+    def test_atomic_write_uses_temp_file_and_rename(self):
+        """Verify save_state writes to temp file then renames."""
+        from clawmetry import sync
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            state_file = os.path.join(tmpdir, "sync-state.json")
+            tmp_state_file = os.path.join(tmpdir, "sync-state.tmp")
+
+            with patch.object(sync, "STATE_FILE", sync.Path(state_file)):
+                with patch.object(sync, "CONFIG_DIR", sync.Path(tmpdir)):
+                    sync.save_state({"last_event_ids": {"x": 999}, "last_sync": 2000})
+
+            self.assertTrue(os.path.exists(state_file))
+            self.assertFalse(os.path.exists(tmp_state_file))
+
+            with open(state_file) as f:
+                data = json.load(f)
+            self.assertEqual(data["last_event_ids"]["x"], 999)
+            self.assertEqual(data["last_sync"], 2000)
+
+    def test_atomic_write_preserves_state_on_failed_rename(self):
+        """Verify old state is preserved when rename fails."""
+        from clawmetry import sync
+
+        with tempfile.TemporaryDirectory() as tmpdir:
+            state_file = os.path.join(tmpdir, "sync-state.json")
+
+            original_state = {
+                "last_event_ids": {"a": 1},
+                "last_sync": 1000,
+                "last_log_offsets": {},
+            }
+            with open(state_file, "w") as f:
+                json.dump(original_state, f)
+
+            original_rename = os.rename
+
+            def failing_rename(src, dst):
+                if ".tmp" in str(src):
+                    raise OSError("Simulated disk full during rename")
+                return original_rename(src, dst)
+
+            with patch.object(sync, "STATE_FILE", sync.Path(state_file)):
+                with patch.object(sync, "CONFIG_DIR", sync.Path(tmpdir)):
+                    with patch("os.rename", failing_rename):
+                        try:
+                            sync.save_state(
+                                {"last_event_ids": {"x": 999}, "last_sync": 2000}
+                            )
+                        except OSError:
+                            pass
+
+            with open(state_file) as f:
+                data = json.load(f)
+
+            self.assertEqual(data["last_event_ids"]["a"], 1)
+            self.assertEqual(data["last_sync"], 1000)
+
+
+if __name__ == "__main__":
+    unittest.main()