diff --git a/src/format.ts b/src/format.ts
new file mode 100644
index 0000000..4cb7b6d
--- /dev/null
+++ b/src/format.ts
@@ -0,0 +1,49 @@
+export function escapeHtml(text: string): string {
+  return text
+    .replace(/&/g, '&')
+    .replace(/</g, '&lt;')
+    .replace(/>/g, '&gt;')
+    .replace(/"/g, '&quot;')
+    .replace(/'/g, '&#39;')
+}
+
+function inlineToHtml(text: string): string {
+  let html = escapeHtml(text)
+  html = html.replace(/`([^`\n]+)`/g, '<code>$1</code>')
+  html = html.replace(/\*\*([^*\n]+)\*\*/g, '<b>$1</b>')
+  return html
+}
+
+export function markdownToHtml(text: string): string {
+  const parts = text.split(/```[\w-]*/)
+  if (parts.length % 2 === 0) return inlineToHtml(text)
+  let html = ''
+  for (let i = 0; i < parts.length; i++) {
+    if (i % 2 === 1) {
+      const code = parts[i]!.replace(/^\n/, '').replace(/\n$/, '')
+      html += `<pre>${escapeHtml(code)}</pre>`
+    } else {
+      html += inlineToHtml(parts[i]!)
+    }
+  }
+  return html
+}
+
+export function splitMessage(text: string, maxLength: number): string[] {
+  if (maxLength < 1) throw new Error('maxLength must be >= 1')
+  if (text.length <= maxLength) return [text]
+  const chunks: string[] = []
+  let rest = text
+  while (rest.length > maxLength) {
+    const window = rest.slice(0, maxLength)
+    const newline = window.lastIndexOf('\n')
+    const ideographic = window.lastIndexOf('。')
+    const sentence = window.lastIndexOf('. ')
+    const breakAt = Math.max(newline, ideographic, sentence)
+    const cut = breakAt > 0 ? (breakAt === sentence ? breakAt + 2 : breakAt + 1) : maxLength
+    chunks.push(rest.slice(0, cut))
+    rest = rest.slice(cut)
+  }
+  chunks.push(rest)
+  return chunks
+}
diff --git a/tests/format.test.ts b/tests/format.test.ts
new file mode 100644
index 0000000..1448ac5
--- /dev/null
+++ b/tests/format.test.ts
@@ -0,0 +1,29 @@
+import assert from 'node:assert/strict'
+import test from 'node:test'
+import { escapeHtml, markdownToHtml, splitMessage } from '../src/format.ts'
+
+test('escapeHtml escapes five special chars', () => {
+  assert.equal(escapeHtml(`&<>"'`), '&amp;&lt;&gt;&quot;&#39;')
+})
+
+test('markdownToHtml converts fence bold and inline code', () => {
+  const html = markdownToHtml('hi **x** and `y`\n```\nz\n```')
+  assert.match(html, /<b>x<\/b>/)
+  assert.match(html, /<code>y<\/code>/)
+  assert.match(html, /<pre>z<\/pre>/)
+})
+
+test('markdownToHtml keeps unbalanced fence without pre', () => {
+  const html = markdownToHtml('start ``` not closed')
+  assert.equal(html.includes('<pre>'), false)
+  assert.match(html, /start/)
+})
+
+test('splitMessage prefers newline within window', () => {
+  const text = 'aaaa\nbbbb\ncccc'
+  assert.deepEqual(splitMessage(text, 6), ['aaaa\n', 'bbbb\n', 'cccc'])
+})
+
+test('splitMessage hard-cuts when no break', () => {
+  assert.deepEqual(splitMessage('abcdefghij', 4), ['abcd', 'efgh', 'ij'])
+})