diff --git a/__tests__/actions/user-settings.test.ts b/__tests__/actions/user-settings.test.ts
new file mode 100644
index 0000000..1fb53ad
--- /dev/null
+++ b/__tests__/actions/user-settings.test.ts
@@ -0,0 +1,82 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+
+vi.mock('next/headers', () => ({ cookies: vi.fn().mockResolvedValue({}) }))
+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/prisma', () => ({
+  prisma: {
+    user: { findUnique: vi.fn() },
+    $transaction: vi.fn(async (fn: (tx: unknown) => Promise<unknown>) => {
+      return fn({
+        user: {
+          findUnique: vi.fn().mockResolvedValue({ settings: {} }),
+          update: vi.fn().mockResolvedValue({}),
+        },
+      })
+    }),
+    $executeRaw: vi.fn().mockResolvedValue(1),
+  },
+}))
+
+import { prisma } from '@/lib/prisma'
+import { getIronSession } from 'iron-session'
+import { updateUserSettingsAction } from '@/actions/user-settings'
+
+const mockPrisma = prisma as unknown as {
+  user: { findUnique: ReturnType<typeof vi.fn> }
+  $transaction: ReturnType<typeof vi.fn>
+  $executeRaw: ReturnType<typeof vi.fn>
+}
+const mockGetIronSession = getIronSession as ReturnType<typeof vi.fn>
+
+beforeEach(() => {
+  vi.clearAllMocks()
+  mockGetIronSession.mockResolvedValue({ userId: 'user-1', isDemo: false })
+  mockPrisma.$executeRaw.mockResolvedValue(1)
+})
+
+describe('updateUserSettingsAction', () => {
+  it('returns 401 when not logged in', async () => {
+    mockGetIronSession.mockResolvedValue({ userId: undefined, isDemo: false })
+    const result = await updateUserSettingsAction({})
+    expect(result).toEqual({ error: 'Niet ingelogd', code: 401 })
+  })
+
+  it('returns 403 for demo accounts', async () => {
+    mockGetIronSession.mockResolvedValue({ userId: 'user-1', isDemo: true })
+    const result = await updateUserSettingsAction({})
+    expect('error' in result && result.code).toBe(403)
+  })
+
+  it('returns 422 when patch is invalid', async () => {
+    const result = await updateUserSettingsAction({
+      views: { sprintBacklog: { filterStatus: 'NONSENSE' } },
+    } as never)
+    expect('error' in result && result.code).toBe(422)
+  })
+
+  it('merges with current settings and emits notify on success', async () => {
+    const existingFindUnique = vi.fn().mockResolvedValue({
+      settings: { views: { sprintBacklog: { sort: 'code' } } },
+    })
+    const update = vi.fn().mockResolvedValue({})
+    mockPrisma.$transaction.mockImplementationOnce(async (fn: (tx: unknown) => Promise<unknown>) => {
+      return fn({ user: { findUnique: existingFindUnique, update } })
+    })
+
+    const result = await updateUserSettingsAction({
+      views: { sprintBacklog: { sortDir: 'desc' } },
+    })
+
+    expect('success' in result && result.success).toBe(true)
+    expect(update).toHaveBeenCalledWith({
+      where: { id: 'user-1' },
+      data: { settings: { views: { sprintBacklog: { sort: 'code', sortDir: 'desc' } } } },
+    })
+    expect(mockPrisma.$executeRaw).toHaveBeenCalled()
+  })
+})
diff --git a/__tests__/lib/user-settings.test.ts b/__tests__/lib/user-settings.test.ts
new file mode 100644
index 0000000..019cc33
--- /dev/null
+++ b/__tests__/lib/user-settings.test.ts
@@ -0,0 +1,115 @@
+import { describe, expect, it } from 'vitest'
+
+import {
+  DEFAULT_USER_SETTINGS,
+  UserSettingsSchema,
+  mergeSettings,
+  parseUserSettings,
+  type UserSettings,
+} from '@/lib/user-settings'
+
+describe('mergeSettings', () => {
