diff --git a/src/lib/tasks.js b/src/lib/tasks.js
new file mode 100644
index 0000000..3a51245
--- /dev/null
+++ b/src/lib/tasks.js
@@ -0,0 +1,41 @@
+/** crypto.randomUUID requires a secure context, and Negotium over plain HTTP
+ *  on a LAN is a real self-hosted deployment shape — hence the fallback. */
+function newId() {
+  if (typeof crypto !== 'undefined' && typeof crypto.randomUUID === 'function') {
+    return crypto.randomUUID()
+  }
+  return `${Date.now()}-${Math.random().toString(36).slice(2)}`
+}
+
+export function createTask(text, now = Date.now()) {
+  return { id: newId(), text, completed: false, createdAt: now }
+}
+
+export function addTask(tasks, text, now = Date.now()) {
+  const trimmed = text.trim()
+  if (!trimmed) return tasks
+  return [...tasks, createTask(trimmed, now)]
+}
+
+export function toggleTask(tasks, id) {
+  return tasks.map((task) => (task.id === id ? { ...task, completed: !task.completed } : task))
+}
+
+export function deleteTask(tasks, id) {
+  return tasks.filter((task) => task.id !== id)
+}
+
+export function reorderTask(tasks, from, to) {
+  if (from === to) return tasks
+  if (from < 0 || from >= tasks.length) return tasks
+  if (to < 0 || to >= tasks.length) return tasks
+
+  const next = [...tasks]
+  const [moved] = next.splice(from, 1)
+  next.splice(to, 0, moved)
+  return next
+}
+
+export function clearCompleted(tasks) {
+  return tasks.filter((task) => !task.completed)
+}
diff --git a/src/lib/tasks.test.js b/src/lib/tasks.test.js
new file mode 100644
index 0000000..1bce4ba
--- /dev/null
+++ b/src/lib/tasks.test.js
@@ -0,0 +1,128 @@
+import { describe, it, expect } from 'vitest'
+import { addTask, toggleTask, deleteTask, reorderTask, clearCompleted } from './tasks.js'
+
+const task = (id, completed = false) => ({ id, text: id, completed })
+
+describe('addTask', () => {
+  it('appends a task', () => {
+    const result = addTask([], 'buy milk')
+    expect(result).toHaveLength(1)
+    expect(result[0].text).toBe('buy milk')
+    expect(result[0].completed).toBe(false)
+  })
+
+  it('trims surrounding whitespace', () => {
+    expect(addTask([], '  spaced  ')[0].text).toBe('spaced')
+  })
+
+  it('rejects whitespace-only input', () => {
+    const before = [task('a')]
+    expect(addTask(before, '   ')).toBe(before)
+  })
+
+  it('rejects empty input', () => {
+    const before = [task('a')]
+    expect(addTask(before, '')).toBe(before)
+  })
+
+  it('assigns unique ids', () => {
+    const one = addTask([], 'a')
+    const two = addTask(one, 'b')
+    expect(two[0].id).not.toBe(two[1].id)
+  })
+
+  it('assigns unique ids even when added in the same millisecond', () => {
+    let tasks = []
+    for (let i = 0; i < 50; i += 1) tasks = addTask(tasks, `task ${i}`, 1_000_000)
+    expect(new Set(tasks.map((t) => t.id)).size).toBe(50)
+  })
+
+  it('records the supplied creation time', () => {
+    expect(addTask([], 'x', 1_234_567)[0].createdAt).toBe(1_234_567)
+  })
+
+  it('does not mutate the input array', () => {
+    const before = [task('a')]
+    addTask(before, 'b')
+    expect(before).toHaveLength(1)
+  })
+})
+
+describe('toggleTask', () => {
+  it('flips completion', () => {
+    expect(toggleTask([task('a')], 'a')[0].completed).toBe(true)
