Auto-Classify and Draft Gmail Replies with Claude API and Google Apps Script

/**
 * Fetch unread emails and process them with Claude
 * Recommended GAS trigger: time-based, every hour
 */
function processUnreadEmails() {
  const apiKey = PropertiesService.getScriptProperties()
    .getProperty('CLAUDE_API_KEY');
  
  // Fetch up to 10 unread threads from the last 24 hours
  const threads = GmailApp.search('is:unread newer_than:1d', 0, 10);
  
  if (threads.length === 0) {
    console.log('No unread emails');
    return;
  }
  
  threads.forEach(thread => {
    const messages = thread.getMessages();
    const latestMessage = messages[messages.length - 1];
    
    // Skip automated emails (newsletters, notifications)
    const from = latestMessage.getFrom();
    if (isAutoSentEmail(from)) {
      latestMessage.markRead();
      return;
    }
    
    const subject = latestMessage.getSubject();
    const body = latestMessage.getPlainBody().slice(0, 2000);
    
    const result = classifyAndDraftReply(apiKey, subject, body, from);
    applyLabelAndCreateDraft(thread, latestMessage, result);
    latestMessage.markRead();
  });
}
 
/**
 * Basic check to skip automated sender addresses
 */
function isAutoSentEmail(from) {
  const autoPatterns = [
    'noreply', 'no-reply', 'donotreply',
    'notification', 'newsletter', 'mailer-daemon'
  ];
  return autoPatterns.some(pattern => 
    from.toLowerCase().includes(pattern)
  );
}

/**
 * Call Claude API to classify the email and generate a reply draft
 * @returns {Object} { category, priority, summary, draftReply }
 */
function classifyAndDraftReply(apiKey, subject, body, from) {
  const prompt = `
Analyze the following email and respond in JSON format only.
 
From: ${from}
Subject: ${subject}
Body:
${body}
 
Respond with this exact JSON structure:
{
  "category": "inquiry|collaboration|technical|spam|other",
  "priority": "high|medium|low",
  "summary": "One sentence summary (under 20 words)",
  "draftReply": "Professional reply draft (under 150 words)"
}
 
Category definitions:
- inquiry: questions about products, services, or pricing
- collaboration: partnership proposals, guest posts, sponsorships
- technical: bug reports, technical questions
- spam: unsolicited sales, irrelevant promotions
- other: anything else
`;
 
  const payload = {
    model: 'claude-haiku-4-5-20251001',
    max_tokens: 1024,
    messages: [
      { role: 'user', content: prompt }
    ]
  };
 
  const options = {
    method: 'post',
    contentType: 'application/json',
    headers: {
      'x-api-key': apiKey,
      'anthropic-version': '2023-06-01'
    },
    payload: JSON.stringify(payload),
    muteHttpExceptions: true
  };
 
  try {
    const response = UrlFetchApp.fetch(
      'https://api.anthropic.com/v1/messages',
      options
    );
    
    if (response.getResponseCode() !== 200) {
      console.error('API error:', response.getContentText());
      return { category: 'other', priority: 'low', draftReply: '' };
    }
    
    const data = JSON.parse(response.getContentText());
    const content = data.content[0].text;
    
    // Extract JSON even if Claude wraps it in a code block
    const jsonMatch = content.match(/\{[\s\S]*\}/);
    if (!jsonMatch) {
      return { category: 'other', priority: 'low', draftReply: '' };
    }
    
    return JSON.parse(jsonMatch[0]);
    
  } catch (error) {
    console.error('Processing error:', error.toString());
    return { category: 'other', priority: 'low', draftReply: '' };
  }
}

/**
 * Apply labels based on classification and create a reply draft
 */
function applyLabelAndCreateDraft(thread, message, result) {
  const { category, priority, summary, draftReply } = result;
  
  // Get or create the category label
  const labelName = `AI-Classified/${category}`;
  let label = GmailApp.getUserLabelByName(labelName);
  if (!label) {
    label = GmailApp.createLabel(labelName);
  }
  thread.addLabel(label);
  
  // Flag high-priority emails separately
  if (priority === 'high') {
    const urgentLabel = GmailApp.getUserLabelByName('AI-Classified/⭐ Needs Action')
      || GmailApp.createLabel('AI-Classified/⭐ Needs Action');
    thread.addLabel(urgentLabel);
    thread.markImportant();
  }
  
  // Create draft reply (skip spam)
  if (category !== 'spam' && draftReply) {
    const replyBody = `${draftReply}\n\n---\n[AI draft | ${summary}]`;
    GmailApp.createDraft(
      message.getFrom(),
      `Re: ${message.getSubject()}`,
      replyBody,
      {
        threadId: thread.getId(),
        inReplyTo: message.getId()
      }
    );
  }
  
  console.log(`Processed: ${category} / ${priority} / ${summary}`);
}