diff --git a/__tests__/actions/update-sprint.test.ts b/__tests__/actions/update-sprint.test.ts
new file mode 100644
index 0000000..f51219d
--- /dev/null
+++ b/__tests__/actions/update-sprint.test.ts
@@ -0,0 +1,148 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+
+vi.mock('next/cache', () => ({ revalidatePath: vi.fn() }))
+vi.mock('next/headers', () => ({
+  cookies: vi.fn().mockResolvedValue({
+    set: vi.fn(),
+    get: vi.fn(),
+    delete: vi.fn(),
+  }),
+}))
+vi.mock('iron-session', () => ({
+  getIronSession: vi.fn().mockResolvedValue({ userId: 'user-1', isDemo: false }),
+}))
+vi.mock('@/lib/session', () => ({
+  sessionOptions: { cookieName: 'test', password: 'test' },
+}))
+vi.mock('@/lib/product-access', () => ({
+  productAccessFilter: vi.fn().mockReturnValue({}),
+  getAccessibleProduct: vi.fn().mockResolvedValue({ id: 'product-1' }),
+}))
+vi.mock('@/lib/rate-limit', () => ({
+  enforceUserRateLimit: vi.fn().mockReturnValue(null),
+}))
+vi.mock('@/lib/code-server', () => ({
+  createWithCodeRetry: vi.fn(),
+  generateNextSprintCode: vi.fn(),
+}))
+vi.mock('@/lib/active-sprint', () => ({
+  setActiveSprintInSettings: vi.fn().mockResolvedValue(undefined),
+}))
+vi.mock('@/lib/prisma', () => ({
+  prisma: {
+    sprint: {
+      findFirst: vi.fn(),
+      update: vi.fn(),
+    },
+    story: {
+      findMany: vi.fn(),
+      updateMany: vi.fn(),
+    },
+    task: {
+      findMany: vi.fn(),
+      updateMany: vi.fn(),
+    },
+    $transaction: vi.fn(),
+  },
+}))
+
+import { prisma } from '@/lib/prisma'
+import { updateSprintAction } from '@/actions/sprints'
+
+type Mocked = {
+  sprint: {
+    findFirst: ReturnType<typeof vi.fn>
+    update: ReturnType<typeof vi.fn>
+  }
+}
+const mockPrisma = prisma as unknown as Mocked
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  mockPrisma.sprint.findFirst.mockReset().mockResolvedValue({
+    id: 'sprint-1',
+    product_id: 'product-1',
+  })
+  mockPrisma.sprint.update.mockReset().mockResolvedValue({})
+})
+
+describe('updateSprintAction', () => {
+  it('updates sprint_goal alone', async () => {
+    const result = await updateSprintAction({
+      sprintId: 'sprint-1',
+      fields: { goal: 'Nieuw doel' },
+    })
+
+    expect('success' in result).toBe(true)
+    expect(mockPrisma.sprint.update).toHaveBeenCalledWith({
+      where: { id: 'sprint-1' },
+      data: { sprint_goal: 'Nieuw doel' },
+    })
+  })
+
+  it('updates dates only', async () => {
+    await updateSprintAction({
+      sprintId: 'sprint-1',
+      fields: { startAt: '2026-06-01', endAt: '2026-06-14' },
+    })
+
+    expect(mockPrisma.sprint.update).toHaveBeenCalledWith({
+      where: { id: 'sprint-1' },
+      data: {
+        start_date: new Date('2026-06-01'),
+        end_date: new Date('2026-06-14'),
+      },
+    })
+  })
+
+  it('accepts null to clear a date', async () => {
+    await updateSprintAction({
+      sprintId: 'sprint-1',
+      fields: { startAt: null },
+    })
+
+    expect(mockPrisma.sprint.update).toHaveBeenCalledWith({
+      where: { id: 'sprint-1' },
+      data: { start_date: null },
+    })
+  })
+
+  it('rejects when sprint not accessible', async () => {
+    mockPrisma.sprint.findFirst.mockResolvedValue(null)
+
+    const result = await updateSprintAction({
+      sprintId: 'sprint-1',
+      fields: { goal: 'x' },
+    })
+
+    expect('error' in result).toBe(true)
+    if ('error' in result) {
+      expect(result.code).toBe(403)
+    }
+    expect(mockPrisma.sprint.update).not.toHaveBeenCalled()
+  })
+
+  it('rejects empty goal', async () => {
+    const result = await updateSprintAction({
+      sprintId: 'sprint-1',
+      fields: { goal: '' },
+    })
+
+    expect('error' in result).toBe(true)
+    expect(mockPrisma.sprint.update).not.toHaveBeenCalled()
+  })
+
+  it('rejects when no fields are supplied', async () => {
+    const result = await updateSprintAction({
+      sprintId: 'sprint-1',
+      fields: {},
+    })
+
+    // Schema-refine should reject; OR action treats empty data as no-op success.
+    // Current implementation: refine forces minstens één veld → 422 error.
+    expect('error' in result).toBe(true)
+    if ('error' in result) {
+      expect(result.code).toBe(422)
+    }
+  })
+})