Agent Mode in Microsoft Excel

Incredible power through natural language

I’ve been so busy these days playing with AI products it’s been hard to find time to write about some of my successes. One such success is that I’m building an iOS app using Cursor with Claude Sonnet 4.5 and Claude Code. I’ve been working on this app for nearly 2 months, and it’s written about 15,000 lines of code for me. I will finish up this app and then write about how I built it and all the wonderful and crazy things that are required to do this well.

But what I want to write about is Agent Mode in Excel.

Agent Mode in Excel is quite possibly the best product of 2025 (according to me). It’s the best product to me because Excel is an amazing app that I don’t have time to learn how to use well. Every spreadsheet I have ever built has been to solve some problem. To help me understand something. And there are so many features and capabilities that are just not accessible. I don’t understand why XLOOKUP is better than VLOOKUP. And I don’t care. It’s because I don’t have time. I never learned Regex and I will never have to because ChatGPT can crank out Regex in seconds that gets things done.

I remember the day I learned how to use a Pivot Table. Nothing about it made sense till someone showed me. I was a changed person after that.

The magic in Excel Agent is that it creates complete spreadsheets, with formulas, sheets, tabs, and checks all its work. There were moments where I felt I was working with an Excel expert - in this case an expert in the US Tax Code and IRS Rules and Regulations.

So what was it that I built?

I built a Tax Estimator that can accurately tell me exactly what I will owe in taxes. This is something only my accountant can do and every other tax tool has failed at.

For the past few years I have used this website as it’s the most comprehensive calculator.

https://www.mortgagecalculator.org/calcs/1040-calculator.php

The problem? It’s a junky website and it doesn’t save my data and it’s tedious to fill out. So I set out on a mission to build this in Excel.

My taxes are complicated. My tax return is over a hundred pages. It’s because my wife and I each have LLCs, she is an owner of a business, we are limited partners in a few funds, we have complex deductions, international income, and get K-1s that I don’t understand and never will.

I want to know what my tax bill is because if I get this wrong the IRS imposes penalties I don’t want.

I started by giving the Excel Agent all the fields on this web page and asking it to create a calculator. It did that.

Then I fed it my tax data from 2024 and asked it to fill it in.

Then I went cell by cell and asked it to create a sheet for each cell so that I could then “input” all the data on the sheet.

Along the way it validated its work, it iterated, it pointed out places where things were wrong.

It suggested things to me, it waited for me to provide input to answer questions.

In the end, what I got is exactly what I want. I have no idea how any of the formulas work. But what I see is a very familiar thing. A spreadsheet that I can now fill in each year and it will tell me exactly what I need to know based on dozens of variables that I can change. You input a number and 20 things on the sheet change.

This is empowering and amazing and you should try it out to experience it.

Here is the final product with my personal information blurred. What you see is a row for each input and then sheets for:

• Dividend income

• Schedule E (Passive income)

• Capital Gains

• Interest Income

• Foreign Tax Credit

• Estimated Tax Payments

• Charitable Contributions

• W-2 Omar

• W-2 Lora

• Schedule C - Lora’s LLC

• Schedule C - Omar’s LLC

I built this spreadsheet over the weekend. It took a few hours, but it’s better than anything I could do on my own and I’ve not run across a spreadsheet like this. Now I have my own personal tax calculator that is a file on my hard disk. I can use it every year and it will save me a lot of headache.

It has formulas like this created based on a prompt to “Calculate the Medicare tax sur-tax on earned income (Form 8959)”

=ROUND(LET(
  wO, XLOOKUP(”W-2 Box 5 Medicare Wages”, tblW2_Omar[Field], tblW2_Omar[Value], 0),
  wL, XLOOKUP(”W-2 Box 5 Medicare Wages”, tblW2_Lora[Field], tblW2_Lora[Value], 0),
  se, ROUND(0.9235 * ‘Business Income (SE)’!B6, 0),
  threshold, 250000,
  wageEx, MAX(0, wO + wL - threshold),
  seEx, MIN(se, MAX(0, se + wO + wL - threshold)),
  0.009 * wageEx + 0.009 * seEx
), 0)

The feeling I have is the feeling of empowerment. This is why I am so bullish on AI. It makes tools like Excel, now 40 years old, more valuable and more accessable to do awesome things.

Great job Excel team. This is going to be a winner.