diff --git a/apps/skilltree/apps/backend/jest.config.js b/apps/skilltree/apps/backend/jest.config.js
new file mode 100644
index 000000000..702581f02
--- /dev/null
+++ b/apps/skilltree/apps/backend/jest.config.js
@@ -0,0 +1,15 @@
+/** @type {import('jest').Config} */
+module.exports = {
+	moduleFileExtensions: ['js', 'json', 'ts'],
+	rootDir: 'src',
+	testRegex: '.*\\.spec\\.ts$',
+	transform: {
+		'^.+\\.(t|j)s$': 'ts-jest',
+	},
+	collectCoverageFrom: ['**/*.(t|j)s', '!**/*.module.ts', '!**/main.ts', '!**/*.dto.ts'],
+	coverageDirectory: '../coverage',
+	testEnvironment: 'node',
+	moduleNameMapper: {
+		'^src/(.*)$': '<rootDir>/$1',
+	},
+};
diff --git a/apps/skilltree/apps/backend/src/skill/skill.service.spec.ts b/apps/skilltree/apps/backend/src/skill/skill.service.spec.ts
new file mode 100644
index 000000000..01e9d3e9b
--- /dev/null
+++ b/apps/skilltree/apps/backend/src/skill/skill.service.spec.ts
@@ -0,0 +1,497 @@
+import { Test, TestingModule } from '@nestjs/testing';
+import { NotFoundException } from '@nestjs/common';
+import { SkillService } from './skill.service';
+import { DATABASE_TOKEN } from '../db/database.module';
+
+// Mock database operations
+// Uses a query builder pattern where each query chain is thenable
+const createMockDb = () => {
+	// Queue for resolved values - each await will pop from this queue
+	const resolveQueue: any[] = [];
+
+	// Create a thenable query result (only used for final await)
+	const createQueryResult = (): any => {
+		return {
+			then: (resolve: (value: any) => void, reject?: (reason: any) => void) => {
+				const value = resolveQueue.shift() ?? [];
+				return Promise.resolve(value).then(resolve, reject);
+			},
+		};
+	};
+
+	// The mock database object - NOT thenable itself
+	const mockDb: any = {
+		// Helper methods
+		_queueResult: (value: any) => {
+			resolveQueue.push(value);
+		},
+		_queueResults: (...values: any[]) => {
+			values.forEach((v) => resolveQueue.push(v));
+		},
+		_clearQueue: () => {
+			resolveQueue.length = 0;
+		},
+	};
+
+	// Create a query builder that returns thenable results
+	const createChainableMethod = () => {
+		const chainable: any = createQueryResult();
+		chainable.select = jest.fn(() => chainable);
+		chainable.from = jest.fn(() => chainable);
+		chainable.where = jest.fn(() => chainable);
+		chainable.orderBy = jest.fn(() => chainable);
+		chainable.limit = jest.fn(() => chainable);
+		chainable.returning = jest.fn(() => chainable);
+		chainable.insert = jest.fn(() => chainable);
+		chainable.values = jest.fn(() => chainable);
+		chainable.update = jest.fn(() => chainable);
+		chainable.set = jest.fn(() => chainable);
+		chainable.delete = jest.fn(() => chainable);
+		chainable.onConflictDoUpdate = jest.fn(() => chainable);
+		return chainable;
+	};
+
+	// Database entry points return new chainable builders
+	mockDb.select = jest.fn(() => createChainableMethod());
+	mockDb.insert = jest.fn(() => createChainableMethod());
+	mockDb.update = jest.fn(() => createChainableMethod());
+	mockDb.delete = jest.fn(() => createChainableMethod());
+
+	return mockDb;
+};
+
+describe('SkillService', () => {
+	let service: SkillService;
+	let mockDb: ReturnType<typeof createMockDb>;
+
+	const testUserId = 'test-user-123';
+	const testSkillId = 'skill-uuid-123';
+
+	const mockSkill = {
+		id: testSkillId,
+		userId: testUserId,
+		name: 'TypeScript',
+		description: 'Learn TypeScript programming',
+		branch: 'intellect',
+		parentId: null,
+		icon: 'code',
+		color: '#3178C6',
+		currentXp: 150,
+		totalXp: 150,
+		level: 1,
+		createdAt: new Date(),
+		updatedAt: new Date(),
+	};
+
+	beforeEach(async () => {
