diff --git a/src/shared/shared/three_cx_client.py b/src/shared/shared/three_cx_client.py
index c579fc5..d463f69 100644
--- a/src/shared/shared/three_cx_client.py
+++ b/src/shared/shared/three_cx_client.py
@@ -47,6 +47,7 @@ class ThreeCXClient:
         self._auth_mode = auth_mode
         self._client_id = auth_kwargs.get("client_id")
         self._client_secret = auth_kwargs.get("client_secret")
+        self._retired_secrets: set[str] = set()
         self._token_expires_at = 0.0
         self._auth_lock = threading.RLock()
         self.session = requests.Session()
@@ -65,12 +66,22 @@ class ThreeCXClient:
             self._authenticate_user(auth_kwargs["username"], auth_kwargs["password"])
 
     def use_client_secret(self, client_secret: str) -> None:
-        """Point this client at ``client_secret`` and log in again if needed."""
-        if client_secret != self._client_secret:
-            with self._auth_lock:
+        """Point this client at ``client_secret`` and log in again if needed.
+
+        The compare-and-set runs under ``_auth_lock``. A secret this client has
+        already replaced is ignored, so a slower caller still holding the
+        pre-rotation secret cannot write it back.
+        """
+        with self._auth_lock:
+            if (
+                client_secret != self._client_secret
+                and client_secret not in self._retired_secrets
+            ):
+                if self._client_secret:
+                    self._retired_secrets.add(self._client_secret)
                 self._client_secret = client_secret
                 self._token_expires_at = 0.0
-        self.ensure_fresh_token()
+            self._refresh_expired_token()
 
     def ensure_fresh_token(self) -> None:
         """Fetch a new access token when the current one is missing or near expiry."""
@@ -79,9 +90,15 @@ class ThreeCXClient:
         if time.monotonic() < self._token_expires_at:
             return
         with self._auth_lock:
-            if time.monotonic() < self._token_expires_at:
-                return
-            self._authenticate_oauth(self._client_id, self._client_secret)
+            self._refresh_expired_token()
+
+    def _refresh_expired_token(self) -> None:
+        """Log in again when the token is due. Caller holds ``_auth_lock``."""
+        if self._auth_mode != "oauth":
+            return
+        if time.monotonic() < self._token_expires_at:
+            return
+        self._authenticate_oauth(self._client_id, self._client_secret)
 
     def _request(self, method, url, **kwargs):
         if self._auth_mode == "oauth":
diff --git a/tests/shared/test_three_cx_client.py b/tests/shared/test_three_cx_client.py
index c1595cb..0a4ff5a 100644
--- a/tests/shared/test_three_cx_client.py
+++ b/tests/shared/test_three_cx_client.py
@@ -286,8 +286,9 @@ def test_oauth_client_reauths_when_client_secret_changes():
     )
     first = tcx.oauth_client("test.3cx.us", "cid", "old-secret")
     second = tcx.oauth_client("test.3cx.us", "cid", "new-secret")
-    assert first is second
-    assert second.session.headers["Authorization"] == "Bearer tok-new"
+    stale = tcx.oauth_client("test.3cx.us", "cid", "old-secret")
+    assert first is second is stale
+    assert stale.session.headers["Authorization"] == "Bearer tok-new"
     posts = _token_posts()
     assert len(posts) == 2
     assert "client_secret=new-secret" in posts[1].request.body