diff --git a/backend/package-lock.json b/backend/package-lock.json
index e435f21c..6b07f232 100644
--- a/backend/package-lock.json
+++ b/backend/package-lock.json
@@ -2753,67 +2753,6 @@
         "url": "https://opencollective.com/mongoose"
       }
     },
-    "node_modules/mongoose/node_modules/agent-base": {
-      "version": "6.0.2",
-      "resolved": "https://registry.npmjs.org/agent-base/-/agent-base-6.0.2.tgz",
-      "integrity": "sha512-RZNwNclF7+MS/8bDg70amg32dyeZGZxiDuQmZxKLAlQjr3jGyLx+4Kkk58UO7D2QdgFIQCovuSuZESne6RG6XQ==",
-      "license": "MIT",
-      "optional": true,
-      "peer": true,
-      "dependencies": {
-        "debug": "4"
-      },
-      "engines": {
-        "node": ">= 6.0.0"
-      }
-    },
-    "node_modules/mongoose/node_modules/gaxios": {
-      "version": "5.1.3",
-      "resolved": "https://registry.npmjs.org/gaxios/-/gaxios-5.1.3.tgz",
-      "integrity": "sha512-95hVgBRgEIRQQQHIbnxBXeHbW4TqFk4ZDJW7wmVtvYar72FdhRIo1UGOLS2eRAKCPEdPBWu+M7+A33D9CdX9rA==",
-      "license": "Apache-2.0",
-      "optional": true,
-      "peer": true,
-      "dependencies": {
-        "extend": "^3.0.2",
-        "https-proxy-agent": "^5.0.0",
-        "is-stream": "^2.0.0",
-        "node-fetch": "^2.6.9"
-      },
-      "engines": {
-        "node": ">=12"
-      }
-    },
-    "node_modules/mongoose/node_modules/gcp-metadata": {
-      "version": "5.3.0",
-      "resolved": "https://registry.npmjs.org/gcp-metadata/-/gcp-metadata-5.3.0.tgz",
-      "integrity": "sha512-FNTkdNEnBdlqF2oatizolQqNANMrcqJt6AAYt99B3y1aLLC8Hc5IOBb+ZnnzllodEEf6xMBp6wRcBbc16fa65w==",
-      "license": "Apache-2.0",
-      "optional": true,
-      "peer": true,
-      "dependencies": {
-        "gaxios": "^5.0.0",
-        "json-bigint": "^1.0.0"
-      },
-      "engines": {
-        "node": ">=12"
-      }
-    },
-    "node_modules/mongoose/node_modules/https-proxy-agent": {
-      "version": "5.0.1",
-      "resolved": "https://registry.npmjs.org/https-proxy-agent/-/https-proxy-agent-5.0.1.tgz",
-      "integrity": "sha512-dFcAjpTQFgoLMzC2VwU+C/CbS7uRL0lWmxDITmqm7C+7F0Odmj6s9l6alZc6AELXhrnggM2CeWSXHGOdX2YtwA==",
-      "license": "MIT",
-      "optional": true,
-      "peer": true,
-      "dependencies": {
-        "agent-base": "6",
-        "debug": "4"
-      },
-      "engines": {
-        "node": ">= 6"
-      }
-    },
     "node_modules/mongoose/node_modules/mongodb": {
       "version": "6.20.0",
       "resolved": "https://registry.npmjs.org/mongodb/-/mongodb-6.20.0.tgz",
diff --git a/backend/tests/achievementController.unit.test.js b/backend/tests/achievementController.unit.test.js
index e69de29b..c831a906 100644
--- a/backend/tests/achievementController.unit.test.js
+++ b/backend/tests/achievementController.unit.test.js
@@ -0,0 +1,165 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+const User = require('../models/User');
+const { getAchievements, saveAchievements } = require('../controllers/achievementController');
+
+const VALID_USER_ID = '507f1f77bcf86cd799439011';
+
+beforeEach(() => {
+  vi.restoreAllMocks();
+  User.findById = vi.fn();
+  User.findByIdAndUpdate = vi.fn();
+});
+
+function makeReq(body = {}) {
+  return {
+    user: { _id: VALID_USER_ID },
+    body,
+  };
+}
+
+function makeRes() {
+  const res = {};
+  res.status = vi.fn().mockReturnValue(res);
+  res.json = vi.fn().mockReturnValue(res);
+  return res;
+}
+
+describe('achievementController', () => {
+  describe('getAchievements', () => {
+    it('returns 404 if user is not found', async () => {
+      User.findById.mockReturnValue({
+        select: vi.fn().mockResolvedValue(null),
+      });
+
+      const req = makeReq();
+      const res = makeRes();
+
+      await getAchievements(req, res);
+
+      expect(res.status).toHaveBeenCalledWith(404);
+      expect(res.json).toHaveBeenCalledWith({ success: false, error: 'User not found' });
+    });
+
+    it('returns user achievements and resets streak if diffDays > 1', async () => {
