diff --git a/packages/core/src/summary.ts b/packages/core/src/summary.ts
new file mode 100644
index 00000000..d788a78d
--- /dev/null
+++ b/packages/core/src/summary.ts
@@ -0,0 +1,139 @@
+import * as fs from 'fs'
+import {promisify} from 'util'
+
+const exists = promisify(fs.exists)
+const appendFile = promisify(fs.appendFile)
+
+export class MarkdownSummary {
+  static ENV_VAR = 'GITHUB_STEP_SUMMARY'
+  private buffer: string
+
+  constructor() {
+    this.buffer = ''
+  }
+
+  /**
+   * Finds the summary file path from the environment, rejects if not found
+   *
+   * @returns step summary file path
+   */
+  private async filePath(): Promise<string> {
+    const filePath = process.env[MarkdownSummary.ENV_VAR]
+    if (!filePath) {
+      throw new Error(
+        `Unable to find environment variable for ${MarkdownSummary.ENV_VAR}`
+      )
+    }
+    if (!(await exists(filePath))) {
+      throw new Error(`Missing summary file at path: ${filePath}`)
+    }
+
+    return filePath
+  }
+
+  /**
+   * Writes any text in the buffer to the summary file
+   *
+   * @returns {MarkdownSummary} markdown summary instance
+   */
+  async write(): Promise<MarkdownSummary> {
+    const filePath = await this.filePath()
+    await appendFile(filePath, this.buffer, {encoding: 'utf8'})
+    this.clear()
+
+    return this
+  }
+
+  /**
+   * Clears the summary buffer without writing to summary file
+   *
+   * @returns {MarkdownSummary} markdown summary instance
+   */
+  clear(): MarkdownSummary {
+    this.buffer = ''
+    return this
+  }
+
+  /**
+   * Adds a newline to the summary
+   *
+   * @returns {MarkdownSummary} markdown summary instance
+   */
+  addNewline(): MarkdownSummary {
+    this.buffer += '\n'
+    return this
+  }
+
+  /**
+   * Adds text to the summary
+   * @param {string} text content to add
+   * @param {boolean} [newline=false] whether or not to add a newline
+   * @returns {MarkdownSummary} markdown summary instance
+   */
+  addText(text: string, newline = false): MarkdownSummary {
+    this.buffer += text
+    return newline ? this.addNewline() : this
+  }
+
+  /**
+   * Adds a markdown codeblock to the summary
+   *
+   * @param {string} code content to render within fenced code block
+   * @param {string} [language=''] optional language to syntax highlight code
+   *
+   * @returns {MarkdownSummary} markdown summary instance
+   */
+  addCodeBlock(code: string, language = ''): MarkdownSummary {
+    this.buffer += `\`\`\`${language}\n${code}\n\`\`\`\n`
+    return this
+  }
+
+  /**
+   * Adds an HTML list to the summary
+   *
+   * @param {string[]} items list of items to render
+   * @param {boolean} [ordered=false] if the rendered list should be ordered or not (default: false)
+   *
+   * @returns {MarkdownSummary} markdown summary instance
+   */
+  addList(items: string[], ordered = false): MarkdownSummary {
+    const listType = `${ordered ? 'o' : 'u'}l`
+    const listElems = items.map(e => `<li>${e}</li>\n`).join()
+    this.buffer += `<${listType}>\n${listElems}</${listType}>\n`
+    return this
+  }
+
+  /**
+   * Adds an HTML list to the summary
+   *
+   * @param {{[key: string]: any}[]} rows list of data rows
+   * @param {string[]} headers list of keys to use as headers
+   *
+   * @returns {MarkdownSummary} markdown summary instance
+   */
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  addTable(rows: {[key: string]: any}[], headers: string[]): MarkdownSummary {
+    const headerElems = headers.map(h => `<th>${h}</th>\n`).join()
+    const rowElems = rows
+      .map(row => {
+        const data = headers.map(h => `<td>${row[h]}</td>\n`).join()
+        return `<tr>${data}</tr>\n`
+      })
+      .join()
+    this.buffer += `<table>\n<tr>${headerElems}</tr>\n${rowElems}</table>\n`
+    return this
+  }
+
+  /**
+   * Adds a collapsable HTML details element to the summary
+   *
+   * @param {string} label text for the closed state
+   * @param {string} content collapsable content
+   *
+   * @returns {MarkdownSummary} markdown summary instance
+   */
+  addDetails(label: string, content: string): MarkdownSummary {
+    this.buffer += `<details><summary>${label}</summary>\n\n${content}</details>\n`
+    return this
+  }
+}