Build your first software tool in Claude Code

Describe a tool out loud. Forty minutes later, open a working app in your browser. No code, no engineer, no budget.

Today you are going to build a real piece of software by describing it. Not a document. Not a prompt. A working tool you can open in your browser, hand to a teammate, and use on Monday. The example here is a deal-margin calculator your sales team can punch numbers into, but the move is the same for any small internal tool: you say what you want in plain English, Claude writes the code, and you judge it by whether it works.

This is the thing most CEOs don't realize they can already do. You will not read a line of code. You will not install a programming language. You describe the outcome the way you would brief a sharp new hire, and you get back something that runs.

This is the how-to companion to what is vibe coding. If you have not read it, the one-line version is this: vibe coding means you describe software and let the AI write it without you reading the code. It is exactly right for a disposable internal tool like the one we are building, and exactly wrong for anything that touches customer data or money. We will stay on the safe side of that line the whole way.

What you'll have when you're done

A single file called index.html sitting in a folder on your laptop. Double-click it and a clean calculator opens in your browser. Your rep types in the cost, the list price, and the discount they want to give. The tool shows the final price and the gross margin, and it turns green when the margin clears your floor and red when it does not. No server, no login, no internet connection required. You can email the file to your whole sales team and every one of them can open it the same way.

My reps kept discounting past the floor

For a long stretch at Headphones.com, margin leaked one "small" discount at a time. A rep would knock 40% off to close a deal on a Friday, and nobody did the math in the room on what that did to the margin until the month closed and it was too late. I didn't need a CRM feature or a finance hire. I needed a dead-simple thing a rep could open mid-call, type three numbers into, and see a red light when the deal dipped below the floor we'd set. That tool didn't exist, and it wasn't worth a budget line. So I described it to Claude and had it in the time it takes to drink a coffee. That is the whole category this unlocks: the useful little tool that was never worth paying for.

What you need first

• A Mac, Windows, or Linux laptop. Any of the three is fine.
• A paid Claude plan. Claude Code is included with Claude Pro, Max, Team, or Enterprise. The free Claude.ai plan does not include it. If you are paying for Claude already, you almost certainly have access.
• Claude Code itself, which you will install in Step 1. It is Anthropic's official tool that runs on your laptop and can read and write files in a folder you point it at.
• Forty minutes. Most of it is you describing what you want and trying the result, not waiting.
• Nothing else. No coding language, no accounts, no API keys. If a tutorial ever tells you to paste an API key into a file for a tool like this, stop; that is the line where you bring in help.

Step-by-step

Step 1Install Claude Code

The terminal is the plain-text app that comes free on every computer. On a Mac it is called Terminal (find it with Spotlight: press Cmd+Space, type "Terminal"). On Windows it is PowerShell. You will type one line into it, once.

Open the terminal and paste this line, then press Enter:

curl -fsSL https://claude.ai/install.sh | bash

That is the official installer for Mac, Linux, and Windows WSL. On Windows PowerShell, use this line instead:

irm https://claude.ai/install.ps1 | iex

If you would rather not touch the terminal at all, Anthropic also ships a Claude Code desktop app for Mac and Windows that you install like any normal program; the rest of these steps work the same way inside it.

When the install finishes, confirm it worked:

claude --version

You should see a version number print out. If you see "command not found," close the terminal window, open a fresh one, and run it again.

Step 2Make a folder and open Claude inside it

A folder is just a folder, the same kind you already use. Claude Code works inside whatever folder you start it in, so we will make a clean one for this tool. Paste these two lines:

mkdir -p ~/tools/margin-calculator && cd ~/tools/margin-calculator

That creates a folder called margin-calculator and moves you into it. Now start Claude:

claude

The first time you run it, it opens your browser to log in. Sign in with the same account as your paid Claude plan, come back to the terminal, and you are in. You will see a prompt waiting for you to type.

Step 3Turn on plan mode, then describe the tool

Before you let Claude build anything, turn on plan mode. It is a safety setting: Claude figures out what it intends to do and writes it back to you in plain English, and it changes nothing until you say go. Press Shift+Tab until the prompt shows it is in plan mode (you will see the mode label change at the bottom).

Now describe the tool. Type or paste this, exactly as a brief, then press Enter:

Build me a single self-contained index.html file. No external libraries,
no internet connection needed, everything in the one file so I can open it
by double-clicking. It is a deal-margin calculator for my sales team.

Three input boxes:
- Our cost per unit, in dollars
- The list price, in dollars
- The discount the rep wants to give, as a percentage

When any box changes, show:
- The final price after the discount
- The gross margin, as both a percentage and a dollar amount
- A big GREEN check if the margin is 35% or higher, a big RED warning
  if it is below 35%

Make it clean and easy to read on a phone. Don't use any frameworks.