+  it('returns the patch when previous is empty', () => {
+    const result = mergeSettings({}, { views: { sprintBacklog: { sort: 'code' } } })
+    expect(result).toEqual({ views: { sprintBacklog: { sort: 'code' } } })
+  })
+
+  it('preserves existing keys when patch only sets new ones', () => {
+    const prev: UserSettings = { views: { sprintBacklog: { sort: 'code' } } }
+    const result = mergeSettings(prev, {
+      views: { pbiList: { sort: 'date' } },
+    })
+    expect(result).toEqual({
+      views: {
+        sprintBacklog: { sort: 'code' },
+        pbiList: { sort: 'date' },
+      },
+    })
+  })
+
+  it('merges nested objects without overwriting siblings', () => {
+    const prev: UserSettings = {
+      views: { sprintBacklog: { sort: 'code', sortDir: 'asc' } },
+    }
+    const result = mergeSettings(prev, {
+      views: { sprintBacklog: { sort: 'priority' } },
+    })
+    expect(result).toEqual({
+      views: { sprintBacklog: { sort: 'priority', sortDir: 'asc' } },
+    })
+  })
+
+  it('replaces arrays instead of appending', () => {
+    const prev: UserSettings = {
+      views: { sprintBacklog: { collapsedPbis: ['a', 'b'] } },
+    }
+    const result = mergeSettings(prev, {
+      views: { sprintBacklog: { collapsedPbis: ['c'] } },
+    })
+    expect(result.views?.sprintBacklog?.collapsedPbis).toEqual(['c'])
+  })
+
+  it('does not mutate the previous object', () => {
+    const prev: UserSettings = { views: { sprintBacklog: { sort: 'code' } } }
+    const snapshot = JSON.parse(JSON.stringify(prev))
+    mergeSettings(prev, { views: { sprintBacklog: { sortDir: 'desc' } } })
+    expect(prev).toEqual(snapshot)
+  })
+
+  it('skips undefined values in the patch', () => {
+    const prev: UserSettings = { views: { sprintBacklog: { sort: 'code' } } }
+    const result = mergeSettings(prev, { views: undefined })
+    expect(result).toEqual(prev)
+  })
+})
+
+describe('parseUserSettings', () => {
+  it('returns defaults for null', () => {
+    expect(parseUserSettings(null)).toEqual(DEFAULT_USER_SETTINGS)
+  })
+
+  it('returns defaults for undefined', () => {
+    expect(parseUserSettings(undefined)).toEqual(DEFAULT_USER_SETTINGS)
+  })
+
+  it('returns defaults for invalid input', () => {
+    expect(parseUserSettings({ views: { sprintBacklog: { filterStatus: 'BOGUS' } } }))
+      .toEqual(DEFAULT_USER_SETTINGS)
+  })
+
+  it('passes valid settings through', () => {
+    const valid = { views: { sprintBacklog: { sort: 'code' as const } } }
+    expect(parseUserSettings(valid)).toEqual(valid)
+  })
+})
+
+describe('UserSettingsSchema', () => {
+  it('rejects unknown top-level keys', () => {
+    const result = UserSettingsSchema.safeParse({ unknown: 1 })
+    expect(result.success).toBe(false)
+  })
+
+  it('accepts an empty object', () => {
+    expect(UserSettingsSchema.safeParse({}).success).toBe(true)
+  })
+
+  it('accepts the full shape', () => {
+    const result = UserSettingsSchema.safeParse({
+      views: {
+        sprintBacklog: {
+          filterPriority: 1,
+          filterStatus: 'OPEN',
+          sort: 'code',
+          sortDir: 'asc',
+          collapsedPbis: ['x'],
+          filterPopoverOpen: true,
+        },
+        pbiList: { sort: 'priority', filterPriority: 'all', filterStatus: 'ready', sortDir: 'desc' },
+        storyPanel: { sort: 'date' },
+        jobsColumns: { 'queue:active': { kinds: ['TASK_IMPLEMENTATION'], statuses: [] } },
+      },
+      devTools: { debugMode: true },
+    })
