diff --git a/apps/server/src/services/llm/providers/stream_handler.spec.ts b/apps/server/src/services/llm/providers/stream_handler.spec.ts
index 550a69ab2..0a20100db 100644
--- a/apps/server/src/services/llm/providers/stream_handler.spec.ts
+++ b/apps/server/src/services/llm/providers/stream_handler.spec.ts
@@ -1,4 +1,4 @@
-import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { describe, it, expect, vi, beforeEach, Mock } from 'vitest';
 import { StreamProcessor, createStreamHandler, processProviderStream, extractStreamStats, performProviderHealthCheck } from './stream_handler.js';
 import type { StreamProcessingOptions, StreamChunk } from './stream_handler.js';
 
@@ -12,11 +12,11 @@ vi.mock('../../log.js', () => ({
 }));
 
 describe('StreamProcessor', () => {
-    let mockCallback: ReturnType<typeof vi.fn>;
+    let mockCallback: Mock<(text: string, done: boolean, chunk?: any) => Promise<void> | void>;
     let mockOptions: StreamProcessingOptions;
 
     beforeEach(() => {
-        mockCallback = vi.fn();
+        mockCallback = vi.fn<(text: string, done: boolean, chunk?: any) => Promise<void> | void>();
         mockOptions = {
             streamCallback: mockCallback,
             providerName: 'TestProvider',
@@ -262,7 +262,7 @@ describe('createStreamHandler', () => {
 
 describe('processProviderStream', () => {
     let mockStreamIterator: AsyncIterable<any>;
-    let mockCallback: ReturnType<typeof vi.fn>;
+    let mockCallback: Mock<(text: string, done: boolean, chunk?: any) => Promise<void> | void>;
 
     beforeEach(() => {
         mockCallback = vi.fn();
diff --git a/apps/website/package.json b/apps/website/package.json
index cf27abca5..c565a4c70 100644
--- a/apps/website/package.json
+++ b/apps/website/package.json
@@ -22,7 +22,8 @@
 		"eslint-config-preact": "2.0.0",
 		"typescript": "5.9.3",
 		"user-agent-data-types": "0.4.2",
-		"vite": "7.1.12"
+		"vite": "7.1.12",
+		"vitest": "4.0.6"
 	},
 	"eslintConfig": {
 		"extends": "preact"
diff --git a/apps/website/vite.config.ts b/apps/website/vite.config.ts
index 02daafee3..becf0a246 100644
--- a/apps/website/vite.config.ts
+++ b/apps/website/vite.config.ts
@@ -1,4 +1,4 @@
-import { defineConfig } from 'vite';
+import { defineConfig } from 'vitest/config';
 import preact from '@preact/preset-vite';
 
 // https://vitejs.dev/config/
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 3bd808a45..dee3e0f53 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -820,6 +820,9 @@ importers:
       vite:
         specifier: 7.1.12
         version: 7.1.12(@types/node@24.9.1)(jiti@2.6.1)(less@4.1.3)(lightningcss@1.30.1)(sass-embedded@1.91.0)(sass@1.91.0)(terser@5.44.0)(tsx@4.20.6)(yaml@2.8.1)
+      vitest:
+        specifier: 4.0.6
+        version: 4.0.6(@types/debug@4.1.12)(@types/node@24.9.1)(@vitest/browser-webdriverio@4.0.6)(@vitest/ui@4.0.6)(happy-dom@20.0.8)(jiti@2.6.1)(jsdom@26.1.0(bufferutil@4.0.9)(utf-8-validate@6.0.5))(less@4.1.3)(lightningcss@1.30.1)(msw@2.7.5(@types/node@24.9.1)(typescript@5.9.3))(sass-embedded@1.91.0)(sass@1.91.0)(terser@5.44.0)(tsx@4.20.6)(yaml@2.8.1)
 
   packages/ckeditor5:
     dependencies: