diff --git a/tests/milestone.test.cjs b/tests/milestone.test.cjs
index cff09ec33..43f2b563e 100644
--- a/tests/milestone.test.cjs
+++ b/tests/milestone.test.cjs
@@ -221,6 +221,166 @@ describe('milestone complete command', () => {
   });
 });
 
+// ─────────────────────────────────────────────────────────────────────────────
+// requirements mark-complete command
+// ─────────────────────────────────────────────────────────────────────────────
+
+describe('requirements mark-complete command', () => {
+  let tmpDir;
+
+  beforeEach(() => {
+    tmpDir = createTempProject();
+  });
+
+  afterEach(() => {
+    cleanup(tmpDir);
+  });
+
+  // ─── helpers ──────────────────────────────────────────────────────────────
+
+  function writeRequirements(tmpDir, content) {
+    fs.writeFileSync(path.join(tmpDir, '.planning', 'REQUIREMENTS.md'), content, 'utf-8');
+  }
+
+  function readRequirements(tmpDir) {
+    return fs.readFileSync(path.join(tmpDir, '.planning', 'REQUIREMENTS.md'), 'utf-8');
+  }
+
+  const STANDARD_REQUIREMENTS = `# Requirements
+
+## Test Coverage
+- [ ] **TEST-01**: core.cjs has tests for loadConfig
+- [ ] **TEST-02**: core.cjs has tests for resolveModelInternal
+- [x] **TEST-03**: core.cjs has tests for escapeRegex (already complete)
+
+## Bug Regressions
+- [ ] **REG-01**: Test confirms loadConfig returns model_overrides
+
+## Infrastructure
+- [ ] **INFRA-01**: GitHub Actions workflow runs tests
+
+## Traceability
+
+| Requirement | Phase | Status |
+|-------------|-------|--------|
+| TEST-01 | Phase 1 | Pending |
+| TEST-02 | Phase 1 | Pending |
+| TEST-03 | Phase 1 | Complete |
+| REG-01 | Phase 1 | Pending |
+| INFRA-01 | Phase 6 | Pending |
+`;
+
+  // ─── tests ────────────────────────────────────────────────────────────────
+
+  test('marks single requirement complete (checkbox + table)', () => {
+    writeRequirements(tmpDir, STANDARD_REQUIREMENTS);
+
+    const result = runGsdTools('requirements mark-complete TEST-01', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.updated, true);
+    assert.ok(output.marked_complete.includes('TEST-01'), 'TEST-01 should be marked complete');
+
+    const content = readRequirements(tmpDir);
+    assert.ok(content.includes('- [x] **TEST-01**'), 'checkbox should be checked');
+    assert.ok(content.includes('| TEST-01 | Phase 1 | Complete |'), 'table row should be Complete');
+    // Other checkboxes unchanged
+    assert.ok(content.includes('- [ ] **TEST-02**'), 'TEST-02 should remain unchecked');
+  });
+
+  test('handles mixed prefixes in single call (TEST-XX, REG-XX, INFRA-XX)', () => {
+    writeRequirements(tmpDir, STANDARD_REQUIREMENTS);
+
+    const result = runGsdTools('requirements mark-complete TEST-01,REG-01,INFRA-01', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.marked_complete.length, 3, 'should mark 3 requirements complete');
+    assert.ok(output.marked_complete.includes('TEST-01'));
+    assert.ok(output.marked_complete.includes('REG-01'));
+    assert.ok(output.marked_complete.includes('INFRA-01'));
+
+    const content = readRequirements(tmpDir);
+    assert.ok(content.includes('- [x] **TEST-01**'), 'TEST-01 checkbox should be checked');
+    assert.ok(content.includes('- [x] **REG-01**'), 'REG-01 checkbox should be checked');
+    assert.ok(content.includes('- [x] **INFRA-01**'), 'INFRA-01 checkbox should be checked');
+    assert.ok(content.includes('| TEST-01 | Phase 1 | Complete |'), 'TEST-01 table should be Complete');
+    assert.ok(content.includes('| REG-01 | Phase 1 | Complete |'), 'REG-01 table should be Complete');
+    assert.ok(content.includes('| INFRA-01 | Phase 6 | Complete |'), 'INFRA-01 table should be Complete');
+  });
+
+  test('accepts space-separated IDs', () => {
+    writeRequirements(tmpDir, STANDARD_REQUIREMENTS);
+
+    const result = runGsdTools('requirements mark-complete TEST-01 TEST-02', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.marked_complete.length, 2, 'should mark 2 requirements complete');
+
+    const content = readRequirements(tmpDir);
+    assert.ok(content.includes('- [x] **TEST-01**'), 'TEST-01 should be checked');
+    assert.ok(content.includes('- [x] **TEST-02**'), 'TEST-02 should be checked');
+  });
+
+  test('accepts bracket-wrapped IDs [REQ-01, REQ-02]', () => {
+    writeRequirements(tmpDir, STANDARD_REQUIREMENTS);
+
+    const result = runGsdTools('requirements mark-complete [TEST-01,TEST-02]', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.marked_complete.length, 2, 'should mark 2 requirements complete');
+
+    const content = readRequirements(tmpDir);
+    assert.ok(content.includes('- [x] **TEST-01**'), 'TEST-01 should be checked');
+    assert.ok(content.includes('- [x] **TEST-02**'), 'TEST-02 should be checked');
+  });
+
+  test('returns not_found for invalid IDs while updating valid ones', () => {
+    writeRequirements(tmpDir, STANDARD_REQUIREMENTS);
+
+    const result = runGsdTools('requirements mark-complete TEST-01,FAKE-99', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.updated, true, 'should still update valid IDs');
+    assert.ok(output.marked_complete.includes('TEST-01'), 'TEST-01 should be marked complete');
+    assert.ok(output.not_found.includes('FAKE-99'), 'FAKE-99 should be in not_found');
+    assert.strictEqual(output.total, 2, 'total should reflect all IDs attempted');
+  });
+
+  test('idempotent — re-marking already-complete requirement does not corrupt', () => {
+    writeRequirements(tmpDir, STANDARD_REQUIREMENTS);
+
+    // TEST-03 already has [x] and Complete in the fixture
+    const result = runGsdTools('requirements mark-complete TEST-03', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    // Regex only matches [ ] (space), not [x], so TEST-03 goes to not_found
+    assert.ok(output.not_found.includes('TEST-03'), 'already-complete ID should be in not_found');
+
+    const content = readRequirements(tmpDir);
+    // File should not be corrupted — no [xx] or doubled markers
+    assert.ok(content.includes('- [x] **TEST-03**'), 'existing [x] should remain intact');
+    assert.ok(!content.includes('[xx]'), 'should not have doubled x markers');
+    assert.ok(!content.includes('- [x] [x]'), 'should not have duplicate checkbox');
+  });
+
+  test('missing REQUIREMENTS.md returns expected error structure', () => {
+    // createTempProject does not create REQUIREMENTS.md — so it's already missing
+
+    const result = runGsdTools('requirements mark-complete TEST-01', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output.updated, false, 'updated should be false');
+    assert.strictEqual(output.reason, 'REQUIREMENTS.md not found', 'should report file not found');
+  });
+});
+
 // ─────────────────────────────────────────────────────────────────────────────
 // validate consistency command
 // ─────────────────────────────────────────────────────────────────────────────