+		mockDb = createMockDb();
+
+		const module: TestingModule = await Test.createTestingModule({
+			providers: [
+				SkillService,
+				{
+					provide: DATABASE_TOKEN,
+					useValue: mockDb,
+				},
+			],
+		}).compile();
+
+		service = module.get<SkillService>(SkillService);
+	});
+
+	afterEach(() => {
+		jest.clearAllMocks();
+		mockDb._clearQueue();
+	});
+
+	describe('findAll', () => {
+		it('should return all skills for a user', async () => {
+			const skills = [mockSkill, { ...mockSkill, id: 'skill-2', name: 'JavaScript' }];
+			mockDb._queueResult(skills);
+
+			const result = await service.findAll(testUserId);
+
+			expect(result).toEqual(skills);
+			expect(mockDb.select).toHaveBeenCalled();
+		});
+
+		it('should return empty array when user has no skills', async () => {
+			mockDb._queueResult([]);
+
+			const result = await service.findAll(testUserId);
+
+			expect(result).toEqual([]);
+		});
+	});
+
+	describe('findByBranch', () => {
+		it('should return skills filtered by branch', async () => {
+			const intellectSkills = [mockSkill];
+			mockDb._queueResult(intellectSkills);
+
+			const result = await service.findByBranch(testUserId, 'intellect');
+
+			expect(result).toEqual(intellectSkills);
+		});
+
+		it('should return empty array for branch with no skills', async () => {
+			mockDb._queueResult([]);
+
+			const result = await service.findByBranch(testUserId, 'body');
+
+			expect(result).toEqual([]);
+		});
+	});
+
+	describe('findById', () => {
+		it('should return skill when found', async () => {
+			mockDb._queueResult([mockSkill]);
+
+			const result = await service.findById(testSkillId, testUserId);
+
+			expect(result).toEqual(mockSkill);
+		});
+
+		it('should return null when skill not found', async () => {
+			mockDb._queueResult([]);
+
+			const result = await service.findById('non-existent', testUserId);
+
+			expect(result).toBeNull();
+		});
+	});
+
+	describe('findByIdOrThrow', () => {
+		it('should return skill when found', async () => {
+			mockDb._queueResult([mockSkill]);
+
+			const result = await service.findByIdOrThrow(testSkillId, testUserId);
+
+			expect(result).toEqual(mockSkill);
+		});
+
+		it('should throw NotFoundException when skill not found', async () => {
+			mockDb._queueResult([]);
+
+			await expect(service.findByIdOrThrow('non-existent', testUserId)).rejects.toThrow(
+				NotFoundException
+			);
+		});
+	});
+
+	describe('create', () => {
+		const createDto = {
+			name: 'React',
+			description: 'Learn React framework',
+			branch: 'intellect' as const,
+			parentId: undefined,
+			icon: 'component',
+			color: '#61DAFB',
+		};
+
+		it('should create a new skill with default XP and level', async () => {
+			const createdSkill = {
+				...createDto,
+				id: 'new-skill-id',
+				userId: testUserId,
+				currentXp: 0,
+				totalXp: 0,
+				level: 0,
+			};
+
+			// Queue results in order of awaits:
+			// 1. insert().values().returning() -> [createdSkill]
+			// 2. updateUserStats: select().from(skills).where() -> [createdSkill]
+			// 3. updateUserStats: select().from(activities).where().orderBy().limit() -> []
+			// 4. calculateStreak: select().from(activities).where().orderBy() -> []
+			// 5. insert().values().onConflictDoUpdate() -> undefined
+			mockDb._queueResults(
+				[createdSkill], // 1. insert skill returning
+				[createdSkill], // 2. select skills
+				[], // 3. select activities (limit)
+				[], // 4. calculateStreak activities
+				undefined // 5. upsert stats
+			);
+
+			const result = await service.create(testUserId, createDto);
+
+			expect(result.name).toBe('React');
+			expect(result.currentXp).toBe(0);
