Build an AI Email Assistant for Gmail

by

🚀 Apps Script + Gemini Mastery — Issue #5

https://github.com/lsvekis/Build-an-AI-Email-Assistant-for-Gmail

Build an AI Email Assistant for Gmail

Draft replies, summarize long threads, extract tasks, detect tone, and create a Gmail sidebar — all powered by Gemini.

⭐ What You Will Build in Issue #5

A complete AI Email Assistant for Gmail that can:

📩 Summarize long email threads
📝 Draft professional replies
📌 Extract tasks, deadlines, and follow-ups
💬 Identify tone (friendly, urgent, formal, etc.)
⚡ Rewrite messages for clarity or politeness
🎯 Suggest subject lines
🧠 Provide quick “reply templates” based on context

You’ll do this using:

  • GmailApp service (safe, built-in)
  • A sidebar UI added via a Gmail Add-on card
  • A central Gemini analysis engine
  • Extracted email metadata (from, to, subject, body)
  • Context-aware prompting

🧠 Learning Objectives

After completing this issue, you will understand:

✔ How to build a Gmail Add-on with Apps Script
✔ How to access email threads + metadata
✔ How to send email context to Gemini safely
✔ How to create AI-powered drafts & summaries
✔ How to generate inline UI cards inside Gmail
✔ How to build reusable AI helpers

This is the core skillset behind AI-powered email assistants like Superhuman, Gemini for Gmail, or Microsoft Copilot — but built by you, inside Apps Script, using your own workflow.

🧩 EXERCISE — Build the Gmail AI Assistant

We will implement:

  1. Gmail Add-on manifest
  2. Entry point for opening the Add-on
  3. Functions to get thread + message content
  4. Gemini-powered actions
  5. UI cards inside Gmail

1️⃣ Update the Manifest for a Gmail Add-on

appsscript.json

{
  "timeZone": "America/Toronto",
  "dependencies": {},
  "exceptionLogging": "STACKDRIVER",
  "oauthScopes": [
    "https://www.googleapis.com/auth/gmail.addons.execute",
    "https://www.googleapis.com/auth/gmail.addons.current.message.readonly",
    "https://www.googleapis.com/auth/gmail.modify",
    "https://www.googleapis.com/auth/gmail.readonly",
    "https://www.googleapis.com/auth/script.external_request"
  ],
  "addOns": {
    "gmail": {
      "name": "AI Email Assistant",
      "logoUrl": "https://www.gstatic.com/images/icons/material/system/2x/mail_outline_black_48dp.png",
      "contextualTriggers": [{
        "unconditional": {},
        "onTriggerFunction": "onGmailMessageOpen"
      }],
      "primaryColor": "#3F51B5",
      "secondaryColor": "#FFFFFF"
    }
  }
}

2️⃣ Create the Add-on Trigger

Code.gs

function onGmailMessageOpen(e) {
  return buildMainCard(e);
}

3️⃣ Build the Gmail Add-on Card UI

UI.gs

function buildMainCard(e) {
  const message = getEmailContent(e);

  const card = CardService.newCardBuilder();

  card.setHeader(
    CardService.newCardHeader()
      .setTitle("AI Email Assistant")
      .setSubtitle("Powered by Gemini")
  );

  // Summarize button
  const summarizeAction = CardService.newAction()
    .setFunctionName("actionSummarize")
    .setParameters({ emailBody: message.body });

  card.addSection(
    CardService.newCardSection()
      .addWidget(CardService.newTextParagraph().setText("<b>What would you like to do?</b>"))
      .addWidget(CardService.newTextButton()
        .setText("Summarize Email")
        .setOnClickAction(summarizeAction)
      )
  );

  // Draft reply button
  const replyAction = CardService.newAction()
    .setFunctionName("actionDraftReply")
    .setParameters({ emailBody: message.body });

  card.addSection(
    CardService.newCardSection()
      .addWidget(CardService.newTextButton()
        .setText("Draft a Reply")
        .setOnClickAction(replyAction)
      )
  );

  // Extract tasks
  const tasksAction = CardService.newAction()
    .setFunctionName("actionExtractTasks")
    .setParameters({ emailBody: message.body });

  card.addSection(
    CardService.newCardSection()
      .addWidget(CardService.newTextButton()
        .setText("Extract Tasks & Deadlines")
        .setOnClickAction(tasksAction)
      )
  );

  return card.build();
}

4️⃣ Email Extraction Helper

EmailHelpers.gs

function getEmailContent(e) {
  const messageId = e.gmail.messageId;
  const message = GmailApp.getMessageById(messageId);

  return {
    from: message.getFrom(),
    to: message.getTo(),
    subject: message.getSubject(),
    body: message.getPlainBody()
  };
}

5️⃣ Gemini-Powered Actions

Actions.gs

function actionSummarize(e) {
  const body = e.emailBody;

  const prompt = `
Summarize the following email. Provide:
- A short summary
- Key points
- Action items (if any)
`;

  return createResultCard(callGemini(prompt, body));
}

function actionDraftReply(e) {
  const body = e.emailBody;

  const prompt = `
Draft a professional email reply to the following message.
Maintain clarity, politeness, and a helpful tone.
`;

  return createResultCard(callGemini(prompt, body));
}

function actionExtractTasks(e) {
  const body = e.emailBody;

  const prompt = `
Extract all tasks, deadlines, commitments, and follow-up actions
from the email below. Output as a task list.
`;

  return createResultCard(callGemini(prompt, body));
}

6️⃣ Common Result Card Generator

UIResult.gs

function createResultCard(resultText) {
  const card = CardService.newCardBuilder();

  card.setHeader(
    CardService.newCardHeader().setTitle("AI Assistant Result")
  );

  card.addSection(
    CardService.newCardSection()
      .addWidget(CardService.newTextParagraph().setText(resultText))
  );

  return card.build();
}

7️⃣ Reuse Your Existing Gemini Helper

GeminiHelpers.gs

function callGemini(prompt, text) {
  const url = `https://generativelanguage.googleapis.com/v1/models/${GEMINI_MODEL}:generateContent?key=${GEMINI_API_KEY}`;

  const payload = {
    contents: [{ parts: [{ text: `${prompt}\n\n${text}` }] }]
  };

  const res = UrlFetchApp.fetch(url, {
    method: "post",
    contentType: "application/json",
    payload: JSON.stringify(payload),
    muteHttpExceptions: true
  });

  const json = JSON.parse(res.getContentText());
  return json.candidates?.[0]?.content?.parts?.[0]?.text || "No response from Gemini.";
}

🔬 Testing Instructions

  1. Install the Add-on as an unpublished Gmail Add-on
  2. Open any email
  3. Look for the AI Email Assistant icon
  4. Test:
    • “Summarize Email”
    • “Draft a Reply”
    • “Extract Tasks”
  5. Verify Gemini generates results smoothly

🎉 What You Built in Issue #5

You now have:

✔ A fully functional Gmail side panel
✔ One-click AI-generated summaries
✔ Automatic draft replies
✔ Task extraction from long threads
✔ Reusable AI-powered components
✔ A foundation for a full Gmail AI copilot

This is a huge step — you’re now building the exact type of workflow automation companies pay thousands to implement.

🔥 Coming Next: Issue #6 — Gemini-Powered Slide Creator for Google Slides

You’ll build an AI tool that:

  • Creates presentations automatically
  • Generates slide structure, titles, bullets
  • Inserts content into a Slide deck
  • Builds entire presentations from a topic