diff --git a/packages/core/__tests__/markdown-summary.test.ts b/packages/core/__tests__/markdown-summary.test.ts
new file mode 100644
index 00000000..efb317f0
--- /dev/null
+++ b/packages/core/__tests__/markdown-summary.test.ts
@@ -0,0 +1,240 @@
+import * as fs from 'fs'
+import * as os from 'os'
+import path from 'path'
+import {markdownSummary, SUMMARY_ENV_VAR} from '../src/markdown-summary'
+
+const testFilePath = path.join(__dirname, 'test', 'test-summary.md')
+
+async function assertSummary(expected: string): Promise<void> {
+  const file = await fs.promises.readFile(testFilePath, {encoding: 'utf8'})
+  expect(file).toEqual(expected)
+}
+
+const fixtures = {
+  text: 'hello world 🌎',
+  code: `func fork() {
+  for {
+    go fork()
+  }
+}`,
+  list: ['foo', 'bar', 'baz', '💣'],
+  table: [
+    [
+      {
+        data: 'foo',
+        header: true
+      },
+      {
+        data: 'bar',
+        header: true
+      },
+      {
+        data: 'baz',
+        header: true
+      },
+      {
+        data: 'tall',
+        rowspan: '3'
+      }
+    ],
+    ['one', 'two', 'three'],
+    [
+      {
+        data: 'wide',
+        colspan: '3'
+      }
+    ]
+  ],
+  details: {
+    label: 'open me',
+    content: '🎉 surprise'
+  },
+  img: {
+    src: 'https://github.com/actions.png',
+    alt: 'actions logo',
+    options: {
+      width: '32',
+      height: '32'
+    }
+  },
+  quote: {
+    text: 'Where the world builds software',
+    cite: 'https://github.com/about'
+  }
+}
+
+describe('@actions/core/src/markdown-summary', () => {
+  beforeAll(() => {
+    process.env[SUMMARY_ENV_VAR] = testFilePath
+  })
+
+  beforeEach(async () => {
+    await fs.promises.writeFile(testFilePath, '', {encoding: 'utf8'})
+  })
+
+  afterAll(async () => {
+    await fs.promises.unlink(testFilePath)
+  })
+
+  it('appends text to summary file', async () => {
+    await fs.promises.writeFile(testFilePath, '# ', {encoding: 'utf8'})
+    await markdownSummary.add(fixtures.text).write()
+    await assertSummary(`# ${fixtures.text}`)
+  })
+
+  it('overwrites text to summary file', async () => {
+    await fs.promises.writeFile(testFilePath, 'overwrite', {encoding: 'utf8'})
+    await markdownSummary.add(fixtures.text).write(true)
+    await assertSummary(fixtures.text)
+  })
+
+  it('chains appends text to summary file', async () => {
+    await fs.promises.writeFile(testFilePath, '', {encoding: 'utf8'})
+    await markdownSummary
+      .add(fixtures.text)
+      .add(fixtures.text)
+      .add(fixtures.text)
+      .write()
+    await assertSummary([fixtures.text, fixtures.text, fixtures.text].join(''))
+  })
+
+  it('empties buffer after write', async () => {
+    await fs.promises.writeFile(testFilePath, '', {encoding: 'utf8'})
+    await markdownSummary.add(fixtures.text).write()
+    await assertSummary(fixtures.text)
+    expect(markdownSummary.isEmptyBuffer()).toBe(true)
+  })
+
+  it('returns summary buffer as string', () => {
+    markdownSummary.add(fixtures.text)
+    expect(markdownSummary.stringify()).toEqual(fixtures.text)
+  })
+
+  it('return correct values for isEmptyBuffer', () => {
+    markdownSummary.add(fixtures.text)
+    expect(markdownSummary.isEmptyBuffer()).toBe(false)
+
+    markdownSummary.emptyBuffer()
+    expect(markdownSummary.isEmptyBuffer()).toBe(true)
+  })
+
+  it('adds EOL', async () => {
+    await markdownSummary
+      .add(fixtures.text)
+      .addEOL()
+      .write()
+    await assertSummary(fixtures.text + os.EOL)
+  })
+
+  it('adds a code block without language', async () => {
+    await markdownSummary.addCodeBlock(fixtures.code).write()
+    const expected = `<pre><code>func fork() {\n  for {\n    go fork()\n  }\n}</code></pre>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds a code block with a language', async () => {
+    await markdownSummary.addCodeBlock(fixtures.code, 'go').write()
