diff --git a/vitest.config.ts b/vitest.config.ts
index ad09a1b..c30412e 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -10,5 +10,18 @@ export default defineConfig({
   test: {
     environment: "node",
     setupFiles: ["./tests/setup.ts"],
+    // Each test file spins up its own throwaway Postgres database by
+    // mutating process.env.DATABASE_URL in tests/setup.ts's beforeAll (see
+    // that file). That per-file DB switch is not safe under Vitest's default
+    // parallel file execution: two files' beforeAll/query timing can
+    // interleave such that both end up issuing real queries against the
+    // same throwaway DB (confirmed empirically once Request gained a real
+    // FK to Employee — tests/employees.test.ts and tests/requests.test.ts
+    // started intermittently deleting/reading each other's rows, causing
+    // spurious FK-violation and row-count failures that disappear when run
+    // sequentially). Disabling file parallelism trades a bit of wall-clock
+    // test time for correctness — each file's throwaway DB is now genuinely
+    // private for the duration of that file's run.
+    fileParallelism: false,
   },
 });