+  })
+
+  it('flips back', () => {
+    expect(toggleTask([task('a', true)], 'a')[0].completed).toBe(false)
+  })
+
+  it('ignores an unknown id', () => {
+    expect(toggleTask([task('a')], 'zzz')[0].completed).toBe(false)
+  })
+
+  it('does not mutate the input array', () => {
+    const before = [task('a')]
+    toggleTask(before, 'a')
+    expect(before[0].completed).toBe(false)
+  })
+})
+
+describe('deleteTask', () => {
+  it('removes the matching task', () => {
+    expect(deleteTask([task('a'), task('b')], 'a').map((t) => t.id)).toEqual(['b'])
+  })
+
+  it('ignores an unknown id', () => {
+    expect(deleteTask([task('a')], 'zzz')).toHaveLength(1)
+  })
+})
+
+describe('reorderTask', () => {
+  const three = [task('a'), task('b'), task('c')]
+
+  it('moves an item later', () => {
+    expect(reorderTask(three, 0, 2).map((t) => t.id)).toEqual(['b', 'c', 'a'])
+  })
+
+  it('moves an item earlier', () => {
+    expect(reorderTask(three, 2, 0).map((t) => t.id)).toEqual(['c', 'a', 'b'])
+  })
+
+  it('moves an item into the middle', () => {
+    expect(reorderTask(three, 0, 1).map((t) => t.id)).toEqual(['b', 'a', 'c'])
+  })
+
+  it('is a no-op when indices match', () => {
+    expect(reorderTask(three, 1, 1)).toBe(three)
+  })
+
+  it('is a no-op for an out-of-range destination', () => {
+    expect(reorderTask(three, 0, 9)).toBe(three)
+  })
+
+  it('is a no-op for an out-of-range source', () => {
+    expect(reorderTask(three, -1, 0)).toBe(three)
+  })
+
+  it('does not mutate the input array', () => {
+    const before = [task('a'), task('b')]
+    reorderTask(before, 0, 1)
+    expect(before.map((t) => t.id)).toEqual(['a', 'b'])
+  })
+})
+
+describe('clearCompleted', () => {
+  it('removes completed tasks', () => {
+    const result = clearCompleted([task('a', true), task('b'), task('c', true)])
+    expect(result.map((t) => t.id)).toEqual(['b'])
+  })
+
+  it('is a no-op when nothing is completed', () => {
+    expect(clearCompleted([task('a')])).toHaveLength(1)
+  })
+
+  it('can empty the list entirely', () => {
+    expect(clearCompleted([task('a', true)])).toEqual([])
+  })
+})
diff --git a/src/lib/undo.js b/src/lib/undo.js
new file mode 100644
index 0000000..a879b6d
--- /dev/null
+++ b/src/lib/undo.js
@@ -0,0 +1,51 @@
+/** Bounded stack of reversible operations. Kept separate from tasks.js so that
+ *  module stays purely functional while this one holds the state. */
+export function createUndoStack(limit = 10) {
+  const entries = []
+
+  return {
+    push(entry) {
+      entries.push(entry)
+      if (entries.length > limit) entries.shift()
+    },
+    pop() {
+      return entries.length > 0 ? entries.pop() : null
+    },
+    get size() {
+      return entries.length
+    },
+    clear() {
+      entries.length = 0
+    },
+  }
+}
+
+/**
+ * Reverses one entry against the current task list.
+ *
+ * Indices are clamped because the list may have changed since the entry was
+ * recorded — a task deleted from position 5 can be restored into a list that
+ * has since shrunk to two items, and landing at the end beats throwing.
+ *
+ * `clearCompleted` entries must record `removed` in ascending index order, so
+ * re-inserting front to back puts each task back where it was.
