Why Do You Need Last Year's AGI to E-File This Year?
Typing in a number from last year’s return can feel like a strange hurdle before filing this year’s, but it’s doing a specific and fairly important job: proving the person filing is actually who they say they are.
The short answer
Prior-year adjusted gross income, or AGI, functions as an identity check for e-filing, standing in for a signature since there’s no physical one on an electronic return. The IRS compares the AGI entered this year against what it has on file from the previous year’s return, and a match confirms the filer has access to that prior return’s information, which is treated as reasonable evidence of identity.
Why AGI specifically
AGI was chosen as the verification figure because it’s a number that appears on every return, is calculated rather than simply stated, and isn’t something a stranger would typically already know. It plays a role somewhat similar to an Identity Protection PIN: both exist to confirm that the person submitting a return is the same person who filed, or is otherwise authorized to file, under that identity. The difference is that AGI verification applies broadly by default, while an IP PIN is a separate, specifically assigned code.
What causes a mismatch
- Using the wrong year’s figure. It’s easy to enter the AGI from two years back instead of the most recent filed return, especially when working from memory rather than a saved document.
- Filing an amended return in between. If a prior-year return was later amended, the AGI on file with the IRS may differ from what appeared on the original as-filed return, depending on which version the system checks against.
- A late-processed prior return. If last year’s return was still processing, was filed very late, or was affected by an identity verification hold, the IRS’s system may not yet reflect the figure a taxpayer expects to use.
What happens after a mismatch
A mismatched AGI typically causes the return to be rejected rather than silently accepted with an error, similar to other e-file rejections tied to identity-matching fields. The usual fix is confirming the exact figure from the actual prior-year return as filed, sometimes by pulling a copy through an IRS account or a saved copy from whatever software or preparer handled it, including free options like IRS Free File if that’s what was used before.
Where to find last year’s figure
The AGI appears on a specific line of the prior year’s return, and it’s worth locating the actual filed copy rather than an estimate, since even a small discrepancy causes a rejection. For a first-time filer, or someone who didn’t file the previous year, entering zero as the prior-year AGI is generally the standard approach, though the exact instructions can vary and are set by the government from year to year.
The takeaway
Prior-year AGI isn’t an arbitrary hurdle; it’s the closest thing e-filing has to a signature check. Keeping a copy of each year’s filed return on hand makes that check a formality rather than a recurring source of rejected filings.