+			expect(result.level).toBe(0);
+		});
+
+		it('should use default icon when not provided', async () => {
+			const dtoWithoutIcon = {
+				name: 'New Skill',
+				description: 'A skill',
+				branch: 'body' as const,
+				parentId: undefined,
+				color: undefined,
+			};
+
+			const createdSkill = {
+				...dtoWithoutIcon,
+				id: 'new-id',
+				userId: testUserId,
+				icon: 'star',
+				currentXp: 0,
+				totalXp: 0,
+				level: 0,
+			};
+
+			mockDb._queueResults([createdSkill], [createdSkill], [], [], undefined);
+
+			const result = await service.create(testUserId, dtoWithoutIcon);
+
+			expect(result.icon).toBe('star');
+		});
+	});
+
+	describe('update', () => {
+		const updateDto = {
+			name: 'Updated TypeScript',
+			description: 'Master TypeScript',
+		};
+
+		it('should update skill and return updated version', async () => {
+			const updatedSkill = { ...mockSkill, ...updateDto };
+
+			// Queue results:
+			// 1. findByIdOrThrow: select().from(skills).where() -> [mockSkill]
+			// 2. update().set().where().returning() -> [updatedSkill]
+			mockDb._queueResults([mockSkill], [updatedSkill]);
+
+			const result = await service.update(testSkillId, testUserId, updateDto);
+
+			expect(result.name).toBe('Updated TypeScript');
+			expect(result.description).toBe('Master TypeScript');
+		});
+
+		it('should throw NotFoundException when updating non-existent skill', async () => {
+			mockDb._queueResult([]);
+
+			await expect(service.update('non-existent', testUserId, updateDto)).rejects.toThrow(
+				NotFoundException
+			);
+		});
+	});
+
+	describe('delete', () => {
+		it('should delete skill successfully', async () => {
+			// Queue results:
+			// 1. findByIdOrThrow: select().from(skills).where() -> [mockSkill]
+			// 2. delete(skills).where() -> undefined
+			// 3. updateUserStats: select().from(skills).where() -> [] (empty after delete)
+			// 4. updateUserStats: select().from(activities).where().orderBy().limit() -> []
+			// 5. calculateStreak: select().from(activities).where().orderBy() -> []
+			// 6. insert().values().onConflictDoUpdate() -> undefined
+			mockDb._queueResults(
+				[mockSkill], // 1. findByIdOrThrow
+				undefined, // 2. delete
+				[], // 3. select skills
+				[], // 4. select activities (limit)
+				[], // 5. calculateStreak
+				undefined // 6. upsert stats
+			);
+
+			await expect(service.delete(testSkillId, testUserId)).resolves.not.toThrow();
+		});
+
+		it('should throw NotFoundException when deleting non-existent skill', async () => {
+			mockDb._queueResult([]);
+
+			await expect(service.delete('non-existent', testUserId)).rejects.toThrow(NotFoundException);
+		});
+	});
+
+	describe('addXp', () => {
+		const addXpDto = {
+			xp: 50,
+			description: 'Completed tutorial',
+			duration: 30,
+		};
+
+		it('should add XP and update skill level when threshold crossed', async () => {
+			// Skill at level 0 with 80 XP, adding 50 should reach level 1
+			const skillAt80Xp = { ...mockSkill, currentXp: 80, totalXp: 80, level: 0 };
+			const updatedSkill = {
+				...skillAt80Xp,
+				currentXp: 130,
+				totalXp: 130,
+				level: 1,
+			};
+			const recentActivity = { timestamp: new Date() };
+
+			// Queue results:
+			// 1. findByIdOrThrow: select().from(skills).where() -> [skillAt80Xp]
+			// 2. update(skills).set().where().returning() -> [updatedSkill]
+			// 3. insert(activities).values() -> undefined
+			// 4. updateUserStats: select().from(skills).where() -> [updatedSkill]
+			// 5. updateUserStats: select().from(activities).where().orderBy().limit() -> [activity]
+			// 6. calculateStreak: select().from(activities).where().orderBy() -> [activity]
