diff --git a/src/vitest.config.ts b/src/vitest.config.ts
index 74779798c..7f285414b 100644
--- a/src/vitest.config.ts
+++ b/src/vitest.config.ts
@@ -13,27 +13,52 @@ export default defineConfig({
     ],
   },
   test: {
-    globals: true,
-    setupFiles: ['./tests/backend/vitest.setup.ts'],
-    include: [
-      'tests/backend-new/specs/**/*.ts',
-      'tests/backend/specs/**/*.ts',
+    projects: [
+      {
+        extends: true,
+        test: {
+          name: 'unit',
+          globals: true,
+          setupFiles: ['./tests/backend/vitest.setup.ts'],
+          include: ['tests/backend-new/specs/**/*.ts'],
+          hookTimeout: 60000,
+          testTimeout: 60000,
+          // Unit tests use vi.mock heavily — they NEED isolation. Each
+          // file gets a fresh module graph so mocks declared at the top
+          // of the file actually apply.
+          isolate: true,
+          fileParallelism: true,
+          pool: 'forks',
+          sequence: {
+            // Run unit project first (group 1), then integration (group 2).
+            // Different groupOrder is required when projects have different maxWorkers.
+            groupOrder: 1,
+          },
+        },
+      },
+      {
+        extends: true,
+        test: {
+          name: 'integration',
+          globals: true,
+          setupFiles: ['./tests/backend/vitest.setup.ts'],
+          include: ['tests/backend/specs/**/*.ts'],
+          hookTimeout: 60000,
+          testTimeout: 120000,
+          // Backend tests share a single Etherpad server instance + rustydb file.
+          // Vitest's default parallel/isolated workers each boot their own server
+          // and crash the second-to-open with `Error: DatabaseAlreadyOpen`. Force
+          // one fork, sequential file execution, no per-file isolation — same
+          // effective model as the old mocha runner.
+          pool: 'forks',
+          fileParallelism: false,
+          isolate: false,
+          sequence: {
+            // Run after the unit project (group 2 runs after group 1).
+            groupOrder: 2,
+          },
+        },
+      },
     ],
-    // Container tests (tests/container/specs/**/*.ts) are excluded from
-    // the default include because they target a separately-booted Etherpad
-    // process (the docker image, port 9001) and ECONNREFUSED locally. They
-    // are invoked explicitly by the `test-container` script which passes
-    // its own include via --include.
-    hookTimeout: 60000,
-    testTimeout: 120000,
-    // Backend tests share a single Etherpad server instance + rustydb file.
-    // Vitest's default parallel/isolated workers each boot their own server
-    // and crash the second-to-open with `Error: DatabaseAlreadyOpen`. Mocha
-    // never hit this because everything ran in one process. Force one fork,
-    // sequential file execution, no per-file isolation — same effective
-    // model as the old mocha runner.
-    pool: 'forks',
-    fileParallelism: false,
-    isolate: false,
   },
 });