Which job is worth giving to the screen?
Computer use lets Claude see your screen, click and type in any program. It's the slowest way for Claude to work, so give it only the jobs nothing faster can reach.
Think of the old accounting package, the booking tool or the till software that only has a window. Name the program and the job.
The answer decides how much checking the job needs. A wrong quantity in an order list is cheap to fix. A wrong bank number is not.
How does Claude work a window?
Guess before you start. The Do part shows you both answers.
Show the answer
The most precise way first. Claude uses a connector if there is one, then the browser for web pages, and clicks and types on the screen only when nothing else reaches the program. The screen is the slowest way in and the easiest to get wrong.
Ledger 98, the old program in this lesson, has no connector and no web page, so the screen is the only way in.
Show the answer
It looks again, if you ask it to. Claude works in a loop: a screenshot, an action, another screenshot. Anthropic's computer use guide warns that Claude sometimes acts as if an action worked without checking, so your prompt asks for a look after every save.
Hand the orders to Claude and watch it type
Five steps, about 15 minutes. Tick each one when it's done.
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Put the kit in a folder and open it in the Code tab.
Unzip the kit into a folder you'll keep, such as Documents. Open that folder in the Code tab with the mode set to Manual, as you did in course 2, lesson 4.
The Harbor Hardware kit
Harbor Hardware is a made-up shop that still types its orders into an old program called Ledger 98. The ZIP holds orders.csv with the five orders, ledger.py (Ledger 98 itself), check.py (a checker that compares what Ledger 98 saved with the orders) and a short README.txt. You never open the two .py files. Claude runs them for you. Unzipping gives you a folder called c4-01.
Download the ZIP (2 KB) -
Turn on computer use, then clear your screen.
This is a switch you click yourself. Open Settings. If the menu lists This computer, go to This computer › System. Otherwise open Capabilities. Under Computer use, turn on Enable computer use.
On a Mac, the same page shows two permissions, Accessibility and Screen Recording. Click any that isn't granted, allow Claude in the System Settings window that opens, and restart Claude Desktop if it asks.
Claude takes screenshots while it works and may hide your other windows. Close mail, banking and anything else private first.
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Give Claude the prompt.
Paste it into the Code tab and press Enter.
Start Ledger 98 for me. It's ledger.py in this folder, an old program with a window. Use what's already on my computer, and tell me what and why before you install anything. Then use computer use to type the five orders from orders.csv into the Ledger 98 window, the way a person would: click a box, type, and click Save order. Never write to ledger.csv yourself, and never change ledger.py, check.py or orders.csv. Every order goes in through the window. After each save, look at the screen and check that the line under the boxes says Order saved for that customer before you start the next one. If the screen looks different, or an order may have saved twice, stop and tell me. Don't try it again on your own. When all five are in, run check.py and show me exactly what it prints.
The first line gives the job and how to start the program.
"Use computer use" names the way in. In the Code tab Claude could also write the ledger file directly, so the next line rules that out: every order goes in through the window.
"After each save, look" asks for a check after every step, and "stop and tell me" says what to do when a check fails. A retry after an unclear screenshot is how an order gets saved twice.
"Run check.py" asks for proof you can read.
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Approve what Claude asks, and watch.
First comes a request to run a command that starts Ledger 98. A small window called Ledger 98 - New order opens. Ledger 98 runs on Python, a free tool many computers already have. If yours lacks it, Claude should say what it wants to install before it asks.
Next Claude asks to use the app. The request may call it Python, because that's what runs Ledger 98. Click Allow for this session. On a Mac it may also ask once before it takes over the full screen. The last request runs the checker.
Read each request before you approve it, and say no to anything you didn't expect. Keep your hands off the mouse while Claude types. You can stop it at any point with the stop button.
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Ask Claude what it did.
Send this in the same session and read the answer:
Tell me in plain words what you did, one line per order, and anything on the screen you weren't sure about. Don't run anything new.
Its five lines should match the five orders, and it should end with the checker's PASS. If it mentions something odd on the screen, read that part before the Check.
If you see this instead
- There's no Enable computer use switch
- Computer use needs a Pro, Max or Ultra plan and a current Claude Desktop. Update the app, restart it and look again in Settings. Team and Enterprise plans don't have it yet.
- Ledger 98 doesn't open, or its window is empty
- Paste: "Ledger 98 didn't open properly. Find out why, and tell me what you need before you install anything." Some computers have a version that can't draw windows, and Claude can usually find one that can.
- Claude says it can't see or use the window
- Ledger 98 may be behind another window. Bring it to the front, and allow it when Claude asks (it may be called Python). On a Mac, check that Screen Recording is allowed on the Settings page from step 2.
- Claude types into the wrong box or skips one
- That's the slip the check is for. Stop it, then paste: "Delete ledger.csv, close Ledger 98, start it again and enter all five orders, checking each save."
Did all five orders land, once each?
Look at the Ledger 98 window. Then open ledger.csv from your folder: it opens in Numbers, Excel or a text editor.
In the Code tab Claude can write files, so the file alone doesn't prove it used the window. The list inside Ledger 98 only grows when someone clicks Save order.
You pass when the list under Saved in this window shows five orders numbered 1 to 5, ledger.csv has the same five lines under its first row, and the checker printed PASS: all 5 orders are in the ledger, once each.
The five are Alma Reyes (Claw hammer, 2), Birch Street Cafe (Paint roller, 6), Cole Marsh (Cordless drill, 1), Dana Whitfield (Wood screws box, 10) and Eli Park (Tape measure, 3).
Then break it on purpose. Type Eli Park, Tape measure and 3 into Ledger 98 yourself and click Save order. Ask Claude: "Run check.py again and show me exactly what it prints." It should print FAIL and Saved 2 times: Eli Park, Tape measure, 3.
Two questions from memory
Why does Claude try a connector or the browser before it uses the screen?
The screen is the slowest way in and the easiest to get wrong. Claude uses the most precise way it has, and clicks and types only when nothing else reaches the program.
Claude says all five orders are in. Why look at Ledger 98's own list as well?
Claude sometimes acts as if a step worked without checking, and in the Code tab it could write the ledger file directly. The list in the window only grows when Save order is clicked, so it shows the orders went in through the program.
Lesson 2 opens by asking the first one again.
Developers run this loop without Claude Desktop, through the computer use tool in Anthropic's API, on a computer set up only for the job. That needs an API key, a paid password for using Claude from a program, and every screenshot Claude looks at is billed.
Course 4 so far
What you've built
- Lesson 1: Old program