+			// 7. insert().values().onConflictDoUpdate() -> undefined
+			mockDb._queueResults(
+				[skillAt80Xp], // 1
+				[updatedSkill], // 2
+				undefined, // 3
+				[updatedSkill], // 4
+				[recentActivity], // 5
+				[recentActivity], // 6
+				undefined // 7
+			);
+
+			const result = await service.addXp(testSkillId, testUserId, addXpDto);
+
+			expect(result.skill.totalXp).toBe(130);
+			expect(result.skill.level).toBe(1);
+			expect(result.leveledUp).toBe(true);
+			expect(result.newLevel).toBe(1);
+		});
+
+		it('should not level up when threshold not crossed', async () => {
+			// Skill at level 1 with 150 XP, adding 50 stays at level 1
+			const updatedSkill = {
+				...mockSkill,
+				currentXp: 200,
+				totalXp: 200,
+				level: 1,
+			};
+			const recentActivity = { timestamp: new Date() };
+
+			mockDb._queueResults(
+				[mockSkill], // findByIdOrThrow
+				[updatedSkill], // update skill
+				undefined, // insert activity
+				[updatedSkill], // select skills
+				[recentActivity], // select activities (limit)
+				[recentActivity], // calculateStreak
+				undefined // upsert stats
+			);
+
+			const result = await service.addXp(testSkillId, testUserId, addXpDto);
+
+			expect(result.leveledUp).toBe(false);
+			expect(result.newLevel).toBe(1);
+		});
+
+		it('should throw NotFoundException when adding XP to non-existent skill', async () => {
+			mockDb._queueResult([]);
+
+			await expect(service.addXp('non-existent', testUserId, addXpDto)).rejects.toThrow(
+				NotFoundException
+			);
+		});
+
+		it('should create activity record when adding XP', async () => {
+			const updatedSkill = { ...mockSkill, currentXp: 200, totalXp: 200 };
+
+			mockDb._queueResults(
+				[mockSkill], // findByIdOrThrow
+				[updatedSkill], // update skill
+				undefined, // insert activity
+				[updatedSkill], // select skills
+				[], // select activities (limit)
+				[], // calculateStreak
+				undefined // upsert stats
+			);
+
+			await service.addXp(testSkillId, testUserId, addXpDto);
+
+			expect(mockDb.insert).toHaveBeenCalled();
+		});
+	});
+
+	describe('getUserStats', () => {
+		it('should return user stats when they exist', async () => {
+			const stats = {
+				userId: testUserId,
+				totalXp: 500,
+				totalSkills: 5,
+				highestLevel: 2,
+				streakDays: 7,
+				lastActivityDate: '2026-01-28',
+			};
+			mockDb._queueResult([stats]);
+
+			const result = await service.getUserStats(testUserId);
+
+			expect(result).toEqual(stats);
+		});
+
+		it('should return default stats when none exist', async () => {
+			mockDb._queueResult([]);
+
+			const result = await service.getUserStats(testUserId);
+
+			expect(result).toEqual({
+				totalXp: 0,
+				totalSkills: 0,
+				highestLevel: 0,
+				streakDays: 0,
+				lastActivityDate: null,
+			});
+		});
+	});
+});
+
+describe('Level Calculation (Unit Tests)', () => {
+	// Test the calculateLevel function directly
+	const LEVEL_THRESHOLDS = [0, 100, 500, 1500, 4000, 10000];
+
+	function calculateLevel(xp: number): number {
+		for (let i = LEVEL_THRESHOLDS.length - 1; i >= 0; i--) {
+			if (xp >= LEVEL_THRESHOLDS[i]) {
+				return i;
+			}
+		}
+		return 0;
+	}
+
+	describe('calculateLevel', () => {
+		it.each([
+			[0, 0],
+			[50, 0],
+			[99, 0],
+			[100, 1],
+			[250, 1],
+			[499, 1],
+			[500, 2],
+			[1000, 2],
+			[1499, 2],
+			[1500, 3],
+			[3999, 3],
+			[4000, 4],
+			[9999, 4],
+			[10000, 5],
+			[50000, 5],
+		])('calculateLevel(%i) should return %i', (xp, expectedLevel) => {
+			expect(calculateLevel(xp)).toBe(expectedLevel);
+		});
+	});
+
+	describe('Level up detection', () => {
+		it('should detect level up from 0 to 1', () => {
