GSTR-1 is the GST return where you report every outward supply — your sales, invoice by invoice — monthly or quarterly under the QRMP scheme. Monthly filers submit by the 11th of the following month. What you report in GSTR-1 flows into your buyers' GSTR-2B for their credit, and since July 2025 it also locks your own GSTR-3B liability, correctable only through GSTR-1A.
In GSTR-1 you report B2B sales invoice-wise with the buyer's GSTIN, B2C sales in summary, exports, credit and debit notes, advances, and an HSN-wise summary. For e-invoicing businesses, most of this auto-populates from the invoices already reported to the IRP. Once filed, your declared liability is carried into GSTR-3B and hard-locked — if you spot an error after filing, the fix is an amendment through GSTR-1A before you file that month's GSTR-3B, not an edit inside GSTR-3B itself.
For an MSME owner the discipline is simple: invoice correctly during the month, close billing a few days early, and file by the 11th. Filing late doesn't just cost you a late fee — your B2B customers don't see the invoice in their GSTR-2B that month, their credit is delayed, and repeat offenders quietly lose customers over it.
Two traps. First, a wrong buyer GSTIN sends the credit to the wrong taxpayer and needs an amendment in a later period. Second, the three-year bar in force since November 2025: returns cannot be filed once three years have passed from the due date, so old pending GSTR-1s are now a permanently closing window.
Act on it
Reviewed to the law in force in FY 2026-27. General information, not advice — confirm the position for your facts before acting.