diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a984580..a234d88 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -16,3 +16,4 @@ jobs:
         with:
           node-version: ${{ matrix.node }}
       - run: npm run check
+      - run: npm test
diff --git a/package.json b/package.json
index 1890958..1e6360d 100644
--- a/package.json
+++ b/package.json
@@ -6,5 +6,5 @@
   "license": "MIT",
   "author": "Okan Tiktas",
   "engines": {"node": ">=20"},
-  "scripts": {"start": "node src/server.js", "check": "node --check src/core.js && node --check src/cli.js && node --check src/server.js && node --check web/app.js"}
+  "scripts": {"start": "node src/server.js", "check": "node --check src/core.js && node --check src/cli.js && node --check src/server.js && node --check web/app.js", "test": "node --test"}
 }
diff --git a/test/core.test.js b/test/core.test.js
new file mode 100644
index 0000000..fb28034
--- /dev/null
+++ b/test/core.test.js
@@ -0,0 +1,99 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  MAX_INPUT_CHARS,
+  parseCommits,
+  suggestVersion,
+  createNotes,
+  createAnnouncement,
+} from '../src/core.js';
+
+test('normalizes copied commit bullets and deduplicates case-insensitively', () => {
+  const commits = parseCommits(
+    '1. feat(ui): Add search\r\n- feat(ui): add search\n* fix: Repair search'
+  );
+  assert.equal(commits.length, 2);
+  assert.deepEqual(commits[0], {
+    type: 'feat', scope: 'ui', breaking: false,
+    message: 'Add search', category: 'features',
+  });
+  assert.equal(commits[1].category, 'fixes');
+});
+
+test('recognizes breaking markers in headers and trailers', () => {
+  const commits = parseCommits(
+    'fix(api)!: Remove old endpoint\nfeat: Update API BREAKING CHANGE: incompatible payload'
+  );
+  assert.equal(commits.length, 2);
+  assert.ok(commits.every(commit => commit.breaking));
+  assert.equal(commits[1].message, 'Update API');
+});
+
+test('accepts standalone BREAKING CHANGE notices', () => {
+  const [commit] = parseCommits('BREAKING CHANGE: remove deprecated configuration');
+  assert.equal(commit.breaking, true);
+  assert.equal(commit.message, 'remove deprecated configuration');
+});
+
+test('filters routine maintenance but preserves breaking maintenance', () => {
+  const commits = parseCommits(
+    'chore: bump packages\nchore!: remove support\nfeat: add export',
+    {includeMaintenance: false}
+  );
+  assert.deepEqual(commits.map(commit => commit.message), [
+    'remove support', 'add export',
+  ]);
+});
+
+test('rejects invalid and oversized commit input', () => {
+  assert.throws(() => parseCommits(null), /text/);
+  assert.throws(() => parseCommits('x'.repeat(MAX_INPUT_CHARS + 1)), /too long/);
+});
+
+test('suggests major, minor, and patch versions using commit severity', () => {
+  assert.equal(suggestVersion(parseCommits('fix!: remove API'), '1.2.3'), '2.0.0');
+  assert.equal(suggestVersion(parseCommits('feat: add API'), 'v1.2.3'), '1.3.0');
+  assert.equal(suggestVersion(parseCommits('docs: update usage'), '1.2.3'), '1.2.4');
+  assert.equal(suggestVersion(parseCommits('miscellaneous update'), '1.2.3'), '1.2.3');
+});
+
+test('rejects unsupported version formats', () => {
+  assert.throws(() => suggestVersion([], '1.2.3-beta'), /version like/);
+  assert.throws(() => suggestVersion([], '01.2.3'), /version like/);
+});
+
+test('groups release notes by category and strips angle brackets', () => {
+  const commits = parseCommits(
+    'feat(ui): Add <beta> view\nfix: Repair crash\nchore: Update tooling'
+  );
+  const notes = createNotes(commits, {version: '1.4.0', title: '<Draft>'});
+  assert.match(notes, /^# Draft — 1\.4\.0/);
+  assert.match(notes, /### Features\n\n- \*\*ui:\*\* Add beta view/);
+  assert.match(notes, /### Fixes\n\n- Repair crash/);
+  assert.match(notes, /### Maintenance\n\n- Update tooling/);
+  assert.doesNotMatch(notes, /<beta>|<Draft>/);
+});
+
+test('allows hiding maintenance in rendered release notes', () => {
+  const commits = parseCommits('feat: add search\nchore: bump versions');
+  const notes = createNotes(commits, {includeMaintenance: false});
+  assert.match(notes, /add search/);
+  assert.doesNotMatch(notes, /bump versions/);
+});
+
+test('renders a useful empty release note and validates its input', () => {
+  assert.match(createNotes([]), /No commit messages were provided/);
+  assert.throws(() => createNotes('not a list'), /must be a list/);
+});
+
+test('announcements cap feature and fix summaries', () => {
+  const commits = parseCommits(
+    'feat: one\nfeat: two\nfeat: three\nfeat: four\n' +
+    'fix: alpha\nfix: beta\nfix: gamma'
+  );
+  const announcement = createAnnouncement(commits, '2.0.0');
+  assert.match(announcement, /Version 2\.0\.0 is here/);
+  assert.match(announcement, /one; two; three — and more/);
+  assert.match(announcement, /alpha; beta — and more/);
+  assert.doesNotMatch(announcement, /four|gamma/);
+});