+			const oldLevel = calculateLevel(90);
+			const newLevel = calculateLevel(110);
+			expect(oldLevel).toBe(0);
+			expect(newLevel).toBe(1);
+			expect(newLevel > oldLevel).toBe(true);
+		});
+
+		it('should not detect level up within same level', () => {
+			const oldLevel = calculateLevel(100);
+			const newLevel = calculateLevel(200);
+			expect(oldLevel).toBe(1);
+			expect(newLevel).toBe(1);
+			expect(newLevel > oldLevel).toBe(false);
+		});
+
+		it('should detect multiple level ups', () => {
+			const oldLevel = calculateLevel(0);
+			const newLevel = calculateLevel(600);
+			expect(oldLevel).toBe(0);
+			expect(newLevel).toBe(2);
+			expect(newLevel - oldLevel).toBe(2);
+		});
+	});
+});
diff --git a/apps/skilltree/apps/web/package.json b/apps/skilltree/apps/web/package.json
index 669146ff4..e8433a797 100644
--- a/apps/skilltree/apps/web/package.json
+++ b/apps/skilltree/apps/web/package.json
@@ -9,20 +9,26 @@
 		"prepare": "svelte-kit sync || echo ''",
 		"check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
 		"check:watch": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json --watch",
-		"type-check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json"
+		"type-check": "svelte-kit sync && svelte-check --tsconfig ./tsconfig.json",
+		"test": "vitest",
+		"test:run": "vitest run",
+		"test:coverage": "vitest run --coverage"
 	},
 	"devDependencies": {
 		"@sveltejs/adapter-node": "^5.0.0",
 		"@sveltejs/kit": "^2.0.0",
 		"@sveltejs/vite-plugin-svelte": "^5.0.0",
 		"@tailwindcss/vite": "^4.1.7",
+		"@testing-library/svelte": "^5.2.6",
 		"@types/node": "^20.0.0",
+		"jsdom": "^25.0.1",
 		"svelte": "^5.0.0",
 		"svelte-check": "^4.0.0",
 		"tailwindcss": "^4.1.7",
 		"tslib": "^2.4.1",
 		"typescript": "^5.0.0",
-		"vite": "^6.0.0"
+		"vite": "^6.0.0",
+		"vitest": "^4.0.18"
 	},
 	"dependencies": {
 		"@manacore/shared-auth": "workspace:*",
diff --git a/apps/skilltree/apps/web/src/lib/types/index.test.ts b/apps/skilltree/apps/web/src/lib/types/index.test.ts
new file mode 100644
index 000000000..5264de55d
--- /dev/null
+++ b/apps/skilltree/apps/web/src/lib/types/index.test.ts
@@ -0,0 +1,378 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import {
+	calculateLevel,
+	xpForNextLevel,
+	xpProgress,
+	createDefaultSkill,
+	createActivity,
+	LEVEL_THRESHOLDS,
+	LEVEL_NAMES,
+	BRANCH_INFO,
+	type Skill,
+	type SkillBranch,
+} from './index';
+
+describe('Level System', () => {
+	describe('calculateLevel', () => {
+		it('should return level 0 for 0 XP', () => {
+			expect(calculateLevel(0)).toBe(0);
+		});
+
+		it('should return level 0 for XP below 100', () => {
+			expect(calculateLevel(50)).toBe(0);
+			expect(calculateLevel(99)).toBe(0);
+		});
+
+		it('should return level 1 for XP between 100 and 499', () => {
+			expect(calculateLevel(100)).toBe(1);
+			expect(calculateLevel(250)).toBe(1);
+			expect(calculateLevel(499)).toBe(1);
+		});
+
+		it('should return level 2 for XP between 500 and 1499', () => {
+			expect(calculateLevel(500)).toBe(2);
+			expect(calculateLevel(1000)).toBe(2);
+			expect(calculateLevel(1499)).toBe(2);
+		});
+
+		it('should return level 3 for XP between 1500 and 3999', () => {
+			expect(calculateLevel(1500)).toBe(3);
+			expect(calculateLevel(2500)).toBe(3);
+			expect(calculateLevel(3999)).toBe(3);
+		});
+
+		it('should return level 4 for XP between 4000 and 9999', () => {
+			expect(calculateLevel(4000)).toBe(4);
+			expect(calculateLevel(7000)).toBe(4);
+			expect(calculateLevel(9999)).toBe(4);
+		});
+
+		it('should return level 5 (max) for XP 10000 and above', () => {
+			expect(calculateLevel(10000)).toBe(5);
+			expect(calculateLevel(50000)).toBe(5);
