diff --git a/src/vitest.js b/src/vitest.js
index daa1460..6fc9c32 100644
--- a/src/vitest.js
+++ b/src/vitest.js
@@ -7,6 +7,9 @@ export default function (options = {}) {
         name: '@hapi/oxc-plugin:vitest',
         async configureVitest({ injectTestProjects }) {
             await injectTestProjects({
+                // Vitest 5 makes inline projects extend the declaring config by default, which
+                // would run the host's own tests a 2nd time under this project.
+                extends: false,
                 define: {
                     __HAPI_OXC_OXLINT__: oxlint,
                     __HAPI_OXC_OXFMT__: oxfmt,
diff --git a/test/vitest.js b/test/vitest.js
index 4d621cf..f0a2b47 100644
--- a/test/vitest.js
+++ b/test/vitest.js
@@ -32,6 +32,7 @@ describe('vitest plugin', () => {
         const injected = await internals.inject(plugin);
 
         expect(injected).toHaveLength(1);
+        expect(injected[0].extends).toBe(false);
         expect(injected[0].test.name).toBe('@hapi/oxc-plugin');
         expect(injected[0].test.include).toHaveLength(1);
         expect(injected[0].test.include[0]).toMatch(/vitest\.test\.js$/);