diff --git a/core/archipelago/src/update.rs b/core/archipelago/src/update.rs
index c08476e5..3e6e22f3 100644
--- a/core/archipelago/src/update.rs
+++ b/core/archipelago/src/update.rs
@@ -2565,8 +2565,14 @@ mod tests {
         assert!(!persisted.update_in_progress);
     }
 
+    /// apply_update takes the global single-flight UPDATE_OP_LOCK, so tests
+    /// that call it must not run concurrently — one would see the other's
+    /// lock and fail with "another update operation is already running".
+    static APPLY_TEST_SERIAL: tokio::sync::Mutex<()> = tokio::sync::Mutex::const_new(());
+
     #[tokio::test]
     async fn test_apply_refuses_unmarked_staging() {
+        let _serial = APPLY_TEST_SERIAL.lock().await;
         // Regression: .198 v1.7.103 bricking — apply ran against a staging
         // dir that a concurrent download was still filling. Without the
         // .download-complete marker, apply must refuse before touching
@@ -2586,6 +2592,7 @@ mod tests {
 
     #[tokio::test]
     async fn test_apply_refuses_staged_bytes_that_mismatch_manifest() {
+        let _serial = APPLY_TEST_SERIAL.lock().await;
         // Marker present (a complete download once existed) but the staged
         // bytes no longer match the manifest — apply must re-verify and
         // refuse rather than install whatever is on disk.