+			expect(calculateLevel(1000000)).toBe(5);
+		});
+
+		it('should handle negative XP by returning 0', () => {
+			expect(calculateLevel(-100)).toBe(0);
+		});
+	});
+
+	describe('xpForNextLevel', () => {
+		it('should return 100 XP for level 0', () => {
+			expect(xpForNextLevel(0)).toBe(100);
+		});
+
+		it('should return 500 XP for level 1', () => {
+			expect(xpForNextLevel(1)).toBe(500);
+		});
+
+		it('should return 1500 XP for level 2', () => {
+			expect(xpForNextLevel(2)).toBe(1500);
+		});
+
+		it('should return 4000 XP for level 3', () => {
+			expect(xpForNextLevel(3)).toBe(4000);
+		});
+
+		it('should return 10000 XP for level 4', () => {
+			expect(xpForNextLevel(4)).toBe(10000);
+		});
+
+		it('should return Infinity for max level (5)', () => {
+			expect(xpForNextLevel(5)).toBe(Infinity);
+		});
+
+		it('should return Infinity for levels beyond max', () => {
+			expect(xpForNextLevel(6)).toBe(Infinity);
+			expect(xpForNextLevel(100)).toBe(Infinity);
+		});
+	});
+
+	describe('xpProgress', () => {
+		it('should return 0% at level threshold', () => {
+			expect(xpProgress(0, 0)).toBe(0);
+			expect(xpProgress(100, 1)).toBe(0);
+			expect(xpProgress(500, 2)).toBe(0);
+		});
+
+		it('should return 50% at midpoint', () => {
+			// Level 0: 0-100, midpoint is 50
+			expect(xpProgress(50, 0)).toBe(50);
+			// Level 1: 100-500, midpoint is 300
+			expect(xpProgress(300, 1)).toBe(50);
+		});
+
+		it('should return close to 100% near next threshold', () => {
+			expect(xpProgress(99, 0)).toBeCloseTo(99, 0);
+			expect(xpProgress(499, 1)).toBeCloseTo(99.75, 1);
+		});
+
+		it('should return 100% for max level', () => {
+			expect(xpProgress(10000, 5)).toBe(100);
+			expect(xpProgress(50000, 5)).toBe(100);
+		});
+
+		it('should clamp progress between 0 and 100', () => {
+			// Progress should never exceed 100
+			expect(xpProgress(200, 0)).toBeLessThanOrEqual(100);
+			// Progress should never be negative
+			expect(xpProgress(-10, 0)).toBeGreaterThanOrEqual(0);
+		});
+
+		it('should calculate correct progress for level 2', () => {
+			// Level 2: 500-1500 (range of 1000)
+			// At 750 XP: (750-500)/(1500-500) = 250/1000 = 25%
+			expect(xpProgress(750, 2)).toBe(25);
+		});
+	});
+
+	describe('LEVEL_THRESHOLDS', () => {
+		it('should have 6 levels (0-5)', () => {
+			expect(LEVEL_THRESHOLDS).toHaveLength(6);
+		});
+
+		it('should start at 0', () => {
+			expect(LEVEL_THRESHOLDS[0]).toBe(0);
+		});
+
+		it('should be in ascending order', () => {
+			for (let i = 1; i < LEVEL_THRESHOLDS.length; i++) {
+				expect(LEVEL_THRESHOLDS[i]).toBeGreaterThan(LEVEL_THRESHOLDS[i - 1]);
+			}
+		});
+
+		it('should have expected values', () => {
+			expect(LEVEL_THRESHOLDS).toEqual([0, 100, 500, 1500, 4000, 10000]);
+		});
+	});
+
+	describe('LEVEL_NAMES', () => {
+		it('should have a name for each level', () => {
+			expect(LEVEL_NAMES).toHaveLength(6);
+		});
+
+		it('should have German names', () => {
+			expect(LEVEL_NAMES[0]).toBe('Unbekannt');
+			expect(LEVEL_NAMES[1]).toBe('Anfänger');
+			expect(LEVEL_NAMES[2]).toBe('Fortgeschritten');
+			expect(LEVEL_NAMES[3]).toBe('Kompetent');
+			expect(LEVEL_NAMES[4]).toBe('Experte');
+			expect(LEVEL_NAMES[5]).toBe('Meister');
+		});
+	});
+});
+
+describe('Branch System', () => {
+	describe('BRANCH_INFO', () => {
+		const branches: SkillBranch[] = [
+			'intellect',
+			'body',
+			'creativity',
+			'social',
+			'practical',
+			'mindset',
+			'custom',
+		];
+
+		it('should have info for all 7 branches', () => {
+			expect(Object.keys(BRANCH_INFO)).toHaveLength(7);
+		});
+
+		it.each(branches)('should have complete info for %s branch', (branch) => {
