Make it check its own work: the one line that catches Claude's mistakes before you do

A confident answer and a true answer aren't the same thing.

Claude writes well. It writes clearly, in full sentences, with numbers and dates in all the right places. That's exactly why its mistakes are easy to miss. A wrong figure in a tidy paragraph looks just as believable as a right one.

The fix is simple, and it takes one line. When Claude finishes a job, tell it to go back over what it just made, check every number against the source, and tell you what it couldn't verify. That one step catches most mistakes before they reach you, let alone a client.

Why confident isn't the same as correct

SI (super intelligence; most people say AI) doesn't sound unsure when it's unsure. It doesn't hedge the way a nervous junior might. It writes the same steady sentence whether the number came straight from your file or got mixed up along the way.

Mistakes usually creep in at the joins. A figure copied from one file into another. Two dates that look alike. A name spelled one way in your notes and another way in the contract. A total worked out from numbers that were never quite the same. None of these are dramatic. They're the same small slips a busy person makes, and they're just as easy to skim past.

The good news is that Claude is much better at catching these when you ask it to look for them. Writing something and checking something are two different jobs. If you only ever ask for the first, you only ever get the first.

Why this matters for an agent

Most of what we hand over in real estate has facts in it that people rely on. Prices, dates, addresses, names, land sizes, settlement terms, inspection times. A vendor report, a property summary, a buyer follow-up, a market update. Get one of those wrong and it isn't just embarrassing. It can confuse a client, cost you trust, or worse.

So the more of your week you hand to Claude, the more this matters. Day 1 was about giving it a job instead of asking it questions. Today is the other half of that: once it's done the job, make it check the job.

What checking actually looks like

When you ask Claude to check its own work against your files, it does something different from writing. It goes back to the source, finds where each fact came from, and compares. If the report says one thing and the file says another, it should flag it. If it can't find where a number came from at all, it should say so.

That last part is the most useful. A list of "things I couldn't verify" tells you exactly where to look. Instead of re-reading the whole document yourself, you check the handful of items it wasn't sure about.

It's not a guarantee. Claude can still miss something, and you're still the one who signs off. But it turns "read every line hoping to spot a problem" into "check these few things it flagged", which is a much better use of your attention.

The Day 17 prompt

Use this straight after Claude has made something for you from your files.

Go back over what you just wrote. Check every number, date and name against the source files. List anything you couldn't verify, and fix anything that's wrong.

Here's what each part does.

• "Go back over what you just wrote." This switches Claude from writing mode into checking mode. It's now reviewing, not producing.
• "Check every number, date and name against the source files." These are the three things most likely to go wrong and most likely to matter. "Against the source files" is the key part. It tells Claude to check against your actual documents, not against what sounds right.
• "List anything you couldn't verify." This is the line that does the heavy lifting. It makes Claude own up to gaps instead of smoothing over them, and it gives you a short list to check yourself.
• "Fix anything that's wrong." If it finds an actual mismatch, it corrects it rather than just pointing at it.

If you'd like to see the fixes before they're made, change the last part to "show me what you'd fix first". That's a good habit while you're still getting a feel for it.

Try it yourself

1. Set up one narrow folder. Copy in a few files that have real facts in them, such as a property's details, some appointment notes and a price list. Copies, not originals.
2. Open the Claude desktop app and add that folder. Just that one folder, nothing else.
3. Give it a job. Ask it to write something from those files, like a one-page property summary or a short follow-up email. Ask it to show you its plan first, then let it go ahead.
4. Paste the Day 17 prompt as your next message, in the same conversation.
5. Read the "couldn't verify" list. Open the source files and check those items yourself.
6. Look at what it changed. Compare the corrected version with the first one so you can see what it caught.

If you want to test it, put a small deliberate mistake into one of your copied files, like two different prices for the same property, and see whether it flags the mismatch. It's a quick way to learn how much to trust the check.

Make it a habit, not a one-off

The real value comes when checking becomes part of every job. A few ways to build that in:

• Add it to the end of your instructions. Instead of a separate message, finish a job request with "when you're done, check every number, date and name against the files and list anything you couldn't verify."
• Use it before anything goes out. Anything heading to a client, a vendor or a buyer should get a check first. It costs you a minute.
• Keep approvals on for anything that sends, spends or deletes. Checking is a safety net, not a replacement for you saying yes. Nothing should go out the door until you've seen it.
• Keep the folder narrow. The fewer files Claude is working from, the easier it is for it to trace every fact back to its source, and the easier it is for you to check it too.

And if the "couldn't verify" list keeps coming back long, that's useful information. It usually means the job needs better source files, or a clearer instruction about where to find things.

Where to go from here

Claude will keep sounding confident. That's just how it writes. Your job isn't to trust it less, it's to build the check into how you work, the same way you'd ask a new assistant to double-check a contract before it goes out.

One line after each job. Every number, date and name against the source. A short list of what it couldn't confirm. That's most of the mistakes caught before anyone else sees them.

Missed yesterday? Day 16: The handbrake

Every prompt from the series is on my free prompts page, with a new one each day as the episodes go up, ready to copy.