Written for Mexico. This analysis applies to Mexican federal tax law — ISR (income tax), IVA (VAT) and SAT rules — and cites Mexican statutes. Amounts are in Mexican pesos (MXN).
For years, filing a return was an act of faith: you reported your numbers and the SAT took them. That is over. Today the authority already has, in real time, every invoice you issue and receive — and your return arrives pre-filled with that data. Declaring something different from what the SAT already knows is not a creative option: it is the fastest way to receive a letter. The only sensible defense is to reconcile it yourself, first.
The return already comes pre-filled with your CFDIs
Ever since the CFDI became mandatory (art. 29 CFF), every invoice you stamp or receive is recorded with the SAT on the spot. On that basis, the authority pre-fills your returns: your monthly IVA and your ISR already arrive with the income from your issued invoices and, increasingly, the deductions from your received ones. The return is no longer a blank page; it is a proposal from the SAT that you confirm or adjust. And that is the point: if you adjust downward without support, or if your CFDIs carry something that doesn't add up, the discrepancy is flagged — not because someone reviewed you, but because the system compared.
What the SAT cross-checks: issued, received and what you declared
The cross-check is common sense, and that is exactly why it is relentless. Your issued invoices are your income: if you invoiced 100 and declared 80, 20 in income is missing. Your received invoices are your deductions and creditable IVA: if you credited IVA from invoices that don't exist, are cancelled, or come from a supplier listed under 69-B, that credit collapses. And everything is compared against what you put in your return. No auditor is needed: the same algorithm that pre-fills your data detects when you declare differently from what your CFDIs say. It is the model we explain in why the SAT no longer reviews only the big players.
The discrepancies that trigger an invitation letter
The most common are four. Invoiced income left undeclared: you issued CFDIs that didn't make it into your return — the most direct discrepancy. IVA credited from cancelled CFDIs: you deducted or credited against invoices the issuer cancelled afterward. Deductions from EFOS: suppliers on the 69-B blacklists, whose IVA and deduction the SAT disregards. Mismatch with your profile: invoicing that doesn't square with your deposits or your lifestyle. None requires an audit to be detected: they fall out of the automatic cross-check and materialize as an invitation letter or deep surveillance (vigilancia profunda) — they are not yet a formal review, but they are the prelude to one, and answering them poorly escalates them.
The monthly reconciliation that saves you the scare
The routine that shields you: each month, download your issued and received CFDIs from the SAT portal, cross-check them against what you are going to declare, and clean up before filing — remove the cancelled ones from the credit, verify that no supplier is on 69-B, confirm that all invoiced income is declared. It is the same exercise the algorithm will run, done by you and in time, while each discrepancy is still fixable with a spontaneous amended return and not with a clarification against the clock. Fifteen minutes a month against the shock of a letter with a deadline. Reconciliation is not bureaucracy: it is the cheapest way to make your return and what the SAT already knows tell the same story.
Frequently asked questions
Where does the SAT get what shows up pre-filled for me?
From your own CFDIs. Every invoice you issue or receive is stamped through the SAT in real time; on that basis, the authority pre-fills your IVA and ISR with your income and, increasingly, your deductions. The pre-filled return is, literally, what the SAT already knows about you — which is why it pays to make your numbers match.
What happens if I credit IVA from a cancelled CFDI?
That credit does not hold up: if the issuer cancelled the invoice, the IVA you credited is left unsupported and the cross-check detects it. That is why, when reconciling, you have to filter received invoices by status and remove the cancelled ones before crediting. Finding it yourself when reconciling is an adjustment; the SAT finding it is a letter.
Is an invitation letter an audit?
No: the invitation letter and deep surveillance (vigilancia profunda) are not audit powers, they are exhortations to self-correct, derived from a cross-check. But ignoring them is the worst path — you confirm to the SAT that the discrepancy exists and you don't intend to fix it, which opens the door to an electronic review or the restriction of your digital seals. Attending to them in time closes the matter cheaply.
How often should I reconcile my invoices?
Ideally every month, before filing your return for the period: you download issued and received invoices, cross-check them against what you are going to declare, and clean up. Monthly reconciliation turns the annual close into a formality and removes surprises. For large volumes it is automated; for most, it is minutes with the SAT portal.
Do your invoices and your returns tell the same story?
The SAT already ran the cross-check; the question is whether you ran it first. We reconcile your issued and received CFDIs against your returns, clean out cancelled invoices and risky suppliers, and hand you the discrepancies with their spontaneous-correction plan — before they arrive as a letter. It is the audit you'd rather run on yourself.