Guide
How to know which document an AI took its answer from before you reply to the SAT (accountants, Mexico 2026)
What a verifiable citation is, why an answer without document and excerpt is useless to an accountant, and a three-minute test for any tool.
Published September 1, 2026 · Updated September 1, 2026
The short answer
A document AI tells you where an answer came from when it hands you three things together: the document's name, the exact excerpt it used, and a link that opens the original at that spot. If it only gives you the figure, or a list of "sources" without the excerpt, you can't verify it and it's no use for replying to the SAT, Mexico's tax authority.
This guide explains what a verifiable citation is, how to test in three minutes whether your AI has one, and what Wurz does about it.
The problem
Why an answer without a citation is no use to an accountant
When an accountant in Mexico uses a figure in a tax return or in the reply to an official notice, that figure carries their signature. If the SAT asks where it came from, "an AI told me" is not an answer. The answer is a CFDI or a bank statement that can be opened and shown.
An AI answer you can't verify is useless for tax work: if the figure is wrong before the SAT, the accountant answers for it, not the tool.
A case that keeps happening: you upload twenty of a client's CFDI to ChatGPT and ask how much they invoiced in March. It replies with a round figure, in a confident tone, with the VAT broken out. When you check it against the XML files, the figure appears in none of them: the model added two February invoices, ignored a credit note and rounded. Nothing in the answer warned you. If that number goes into the client's return, the problem is yours, and you find out when the tax authority's letter arrives.
The definition
What "verifiable citation" means (and what it doesn't)
A verifiable citation has three parts, and all three are required.
- 1
The document, by name
"One of your invoices" doesn't count; the CFDI with its folio and issuer does.
- 2
The excerpt
The exact line of that document the figure came from, as written, not rephrased.
- 3
The original one click away
Opening the document at that spot and seeing with your own eyes that the excerpt is there and says that.
If any of the three is missing, what you have is a reference, and a reference can't be defended before anyone.
The most common confusion: many tools say they "cite their sources" and what they deliver is a list of file names at the foot of the answer. That tells you which documents it read, not which one each figure came from or what it said. To check, you'd have to open each one and search yourself, which is exactly the work you wanted to save.
The difference between summarizing and citing: a summary tells you what the document says; a citation tells you where it says it, so you can check it yourself.
One more detail. The excerpt has to come from the document, not from the model. Some tools ask the same model to "cite", and the model writes a sentence that resembles the document's or invents one that sounds right. A real citation is checked against the original text by a program before you're shown it.
The test
How to test any AI in three minutes
If you already use a tool that shows you the excerpt and opens the original, skip this part. For the rest, the test works with ChatGPT, NotebookLM or any tool that takes documents. You need three or four PDFs or XML files from the same client whose contents you already know.
- 1
Ask something with a single correct answer
Upload the documents and ask something concrete: "what's the subtotal of the January invoice?", "what's the closing date on the bank statement?".
- 2
Check whether it names the document
If it says "according to your invoices" without saying which, you won't be able to defend it.
- 3
Check whether it shows the excerpt
If it only gives the figure, ask "show me the exact excerpt you took it from" and compare it with the PDF open beside it. If the sentence isn't in the document word for word, it made it up.
- 4
Try to open the original from the answer
If there's no way to reach the document at that spot with one click, every check costs you opening the file and searching by hand. With twenty figures in a working paper, that's an hour.
- 5
Ask something that isn't in the documents
"How much income tax did this client pay in 2019?" when you only uploaded 2026. A serious tool tells you it can't find it. One that makes it up has just shown you how it'll behave when you're not checking.
- 6
Mix two clients' documents and ask again
If it takes a figure from the wrong client without warning you, it's no use for a firm, however well it cites.
If your tool passes all six, use it with confidence. If it fails 3 or 5, everything it tells you has to be redone by hand, and that's the afternoon you wanted to save.
What the test doesn't tell you is how long the result lasts. These tools change every month, and the one that admits today it can't find the 2019 income tax may invent it in October's version. We have no better answer than repeating the test every time they update.
Where Wurz comes in
What Wurz does
Wurz is built for a firm with many clients. The first thing is one space per client and period: each client's CFDI, XML files, bank statements and scanned PDFs live in their own space, and when you ask, Wurz only looks at that client and that period. So a question about one client's March isn't answered with another client's invoice.
Everything else follows from that. Wurz asks that client's documents and answers with the exact citation, which document and which excerpt, without keeping your files. Before showing you the answer, a program looks for each quoted sentence inside the excerpt it claims to come from. Every Wurz answer includes the document's literal quote, so you can check it letter by letter against the original before using it. One click opens the document at that spot. If it's a scan or a photo, Wurz runs OCR on it when it arrives and cites it just the same.
That space is shared with your team or with the client, with permissions per person, so whoever asks sees the same documents and the same citations you do.
The files stay in your Google Drive, OneDrive or Dropbox. Wurz reads them there; it doesn't copy them anywhere or upload them to ChatGPT. There's a free plan to try it with real documents, and paid plans start at $349 MXN a month.
What Wurz does not do, so there's no confusion: it doesn't stamp invoices, doesn't post entries or file returns, and doesn't interpret the law or give tax opinions. It finds and cites what your documents say. The judgment stays with the accountant.
And if your doubt is a different one, what uploading client documents to ChatGPT implies, that question has its own guide: Is it safe to upload your clients' information to ChatGPT?
Common questions
What people ask most about this
The AI made up a figure from an invoice. What do I do?
Don't use it. Open the CFDI and compare: if the figure or the line item isn't there word for word, the model generated it. Then demand that every figure come with the document's name and the exact excerpt, and check the excerpt against the original. A tool that doesn't show you the excerpt gives you no way to catch the next invention in time.
What does it mean for an AI to "cite its sources"?
It depends on the tool. For many, "citing" means listing file names at the foot of the answer. For an accountant, citing means saying which document and which excerpt each figure came from, with the original one click away to check it. Only the second is any use for replying to the SAT or to the client.
How do I verify that what the AI told me is really in the document?
Ask it for the literal excerpt and compare it with the document open beside it. If the sentence isn't there as written, the model wrote it. With a tool that checks the citation against the original, that comparison is done by the program before you're shown the answer, and you just confirm with one click.
Is there an AI that answers with the document and the page the figure came from?
Yes. NotebookLM and Acrobat's assistant point to excerpts inside the files you upload. For the Mexican case, with CFDI, XML files, bank statements and scans separated by client and period, Wurz answers with the document and the exact excerpt, checked against the original, without your files leaving your Drive.
Can I ask ChatGPT to cite the document?
You can, and sometimes it gives you a sentence that looks like it came from the document. Nobody checks it: ChatGPT can write a plausible quote that isn't in the PDF. And uploading client documents to ChatGPT has its own implications; there's a separate guide on that.
For today
Before you use the next figure
Run the six-point test with three invoices from a client you know, in the tool you already use. If you want to see what an answer with its document and excerpt looks like, there's a demo on the product page, and the free plan can be tried with your own documents.
Try it with your real documents
Upload a handful of receipts from your most demanding client and ask the question that normally takes you half an hour. That's how you'll know if it's worth it.
No credit card. Nothing to install.