+      const mockUser = {
+        _id: VALID_USER_ID,
+        unlockedAchievements: ['First Interview'],
+        lastPracticeDate: new Date(Date.now() - 3 * 24 * 60 * 60 * 1000), // 3 days ago
+        currentStreak: 5,
+        save: vi.fn().mockResolvedValue(true),
+      };
+
+      User.findById.mockReturnValue({
+        select: vi.fn().mockResolvedValue(mockUser),
+      });
+
+      const req = makeReq();
+      const res = makeRes();
+
+      await getAchievements(req, res);
+
+      expect(mockUser.currentStreak).toBe(0);
+      expect(mockUser.save).toHaveBeenCalled();
+      expect(res.json).toHaveBeenCalledWith({
+        success: true,
+        unlockedAchievements: ['First Interview'],
+      });
+    });
+
+    it('does not reset streak if last practice was within 1 day', async () => {
+      const mockUser = {
+        _id: VALID_USER_ID,
+        unlockedAchievements: ['Interview Pro'],
+        lastPracticeDate: new Date(), // today
+        currentStreak: 5,
+        save: vi.fn().mockResolvedValue(true),
+      };
+
+      User.findById.mockReturnValue({
+        select: vi.fn().mockResolvedValue(mockUser),
+      });
+
+      const req = makeReq();
+      const res = makeRes();
+
+      await getAchievements(req, res);
+
+      expect(mockUser.currentStreak).toBe(5);
+      expect(mockUser.save).not.toHaveBeenCalled();
+      expect(res.json).toHaveBeenCalledWith({
+        success: true,
+        unlockedAchievements: ['Interview Pro'],
+      });
+    });
+
+    it('returns 500 when database error occurs', async () => {
+      User.findById.mockReturnValue({
+        select: vi.fn().mockRejectedValue(new Error('DB Error')),
+      });
+
+      const req = makeReq();
+      const res = makeRes();
+
+      await getAchievements(req, res);
+
+      expect(res.status).toHaveBeenCalledWith(500);
+      expect(res.json).toHaveBeenCalledWith({ success: false, error: 'A server error occurred' });
+    });
+  });
+
+  describe('saveAchievements', () => {
+    it('returns 400 if unlockedAchievements is missing or not an array', async () => {
+      const req = makeReq({ unlockedAchievements: 'First Interview' });
+      const res = makeRes();
+
+      await saveAchievements(req, res);
+
+      expect(res.status).toHaveBeenCalledWith(400);
+      expect(res.json).toHaveBeenCalledWith({
+        success: false,
+        error: 'unlockedAchievements must be a valid array',
+      });
+    });
+
+    it('returns 400 if unlockedAchievements contains invalid IDs', async () => {
+      const req = makeReq({ unlockedAchievements: ['Invalid Achievement'] });
+      const res = makeRes();
+
+      await saveAchievements(req, res);
+
+      expect(res.status).toHaveBeenCalledWith(400);
+      expect(res.json).toHaveBeenCalledWith({
+        success: false,
+        error: 'Unknown achievement ID(s): Invalid Achievement',
+      });
+    });
+
+    it('successfully saves achievements for a valid request', async () => {
+      User.findByIdAndUpdate.mockResolvedValue({});
+
+      const req = makeReq({ unlockedAchievements: ['First Interview', 'Interview Pro'] });
+      const res = makeRes();
+
+      await saveAchievements(req, res);
+
+      expect(User.findByIdAndUpdate).toHaveBeenCalledWith(
+        VALID_USER_ID,
+        { $addToSet: { unlockedAchievements: { $each: ['First Interview', 'Interview Pro'] } } }
+      );
+      expect(res.json).toHaveBeenCalledWith({ success: true });
+    });
+
+    it('returns 500 when database error occurs during save', async () => {
+      User.findByIdAndUpdate.mockRejectedValue(new Error('DB Error'));
+
+      const req = makeReq({ unlockedAchievements: ['First Interview'] });
+      const res = makeRes();
+
+      await saveAchievements(req, res);
+
+      expect(res.status).toHaveBeenCalledWith(500);
+      expect(res.json).toHaveBeenCalledWith({ success: false, error: 'A server error occurred' });
+    });
+  });
+});