+			const info = BRANCH_INFO[branch];
+			expect(info).toBeDefined();
+			expect(info.name).toBeTruthy();
+			expect(info.icon).toBeTruthy();
+			expect(info.color).toBeTruthy();
+			expect(info.description).toBeTruthy();
+		});
+
+		it('should have German names', () => {
+			expect(BRANCH_INFO.intellect.name).toBe('Intellekt');
+			expect(BRANCH_INFO.body.name).toBe('Körper');
+			expect(BRANCH_INFO.creativity.name).toBe('Kreativität');
+			expect(BRANCH_INFO.social.name).toBe('Sozial');
+			expect(BRANCH_INFO.practical.name).toBe('Praktisch');
+			expect(BRANCH_INFO.mindset.name).toBe('Mindset');
+			expect(BRANCH_INFO.custom.name).toBe('Eigene');
+		});
+
+		it('should have correct icons', () => {
+			expect(BRANCH_INFO.intellect.icon).toBe('brain');
+			expect(BRANCH_INFO.body.icon).toBe('dumbbell');
+			expect(BRANCH_INFO.creativity.icon).toBe('palette');
+			expect(BRANCH_INFO.social.icon).toBe('users');
+			expect(BRANCH_INFO.practical.icon).toBe('wrench');
+			expect(BRANCH_INFO.mindset.icon).toBe('heart');
+			expect(BRANCH_INFO.custom.icon).toBe('star');
+		});
+	});
+});
+
+describe('Factory Functions', () => {
+	describe('createDefaultSkill', () => {
+		beforeEach(() => {
+			vi.useFakeTimers();
+			vi.setSystemTime(new Date('2026-01-28T12:00:00Z'));
+		});
+
+		it('should create a skill with default values', () => {
+			const skill = createDefaultSkill();
+
+			expect(skill.id).toBeTruthy();
+			expect(skill.name).toBe('');
+			expect(skill.description).toBe('');
+			expect(skill.branch).toBe('custom');
+			expect(skill.parentId).toBeNull();
+			expect(skill.icon).toBe('star');
+			expect(skill.color).toBeNull();
+			expect(skill.currentXp).toBe(0);
+			expect(skill.totalXp).toBe(0);
+			expect(skill.level).toBe(0);
+		});
+
+		it('should generate unique IDs', () => {
+			const skill1 = createDefaultSkill();
+			const skill2 = createDefaultSkill();
+			expect(skill1.id).not.toBe(skill2.id);
+		});
+
+		it('should set timestamps', () => {
+			const skill = createDefaultSkill();
+			expect(skill.createdAt).toBeTruthy();
+			expect(skill.updatedAt).toBeTruthy();
+			expect(skill.createdAt).toBe(skill.updatedAt);
+		});
+
+		it('should merge partial values', () => {
+			const skill = createDefaultSkill({
+				name: 'TypeScript',
+				description: 'Learn TypeScript',
+				branch: 'intellect',
+				totalXp: 500,
+				level: 2,
+			});
+
+			expect(skill.name).toBe('TypeScript');
+			expect(skill.description).toBe('Learn TypeScript');
+			expect(skill.branch).toBe('intellect');
+			expect(skill.totalXp).toBe(500);
+			expect(skill.level).toBe(2);
+			// Default values should still be set
+			expect(skill.icon).toBe('star');
+			expect(skill.parentId).toBeNull();
+		});
+
+		it('should allow overriding all fields', () => {
+			const customSkill: Partial<Skill> = {
+				id: 'custom-id',
+				name: 'Custom Skill',
+				description: 'A custom skill',
+				branch: 'body',
+				parentId: 'parent-123',
+				icon: 'star',
+				color: '#FF0000',
+				currentXp: 100,
+				totalXp: 100,
+				level: 1,
+			};
+
+			const skill = createDefaultSkill(customSkill);
+
+			expect(skill.id).toBe('custom-id');
+			expect(skill.name).toBe('Custom Skill');
+			expect(skill.parentId).toBe('parent-123');
+			expect(skill.color).toBe('#FF0000');
+		});
+	});
+
+	describe('createActivity', () => {
+		beforeEach(() => {
+			vi.useFakeTimers();
+			vi.setSystemTime(new Date('2026-01-28T12:00:00Z'));
+		});
+
+		it('should create an activity with required fields', () => {
+			const activity = createActivity('skill-123', 50, 'Practiced for 30 minutes');
+
+			expect(activity.id).toBeTruthy();
