diff --git a/package.json b/package.json
index 0ab1e41b..14623c0e 100644
--- a/package.json
+++ b/package.json
@@ -5,6 +5,8 @@
   "scripts": {
     "build": "pnpm --filter vinext run build",
     "test": "vitest run",
+    "test:unit": "vitest run --project unit",
+    "test:integration": "vitest run --project integration",
     "test:watch": "vitest",
     "generate:google-fonts": "node scripts/generate-google-fonts.js",
     "typecheck": "tsgo --noEmit",
diff --git a/vitest.config.ts b/vitest.config.ts
index c1c376aa..29c03a1e 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -1,24 +1,62 @@
 import { randomUUID } from "node:crypto";
 import { defineConfig } from "vitest/config";
 
+/**
+ * Integration test files that spin up Vite dev servers against shared fixture
+ * directories. These must run serially to avoid Vite deps optimizer cache
+ * races (node_modules/.vite/*) that produce "outdated pre-bundle" 500s.
+ *
+ * When adding a new test file that calls startFixtureServer(), createServer()
+ * from Vite, or spawns a dev server subprocess, add it here.
+ */
+const integrationTests = [
+  "tests/app-router.test.ts",
+  "tests/pages-router.test.ts",
+  "tests/features.test.ts",
+  "tests/cjs.test.ts",
+  "tests/ecosystem.test.ts",
+  "tests/static-export.test.ts",
+  "tests/postcss-resolve.test.ts",
+  "tests/nextjs-compat/**/*.test.ts",
+];
+
+// GitHub Actions reporter adds inline failure annotations in PR diffs.
+const reporters: string[] = process.env.CI
+  ? ["default", "github-actions"]
+  : ["default"];
+
+const env = {
+  // Mirrors the Vite `define` in index.ts that inlines a build-time UUID.
+  // Setting it here means tests exercise the same code path as production.
+  __VINEXT_DRAFT_SECRET: randomUUID(),
+};
+
 export default defineConfig({
   test: {
-    include: ["tests/**/*.test.ts"],
-    testTimeout: 30000,
-    env: {
-      // Mirrors the Vite `define` in index.ts that inlines a build-time UUID.
-      // Setting it here means tests exercise the same code path as production.
-      __VINEXT_DRAFT_SECRET: randomUUID(),
-    },
-    // Multiple suites spin up Vite dev servers against the same fixture dirs.
-    // Running test files in parallel can race on Vite's deps optimizer cache
-    // (node_modules/.vite/*) and produce "outdated pre-bundle" 500s.
+    reporters,
+    // fileParallelism is a global setting in vitest 3.x (cannot be set
+    // per-project). Integration tests need serial execution, so we keep
+    // it disabled globally. The win from the project split is that
+    // `vitest run --project unit` lets you skip integration tests entirely
+    // for a fast feedback loop (~seconds vs ~2 minutes).
     fileParallelism: false,
-    // GitHub Actions reporter adds inline failure annotations in PR diffs.
-    // It's auto-enabled with the default reporter, but being explicit ensures
-    // it survives any future reporter config changes.
-    reporters: process.env.CI
-      ? ["default", "github-actions"]
-      : ["default"],
+    projects: [
+      {
+        test: {
+          name: "unit",
+          include: ["tests/**/*.test.ts"],
+          exclude: integrationTests,
+          env,
+        },
+      },
+      {
+        test: {
+          name: "integration",
+          include: integrationTests,
+          testTimeout: 30000,
+          env,
+        },
+      },
+    ],
   },
 });