AI in Google Sheets: Custom Functions vs Add-ons vs Direct API

You want AI inside your spreadsheet. Not in a separate tab. Not through copy-pasting from ChatGPT. Actually inside Google Sheets, returning results right in your cells.

There are three ways to do this. Each one comes with real tradeoffs. This is an honest look at all of them.

Option 1: Custom functions with Apps Script

Google Sheets lets you write JavaScript in the built-in Apps Script editor (Extensions > Apps Script). You can make a custom function that calls an AI API and puts the result in a cell.

Here is what a basic version looks like:

function AI(prompt) {
  const apiKey = PropertiesService.getUserProperties().getProperty('OPENAI_KEY');
  const response = UrlFetchApp.fetch('https://api.openai.com/v1/chat/completions', {
    method: 'post',
    headers: {
      'Authorization': 'Bearer ' + apiKey,
      'Content-Type': 'application/json'
    },
    payload: JSON.stringify({
      model: 'gpt-4o',
      messages: [{ role: 'user', content: prompt }]
    })
  });
  return JSON.parse(response.getContentText()).choices[0].message.content;
}

Now you can type =AI("Write a tagline for a coffee shop") in a cell. It works.

The tradeoffs:

That 30-second limit is the big one. Complex prompts or slow API responses will time out and return an error. You also need to build your own caching layer. Without it, formulas re-run every time the sheet recalculates. And that happens constantly.

Best for: Developers who want a quick prototype, need a single specific AI call, and are comfortable maintaining Apps Script code.

Option 2: Marketplace add-ons

Add-ons like SheetMagic install from the Google Workspace Marketplace in a few minutes. They give you pre-built custom functions that handle all the infrastructure. API calls, caching, error handling, retries, and rate limiting are all taken care of. You just write formulas.

=AITEXT("Write a tagline for a coffee shop")
=AIIMAGE("Logo for a minimalist coffee brand, flat design")
=GPTV(A2, "What product is shown in this image?")
=AILIST("5 marketing angles for a coffee subscription service")

The tradeoffs:

The main cost is the subscription. But think about what you get. The custom function approach means building and maintaining caching, error handling, multi-provider support, and media generation yourself. At any real scale, the engineering time costs more than $19/month.

Best for: Anyone who wants AI in their spreadsheet without writing or maintaining code. Teams that need multiple AI features (text, images, vision, scraping) in one tool.

Option 3: Direct API integration

Skip the spreadsheet layer entirely. Write a Python script, a Node.js service, or use a platform like Zapier or Make to call AI APIs. Then push results back into Google Sheets through the Sheets API.

import openai
import gspread

client = openai.OpenAI()
gc = gspread.service_account(filename='credentials.json')
sheet = gc.open('My Sheet').sheet1

for i, row in enumerate(sheet.get_all_values()[1:], start=2):
    prompt = f"Write a tagline for: {row[0]}"
    response = client.chat.completions.create(
        model="gpt-4o",
        messages=[{"role": "user", "content": prompt}]
    )
    sheet.update_cell(i, 2, response.choices[0].message.content)

The tradeoffs:

This is the right choice when you're building a production pipeline that happens to output to a spreadsheet. It's way too much when a marketing manager just wants to generate 50 product descriptions and tweak prompts on the fly.

Best for: Engineering teams building automated data pipelines. Cases where the spreadsheet is an output destination, not the workspace.

The comparison

Which one should you pick?

Pick custom functions if you're a developer who needs one specific AI call, you enjoy maintaining Apps Script, and you don't need image generation, vision, or web scraping.

Pick an add-on if you want AI in your spreadsheet today, you need more than just text generation, and you'd rather pay $19/month than build and maintain infrastructure.

Pick direct API if the spreadsheet is just an output format for an automated pipeline, you need to process tens of thousands of rows in batch, or you need fine-grained control over every API parameter.

Most people reading this want AI inside their spreadsheet as a working tool. Not as a software engineering project. That's where add-ons win.

Have questions about which approach is right for your use case? visit the FAQ.