Notice what that brief does. It states the outcome, lists the inputs and outputs like you would for a new hire, and sets two hard constraints ("one file," "no frameworks") that keep the result simple enough to run anywhere. Vague one-line asks make Claude guess. A brief like this does not.

Step 4Read the plan, then let it build

Claude will respond with a short plan: it intends to create index.html, with the three inputs, the margin math, and the green/red guardrail. Read it. You cannot read code, but you can absolutely read "I'm going to create one file with three inputs and a margin check," and know whether that is what you asked for.

If it matches, approve it (Claude tells you which key to press, usually Enter or "yes"). Claude will then write the file. As it works, it may pause to ask permission before creating the file. That pause is the system working, not getting in your way. For a single HTML file in a fresh, empty folder, saying yes is safe; read the one-line summary and approve.

Step 5Open your tool and test it

Once Claude says the file is written, ask it to open the tool:

Open index.html in my browser.

Claude will run the command that opens the file (or you can open the folder in Finder or File Explorer and double-click index.html yourself). Your calculator appears in the browser.

Now test it with a deal you already know the answer to. Cost of 40, list price of 100, 30% discount. The final price should read $70 and the margin should read $30, which is about 43%, and the light should be green. Push the discount to 65% and the margin drops below your floor and the light turns red. That red light is the entire point: it is the thing your rep sees mid-call that makes them stop and ask.

Step 6Bank a working copy, then change one thing

The moment it works, save a copy you can always return to. Tell Claude:

Set up version control in this folder and save this working version as the
first checkpoint, so I can always get back to it if I break something.

Claude will run the handful of commands that do this (this is git, the standard tool for saving versions of files; you do not need to learn it, just ask Claude to use it). Now you are safe to experiment, because you can always say "go back to the saved version."

Then change one thing to feel the loop:

Make the margin floor 40% instead of 35%, and add a fourth box at the top
for the rep's name so it shows on the calculator. Then explain what you
changed, like I'm not technical.

That last sentence is a habit worth keeping: every change is a free lesson if you ask Claude to explain it in plain English. Re-open the file, check the new floor turns red in the right place, and you have done a full build-and-iterate loop on a real piece of software.

How you'll know it's working

Open the file on a phone, not just your laptop. The inputs should still be tappable and readable, and the green/red light should still be obvious at a glance. Then run three deals you know cold and confirm the margin matches your own math each time. When you can hand the file to one rep, watch them punch in a live deal, and see the light turn red on a discount that should never have been offered, the tool is working. That is also the moment it starts paying for the forty minutes.

When it breaks

• The page opens blank, or nothing happens when you double-click. Do not try to fix it yourself. Go back to the Claude session and say: "The page is blank when I open index.html. Fix it and tell me in plain English what was wrong." Let Claude debug its own work.
• The math looks off. Verify by hand on two deals you know the answer to, then tell Claude the exact numbers: "Cost 40, list 100, 30% off should show $70 and 43% margin. It is showing something else." Specific numbers get a specific fix; "the math is wrong" does not.
• Claude keeps adding things you didn't ask for. This is scope creep, and plan mode is the cure. Stay in plan mode, approve the scope before each change, and say: "One file, no libraries, nothing I did not ask for. Just the change I described."
• You changed something and now it is worse than before. This is exactly why you banked a checkpoint in Step 6. Tell Claude: "Undo the last change and go back to the saved working version." Then make a smaller change.
• A teammate says they can't open it. For a single self-contained HTML file, the fix is almost always the simplest one: send them the actual index.html file (not a link, not a screenshot) and have them double-click it. No install, no setup. That is the whole reason we kept it to one file with no frameworks.
• You start wanting it to do something real, like pull live prices, save deals to a shared place, or sit on a website your customers can reach. That is the boundary from what is vibe coding. The moment a tool touches customer data, money, or the open internet, stop vibe coding it and get someone who can read the code involved. The throwaway calculator is yours to build alone; the production system is not.

Where this fits in your harness

This is your first build, and the muscle it trains is the one that matters: describe an outcome, judge the result, iterate in plain English. The next tool costs you a fraction of the effort, because you already know the shape. A common second build is a CSV cleaner: drag a messy export in, get a tidy one out.

It compounds with the two foundations you may already have. A CLAUDE.md file gives Claude your business context, so your tools come back already speaking in your numbers and your terms. And once you have built the same kind of tool twice, you turn the routine into a skill so you never walk Claude through it again (here is how). For the working habits that make all of this faster, the non-technical CEO's playbook is the companion piece. And the strategic case for working this way at all, in a terminal instead of a chat tab, is why CEOs should use Claude Code in the terminal.

Build the calculator today. Send the file to one rep, watch them run a live deal through it, and reply to tell me what turned red. That red light is the moment the leverage stops being theoretical.

Andrew