+    const expected = `<pre lang="go"><code>func fork() {\n  for {\n    go fork()\n  }\n}</code></pre>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds an unordered list', async () => {
+    await markdownSummary.addList(fixtures.list).write()
+    const expected = `<ul><li>foo</li><li>bar</li><li>baz</li><li>💣</li></ul>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds an ordered list', async () => {
+    await markdownSummary.addList(fixtures.list, true).write()
+    const expected = `<ol><li>foo</li><li>bar</li><li>baz</li><li>💣</li></ol>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds a table', async () => {
+    await markdownSummary.addTable(fixtures.table).write()
+    const expected = `<table><tr><th>foo</th><th>bar</th><th>baz</th><td rowspan="3">tall</td></tr><tr><td>one</td><td>two</td><td>three</td></tr><tr><td colspan="3">wide</td></tr></table>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds a details element', async () => {
+    await markdownSummary
+      .addDetails(fixtures.details.label, fixtures.details.content)
+      .write()
+    const expected = `<details><summary>open me</summary>🎉 surprise</details>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds an image with alt text', async () => {
+    await markdownSummary.addImage(fixtures.img.src, fixtures.img.alt).write()
+    const expected = `<img src="https://github.com/actions.png" alt="actions logo">${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds an image with custom dimensions', async () => {
+    await markdownSummary
+      .addImage(fixtures.img.src, fixtures.img.alt, fixtures.img.options)
+      .write()
+    const expected = `<img src="https://github.com/actions.png" alt="actions logo" width="32" height="32">${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds an image with custom dimensions', async () => {
+    await markdownSummary
+      .addImage(fixtures.img.src, fixtures.img.alt, fixtures.img.options)
+      .write()
+    const expected = `<img src="https://github.com/actions.png" alt="actions logo" width="32" height="32">${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds headings h1...h6', async () => {
+    for (const i of [1, 2, 3, 4, 5, 6]) {
+      markdownSummary.addHeading('heading', i)
+    }
+    await markdownSummary.write()
+    const expected = `<h1>heading</h1>${os.EOL}<h2>heading</h2>${os.EOL}<h3>heading</h3>${os.EOL}<h4>heading</h4>${os.EOL}<h5>heading</h5>${os.EOL}<h6>heading</h6>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds h1 if heading level not specified', async () => {
+    await markdownSummary.addHeading('heading').write()
+    const expected = `<h1>heading</h1>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('uses h1 if heading level is garbage or out of range', async () => {
+    await markdownSummary
+      .addHeading('heading', 'foobar')
+      .addHeading('heading', 1337)
+      .addHeading('heading', -1)
+      .addHeading('heading', Infinity)
+      .write()
+    const expected = `<h1>heading</h1>${os.EOL}<h1>heading</h1>${os.EOL}<h1>heading</h1>${os.EOL}<h1>heading</h1>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds a separator', async () => {
+    await markdownSummary.addSeparator().write()
+    const expected = `<hr>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds a break', async () => {
+    await markdownSummary.addBreak().write()
+    const expected = `<br>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds a quote', async () => {
+    await markdownSummary.addQuote(fixtures.quote.text).write()
+    const expected = `<blockquote>Where the world builds software</blockquote>${os.EOL}`
+    await assertSummary(expected)
+  })
+
+  it('adds a quote with citation', async () => {
+    await markdownSummary
+      .addQuote(fixtures.quote.text, fixtures.quote.cite)
+      .write()
+    const expected = `<blockquote cite="https://github.com/about">Where the world builds software</blockquote>${os.EOL}`
+    await assertSummary(expected)
+  })
+})