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diff --git a/web/test/filter-tabs.test.mjs b/web/test/filter-tabs.test.mjs
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+++ b/web/test/filter-tabs.test.mjs
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+// filter-tabs.test.mjs — TDD contract tests for filterTasksByTab
+//
+// filterTasksByTab is defined inline here to establish expected behaviour.
+// Once filterTasksByTab is exported from web/app.js, remove the inline
+// definition and import it instead.
+//
+// Run with: node --test web/test/filter-tabs.test.mjs
+
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import { filterTasksByTab } from '../app.js';
+
+// ── Helpers ────────────────────────────────────────────────────────────────────
+
+function makeTask(state) {
+  return { id: state, name: `task-${state}`, state };
+}
+
+const ALL_STATES = [
+  'PENDING', 'QUEUED', 'RUNNING', 'READY', 'BLOCKED',
+  'COMPLETED', 'FAILED', 'TIMED_OUT', 'CANCELLED', 'BUDGET_EXCEEDED',
+];
+
+// ── Tests ──────────────────────────────────────────────────────────────────────
+
+describe('filterTasksByTab — active tab', () => {
+  it('includes PENDING, QUEUED, RUNNING, READY, BLOCKED', () => {
+    const tasks = ALL_STATES.map(makeTask);
+    const result = filterTasksByTab(tasks, 'active');
+    for (const state of ['PENDING', 'QUEUED', 'RUNNING', 'READY', 'BLOCKED']) {
+      assert.ok(result.some(t => t.state === state), `${state} should be included`);
+    }
+  });
+
+  it('excludes COMPLETED, FAILED, TIMED_OUT, CANCELLED, BUDGET_EXCEEDED', () => {
+    const tasks = ALL_STATES.map(makeTask);
+    const result = filterTasksByTab(tasks, 'active');
+    for (const state of ['COMPLETED', 'FAILED', 'TIMED_OUT', 'CANCELLED', 'BUDGET_EXCEEDED']) {
+      assert.ok(!result.some(t => t.state === state), `${state} should be excluded`);
+    }
+  });
+
+  it('returns empty array for empty input', () => {
+    assert.deepEqual(filterTasksByTab([], 'active'), []);
+  });
+});
+
+describe('filterTasksByTab — done tab', () => {
+  it('includes COMPLETED, FAILED, TIMED_OUT, CANCELLED, BUDGET_EXCEEDED', () => {
+    const tasks = ALL_STATES.map(makeTask);
+    const result = filterTasksByTab(tasks, 'done');
+    for (const state of ['COMPLETED', 'FAILED', 'TIMED_OUT', 'CANCELLED', 'BUDGET_EXCEEDED']) {
+      assert.ok(result.some(t => t.state === state), `${state} should be included`);
+    }
+  });
+
+  it('excludes PENDING, QUEUED, RUNNING, READY, BLOCKED', () => {
+    const tasks = ALL_STATES.map(makeTask);
+    const result = filterTasksByTab(tasks, 'done');
+    for (const state of ['PENDING', 'QUEUED', 'RUNNING', 'READY', 'BLOCKED']) {
+      assert.ok(!result.some(t => t.state === state), `${state} should be excluded`);
+    }
+  });
+
+  it('returns empty array for empty input', () => {
+    assert.deepEqual(filterTasksByTab([], 'done'), []);
+  });
+});
+
+describe('filterTasksByTab — all tab', () => {
+  it('returns all tasks unchanged', () => {
+    const tasks = ALL_STATES.map(makeTask);
+    const result = filterTasksByTab(tasks, 'all');
+    assert.equal(result.length, ALL_STATES.length);
+    assert.strictEqual(result, tasks, 'should return the same array reference');
+  });
+
+  it('returns empty array for empty input', () => {
+    assert.deepEqual(filterTasksByTab([], 'all'), []);
+  });
+});
+
+describe('filterTasksByTab — unknown tab', () => {
+  it('returns all tasks as defensive fallback', () => {
+    const tasks = ALL_STATES.map(makeTask);
+    const result = filterTasksByTab(tasks, 'unknown-tab');
+    assert.equal(result.length, ALL_STATES.length);
+    assert.strictEqual(result, tasks, 'should return the same array reference');
+  });
+});