diff --git a/packages/client/vitest.integration.config.ts b/packages/client/vitest.integration.config.ts
index a7a22522e1..1d03d78df6 100644
--- a/packages/client/vitest.integration.config.ts
+++ b/packages/client/vitest.integration.config.ts
@@ -7,12 +7,24 @@ export default defineConfig({
     environment: 'node',
     testTimeout: 30000, // 30 seconds for integration tests
     hookTimeout: 30000,
-    // Run integration tests sequentially to avoid race conditions
     pool: 'forks',
-    poolOptions: {
-      forks: {
-        singleFork: true
-      }
-    }
+    // Run integration test files one at a time: they all drive the SAME live
+    // server and the same test data, so running files in parallel races.
+    //
+    // Vitest 4 removed `test.poolOptions` ("Pool Rework" in the migration
+    // guide) — the `poolOptions.forks.singleFork: true` that used to live here
+    // was never read, it only produced a DEPRECATED warning, so this suite has
+    // been running with the default parallelism all along (#5564).
+    //
+    // `fileParallelism: false` is the top-level option that actually enforces
+    // it: per the docs it "will override `maxWorkers` option to 1", which is
+    // the parallelism half of what the guide maps `singleFork` to.
+    //
+    // The guide's other half — `isolate: false` — is deliberately NOT carried
+    // over: it was equally inert here, nothing in this suite depends on
+    // sharing a module registry across files, and turning it off would let
+    // module state leak between files, which is the very class of cross-file
+    // interference this declaration exists to prevent.
+    fileParallelism: false
   }
 });