diff --git a/vitest.config.ts b/vitest.config.ts
index 57734df..ad0b36b 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -10,5 +10,11 @@ export default defineConfig({
     testTimeout: 15000,
     hookTimeout: 15000,
     exclude: ["dist/**", "node_modules/**", ".claude/**"],
+    // Tests run against a real shared MySQL test DB. Running test files in
+    // parallel lets two files (e.g. numbering + manual-create) hit the same
+    // `number_sequences` row with SELECT ... FOR UPDATE + deleteMany at once,
+    // which deadlocks (MySQL 1213). Serialise files so DB-touching suites
+    // don't contend. Tests within a file already run sequentially.
+    fileParallelism: false,
   },
 });