+    expect(result.success).toBe(true)
+  })
+})
diff --git a/__tests__/stores/user-settings.test.ts b/__tests__/stores/user-settings.test.ts
new file mode 100644
index 0000000..019d618
--- /dev/null
+++ b/__tests__/stores/user-settings.test.ts
@@ -0,0 +1,105 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+
+const updateAction = vi.fn()
+
+vi.mock('@/actions/user-settings', () => ({
+  updateUserSettingsAction: (...args: unknown[]) => updateAction(...args),
+}))
+
+import { useUserSettingsStore } from '@/stores/user-settings/store'
+
+function resetStore() {
+  useUserSettingsStore.setState((s) => {
+    s.entities.settings = {}
+    s.context.hydrated = false
+    s.context.isDemo = false
+    s.pendingMutations = {}
+  })
+}
+
+beforeEach(() => {
+  resetStore()
+  updateAction.mockReset()
+})
+
+afterEach(() => {
+  resetStore()
+})
+
+describe('useUserSettingsStore', () => {
+  it('hydrate sets entities and context', () => {
+    useUserSettingsStore.getState().hydrate(
+      { views: { sprintBacklog: { sort: 'code' } } },
+      false,
+    )
+    const s = useUserSettingsStore.getState()
+    expect(s.entities.settings.views?.sprintBacklog?.sort).toBe('code')
+    expect(s.context.hydrated).toBe(true)
+    expect(s.context.isDemo).toBe(false)
+  })
+
+  it('setPref updates state optimistically and settles on success', async () => {
+    useUserSettingsStore.getState().hydrate({}, false)
+    updateAction.mockResolvedValueOnce({
+      success: true,
+      settings: { views: { sprintBacklog: { filterStatus: 'all' } } },
+    })
+
+    await useUserSettingsStore
+      .getState()
+      .setPref(['views', 'sprintBacklog', 'filterStatus'], 'all')
+
+    const s = useUserSettingsStore.getState()
+    expect(s.entities.settings.views?.sprintBacklog?.filterStatus).toBe('all')
+    expect(Object.keys(s.pendingMutations)).toHaveLength(0)
+    expect(updateAction).toHaveBeenCalledWith({
+      views: { sprintBacklog: { filterStatus: 'all' } },
+    })
+  })
+
+  it('setPref rolls back on server error', async () => {
+    useUserSettingsStore.getState().hydrate(
+      { views: { sprintBacklog: { sort: 'code' } } },
+      false,
+    )
+    updateAction.mockResolvedValueOnce({ error: 'boom', code: 422 })
+
+    await useUserSettingsStore
+      .getState()
+      .setPref(['views', 'sprintBacklog', 'sort'], 'priority')
+
+    const s = useUserSettingsStore.getState()
+    expect(s.entities.settings.views?.sprintBacklog?.sort).toBe('code')
+    expect(Object.keys(s.pendingMutations)).toHaveLength(0)
+  })
+
+  it('setPref skips server-call for demo accounts', async () => {
+    useUserSettingsStore.getState().hydrate({}, true)
+
+    await useUserSettingsStore
+      .getState()
+      .setPref(['devTools', 'debugMode'], true)
+
+    const s = useUserSettingsStore.getState()
+    expect(s.entities.settings.devTools?.debugMode).toBe(true)
+    expect(updateAction).not.toHaveBeenCalled()
+  })
+
+  it('applyServerPatch merges without optimistic state', () => {
+    useUserSettingsStore.getState().hydrate(
+      { views: { sprintBacklog: { sort: 'code' } } },
+      false,
+    )
+
+    useUserSettingsStore.getState().applyServerPatch({
+      views: { sprintBacklog: { sortDir: 'desc' } },
+    })
+
+    const s = useUserSettingsStore.getState()
+    expect(s.entities.settings.views?.sprintBacklog).toEqual({
+      sort: 'code',
+      sortDir: 'desc',
+    })
+    expect(Object.keys(s.pendingMutations)).toHaveLength(0)
+  })
+})