+			expect(activity.skillId).toBe('skill-123');
+			expect(activity.xpEarned).toBe(50);
+			expect(activity.description).toBe('Practiced for 30 minutes');
+			expect(activity.duration).toBeNull();
+			expect(activity.timestamp).toBeTruthy();
+		});
+
+		it('should include duration when provided', () => {
+			const activity = createActivity('skill-123', 100, 'Long session', 60);
+
+			expect(activity.duration).toBe(60);
+		});
+
+		it('should set duration to null when undefined', () => {
+			const activity = createActivity('skill-123', 25, 'Quick practice', undefined);
+
+			expect(activity.duration).toBeNull();
+		});
+
+		it('should generate unique IDs', () => {
+			const activity1 = createActivity('skill-1', 10, 'Activity 1');
+			const activity2 = createActivity('skill-1', 10, 'Activity 2');
+
+			expect(activity1.id).not.toBe(activity2.id);
+		});
+
+		it('should set current timestamp', () => {
+			const activity = createActivity('skill-123', 10, 'Test');
+			const expectedTime = new Date('2026-01-28T12:00:00Z').toISOString();
+
+			expect(activity.timestamp).toBe(expectedTime);
+		});
+
+		it('should handle zero XP', () => {
+			const activity = createActivity('skill-123', 0, 'Just a note');
+
+			expect(activity.xpEarned).toBe(0);
+		});
+
+		it('should handle large XP values', () => {
+			const activity = createActivity('skill-123', 10000, 'Major milestone');
+
+			expect(activity.xpEarned).toBe(10000);
+		});
+	});
+});
+
+describe('Edge Cases', () => {
+	describe('Level calculations with boundary values', () => {
+		it('should handle exact threshold values', () => {
+			expect(calculateLevel(100)).toBe(1);
+			expect(calculateLevel(500)).toBe(2);
+			expect(calculateLevel(1500)).toBe(3);
+			expect(calculateLevel(4000)).toBe(4);
+			expect(calculateLevel(10000)).toBe(5);
+		});
+
+		it('should handle one below threshold', () => {
+			expect(calculateLevel(99)).toBe(0);
+			expect(calculateLevel(499)).toBe(1);
+			expect(calculateLevel(1499)).toBe(2);
+			expect(calculateLevel(3999)).toBe(3);
+			expect(calculateLevel(9999)).toBe(4);
+		});
+	});
+
+	describe('Progress calculations at boundaries', () => {
+		it('should handle zero XP at level 0', () => {
+			expect(xpProgress(0, 0)).toBe(0);
+		});
+
+		it('should handle XP exactly at level-up', () => {
+			// When XP equals next threshold, progress should be 100% for current level
+			// But calculateLevel would put them at next level
+			// This tests the edge case
+			expect(xpProgress(100, 0)).toBe(100);
+		});
+	});
+});
diff --git a/apps/skilltree/apps/web/src/test/setup.ts b/apps/skilltree/apps/web/src/test/setup.ts
new file mode 100644
index 000000000..7c13569ef
--- /dev/null
+++ b/apps/skilltree/apps/web/src/test/setup.ts
@@ -0,0 +1,9 @@
+// Test setup file
+import { vi } from 'vitest';
+
+// Mock crypto.randomUUID for tests
+if (typeof crypto === 'undefined') {
+	global.crypto = {
+		randomUUID: () => 'test-uuid-' + Math.random().toString(36).substr(2, 9),
+	} as Crypto;
+}
diff --git a/apps/skilltree/apps/web/vitest.config.ts b/apps/skilltree/apps/web/vitest.config.ts
new file mode 100644
index 000000000..0608bf0c4
--- /dev/null
+++ b/apps/skilltree/apps/web/vitest.config.ts
@@ -0,0 +1,18 @@
+import { defineConfig } from 'vitest/config';
+import { svelte } from '@sveltejs/vite-plugin-svelte';
+import { resolve } from 'path';
+
+export default defineConfig({
+	plugins: [svelte({ hot: !process.env.VITEST })],
+	test: {
+		include: ['src/**/*.{test,spec}.{js,ts}'],
+		globals: true,
+		environment: 'jsdom',
+		setupFiles: ['./src/test/setup.ts'],
+	},
+	resolve: {
+		alias: {
+			$lib: resolve('./src/lib'),
+		},
+	},
+});