diff --git a/crdt-sync/crdt_sync/_credentials.py b/crdt-sync/crdt_sync/_credentials.py
index f87ec87..392634c 100644
--- a/crdt-sync/crdt_sync/_credentials.py
+++ b/crdt-sync/crdt_sync/_credentials.py
@@ -15,10 +15,10 @@ from dataclasses import dataclass
 import datetime as dt
 import json
 import logging
-from typing import TYPE_CHECKING, Protocol
-
-if TYPE_CHECKING:
-    from pathlib import Path
+import os
+from pathlib import Path
+import tempfile
+from typing import Protocol
 
 _logger = logging.getLogger(__name__)
 
@@ -132,12 +132,22 @@ class FileCredentialStore:
     def save(self, credentials: FirebaseCredentials) -> None:
         """Write ``credentials`` atomically, readable only by this user."""
         self._path.parent.mkdir(parents=True, exist_ok=True)
-        temp = self._path.with_suffix(f"{self._path.suffix}.tmp")
-        # Create with 0600 from the outset -- writing then chmod'ing would
-        # leave the refresh token world-readable for the gap in between.
-        temp.touch(mode=0o600)
-        temp.write_text(json.dumps(credentials.to_json()), encoding="utf-8")
-        temp.replace(self._path)
+        # A unique temp name per write, not a fixed ``.tmp``: two processes
+        # (a long-running stream and a periodic sync) share one cache, and a
+        # fixed name let one rename the other's half-written file into place.
+        # mkstemp creates it 0600 from the outset -- writing then chmod'ing
+        # would leave the refresh token world-readable for the gap between.
+        fd, name = tempfile.mkstemp(
+            dir=self._path.parent, prefix=f".{self._path.name}.", suffix=".tmp"
+        )
+        temp = Path(name)
+        try:
+            with os.fdopen(fd, "w", encoding="utf-8") as handle:
+                handle.write(json.dumps(credentials.to_json()))
+            temp.replace(self._path)
+        except BaseException:
+            temp.unlink(missing_ok=True)
+            raise
 
     def clear(self) -> None:
         """Delete the credentials file if it exists."""
diff --git a/crdt-sync/crdt_sync/tests/test_firebase_auth_session.py b/crdt-sync/crdt_sync/tests/test_firebase_auth_session.py
index 848d282..a50181c 100644
--- a/crdt-sync/crdt_sync/tests/test_firebase_auth_session.py
+++ b/crdt-sync/crdt_sync/tests/test_firebase_auth_session.py
@@ -21,6 +21,7 @@ from crdt_sync import (
     FirebaseTokenProvider,
     MemoryCredentialStore,
 )
+from crdt_sync import _credentials as fa_credentials
 from crdt_sync import _firebase_auth as fa
 
 if TYPE_CHECKING:
@@ -172,3 +173,47 @@ class TestFileCredentialStore:
         loaded = store.load()
         assert loaded is not None
         assert loaded.id_token == _ID_2
+
+    def test_leaves_no_temp_file_behind(self, tmp_path: Path) -> None:
+        """A finished save leaves only the credentials file in the directory."""
+        path = tmp_path / "creds.json"
+        FileCredentialStore(path).save(_credentials())
+        assert [p.name for p in tmp_path.iterdir()] == ["creds.json"]
+
+    def test_each_write_uses_its_own_temp_file(self, tmp_path: Path) -> None:
+        """Two writers never share a temp name, so neither renames the other's.
+
+        A fixed ``creds.json.tmp`` let a stream and a periodic sync writing
+        the same cache collide; mkstemp hands each save a fresh name.
+        """
+        path = tmp_path / "creds.json"
+        seen: list[str] = []
+        real_mkstemp = fa_credentials.tempfile.mkstemp
+
+        def spy(**kwargs: str | Path) -> tuple[int, str]:
+            fd, name = real_mkstemp(**kwargs)
+            seen.append(name)
+            return fd, name
+
+        with patch.object(fa_credentials.tempfile, "mkstemp", side_effect=spy):
+            FileCredentialStore(path).save(_credentials())
+            FileCredentialStore(path).save(_credentials(id_value=_ID_2))
+        assert len(set(seen)) == 2
+        assert all(name != f"{path}.tmp" for name in seen)
+
+    def test_a_failed_write_keeps_the_old_file_and_cleans_up(
+        self, tmp_path: Path
+    ) -> None:
+        """An interrupted save neither clobbers the cache nor leaks a temp."""
+        path = tmp_path / "creds.json"
+        store = FileCredentialStore(path)
+        store.save(_credentials())
+        with (
+            patch.object(fa_credentials.json, "dumps", side_effect=OSError("full")),
+            pytest.raises(OSError, match="full"),
+        ):
+            store.save(_credentials(id_value=_ID_2))
+        assert [p.name for p in tmp_path.iterdir()] == ["creds.json"]
+        loaded = store.load()
+        assert loaded is not None
+        assert loaded.id_token == _ID_1
diff --git a/crdt-sync/pyproject.toml b/crdt-sync/pyproject.toml
index 6b7a13f..7fb4cd4 100644
--- a/crdt-sync/pyproject.toml
+++ b/crdt-sync/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "crdt-sync"
-version = "0.9.1"
+version = "0.9.2"
 description = "Shared CRDT merge scheme + GitHub-Contents-API sync transport for cross-device apps"
 requires-python = ">=3.14"
 dependencies = ["requests==2.34.2"]