+ */
+export function applyUndo(tasks, entry) {
+  if (!entry) return tasks
+
+  if (entry.type === 'delete') {
+    const next = [...tasks]
+    next.splice(Math.min(entry.index, next.length), 0, entry.task)
+    return next
+  }
+
+  if (entry.type === 'clearCompleted') {
+    const next = [...tasks]
+    for (const { task, index } of entry.removed) {
+      next.splice(Math.min(index, next.length), 0, task)
+    }
+    return next
+  }
+
+  return tasks
+}
diff --git a/src/lib/undo.test.js b/src/lib/undo.test.js
new file mode 100644
index 0000000..11a0edf
--- /dev/null
+++ b/src/lib/undo.test.js
@@ -0,0 +1,100 @@
+import { describe, it, expect } from 'vitest'
+import { createUndoStack, applyUndo } from './undo.js'
+
+const task = (id, completed = false) => ({ id, text: id, completed })
+
+describe('createUndoStack', () => {
+  it('pops the most recent entry', () => {
+    const stack = createUndoStack()
+    stack.push({ type: 'delete', task: task('a'), index: 0 })
+    stack.push({ type: 'delete', task: task('b'), index: 1 })
+    expect(stack.pop().task.id).toBe('b')
+  })
+
+  it('returns null when empty', () => {
+    expect(createUndoStack().pop()).toBe(null)
+  })
+
+  it('reports its size', () => {
+    const stack = createUndoStack()
+    expect(stack.size).toBe(0)
+    stack.push({ type: 'delete', task: task('a'), index: 0 })
+    expect(stack.size).toBe(1)
+  })
+
+  it('is bounded, discarding the oldest entries', () => {
+    const stack = createUndoStack(3)
+    for (const id of ['a', 'b', 'c', 'd']) {
+      stack.push({ type: 'delete', task: task(id), index: 0 })
+    }
+    expect(stack.size).toBe(3)
+    expect(stack.pop().task.id).toBe('d')
+    expect(stack.pop().task.id).toBe('c')
+    expect(stack.pop().task.id).toBe('b')
+    expect(stack.pop()).toBe(null)
+  })
+
+  it('clears', () => {
+    const stack = createUndoStack()
+    stack.push({ type: 'delete', task: task('a'), index: 0 })
+    stack.clear()
+    expect(stack.size).toBe(0)
+  })
+})
+
+describe('applyUndo', () => {
+  it('restores a deleted task at its original index', () => {
+    const after = [task('a'), task('c')]
+    const entry = { type: 'delete', task: task('b'), index: 1 }
+    expect(applyUndo(after, entry).map((t) => t.id)).toEqual(['a', 'b', 'c'])
+  })
+
+  it('restores a task deleted from the front', () => {
+    const entry = { type: 'delete', task: task('a'), index: 0 }
+    expect(applyUndo([task('b')], entry).map((t) => t.id)).toEqual(['a', 'b'])
+  })
+
+  it('restores at the end when the list has since shrunk', () => {
+    const entry = { type: 'delete', task: task('b'), index: 5 }
+    expect(applyUndo([task('a')], entry).map((t) => t.id)).toEqual(['a', 'b'])
+  })
+
+  it('restores a cleared batch in original positions', () => {
+    const after = [task('b')]
+    const entry = {
+      type: 'clearCompleted',
+      removed: [
+        { task: task('a', true), index: 0 },
+        { task: task('c', true), index: 2 },
+      ],
+    }
+    expect(applyUndo(after, entry).map((t) => t.id)).toEqual(['a', 'b', 'c'])
+  })
+
+  it('restores a batch cleared from an entirely completed list', () => {
+    const entry = {
+      type: 'clearCompleted',
+      removed: [
+        { task: task('a', true), index: 0 },
+        { task: task('b', true), index: 1 },
+      ],
+    }
+    expect(applyUndo([], entry).map((t) => t.id)).toEqual(['a', 'b'])
+  })
+
+  it('is a no-op for a null entry', () => {
+    const tasks = [task('a')]
+    expect(applyUndo(tasks, null)).toBe(tasks)
+  })
+
+  it('is a no-op for an unrecognised entry type', () => {
+    const tasks = [task('a')]
+    expect(applyUndo(tasks, { type: 'nonsense' })).toBe(tasks)
+  })
+
+  it('does not mutate the input array', () => {
+    const before = [task('a')]
+    applyUndo(before, { type: 'delete', task: task('b'), index: 0 })
+    expect(before).toHaveLength(1)
+  })
+})