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diff --git a/web/test/sort-tasks.test.mjs b/web/test/sort-tasks.test.mjs
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+// sort-tasks.test.mjs — TDD contract tests for sortTasksByDate
+//
+// sortTasksByDate is defined inline here to establish expected behaviour.
+// Once sortTasksByDate is exported from web/app.js or a shared module,
+// remove the inline definition and import it instead.
+//
+// Run with: node --test web/test/sort-tasks.test.mjs
+
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+
+// ── Implementation under contract ─────────────────────────────────────────────
+// Remove this block once sortTasksByDate is available from app.js.
+
+function sortTasksByDate(tasks) {
+  return [...tasks].sort((a, b) => {
+    if (!a.created_at && !b.created_at) return 0;
+    if (!a.created_at) return 1;
+    if (!b.created_at) return -1;
+    return new Date(a.created_at) - new Date(b.created_at);
+  });
+}
+
+// ── Helpers ────────────────────────────────────────────────────────────────────
+
+function makeTask(id, created_at, state = 'PENDING') {
+  return { id, name: `task-${id}`, state, created_at };
+}
+
+// ── Tests ──────────────────────────────────────────────────────────────────────
+
+describe('sortTasksByDate', () => {
+  it('sorts tasks oldest-first by created_at', () => {
+    const tasks = [
+      makeTask('c', '2026-03-06T12:00:00Z'),
+      makeTask('a', '2026-03-04T08:00:00Z'),
+      makeTask('b', '2026-03-05T10:00:00Z'),
+    ];
+    const result = sortTasksByDate(tasks);
+    assert.equal(result[0].id, 'a', 'oldest should be first');
+    assert.equal(result[1].id, 'b');
+    assert.equal(result[2].id, 'c', 'newest should be last');
+  });
+
+  it('returns a new array (does not mutate input)', () => {
+    const tasks = [
+      makeTask('b', '2026-03-05T10:00:00Z'),
+      makeTask('a', '2026-03-04T08:00:00Z'),
+    ];
+    const original = [...tasks];
+    const result = sortTasksByDate(tasks);
+    assert.notStrictEqual(result, tasks, 'should return a new array');
+    assert.deepEqual(tasks, original, 'input should not be mutated');
+  });
+
+  it('returns an empty array when given an empty array', () => {
+    assert.deepEqual(sortTasksByDate([]), []);
+  });
+
+  it('returns a single-element array unchanged', () => {
+    const tasks = [makeTask('x', '2026-03-01T00:00:00Z')];
+    const result = sortTasksByDate(tasks);
+    assert.equal(result.length, 1);
+    assert.equal(result[0].id, 'x');
+  });
+
+  it('places tasks with null created_at after tasks with a date', () => {
+    const tasks = [
+      makeTask('no-date', null),
+      makeTask('has-date', '2026-03-01T00:00:00Z'),
+    ];
+    const result = sortTasksByDate(tasks);
+    assert.equal(result[0].id, 'has-date', 'task with date should come first');
+    assert.equal(result[1].id, 'no-date', 'task without date should come last');
+  });
+
+  it('works with mixed states (not just PENDING)', () => {
+    const tasks = [
+      makeTask('r', '2026-03-06T00:00:00Z', 'RUNNING'),
+      makeTask('p', '2026-03-04T00:00:00Z', 'PENDING'),
+      makeTask('q', '2026-03-05T00:00:00Z', 'QUEUED'),
+    ];
+    const result = sortTasksByDate(tasks);
+    assert.equal(result[0].id, 'p');
+    assert.equal(result[1].id, 'q');
+    assert.equal(result[2].id, 'r');
+  });
+});