If you've filed GSTR-1 for more than a quarter, you know the feeling: the JSON uploads cleanly to the offline utility, you generate the file, hit "validate" on the portal, and the error log spits out 47 line items in red. The deadline is in three hours. You start fixing them one by one.
After a few hundred filings, the same handful of mistakes keep showing up. Here are the seven that cause roughly 80% of the rejections we see — and how to catch each one before it costs you a Sunday.
1. Place of Supply set to buyer's state on an inter-state invoice
This is the most common one. The invoice is inter-state — say, Karnataka seller, Tamil Nadu buyer — and someone enters "Tamil Nadu" as the Place of Supply but applies CGST + SGST instead of IGST. The portal rejects it because the tax type contradicts the supply nature.
Fix: The rule is simple — if supplier state ≠ buyer state, the supply is inter-state and tax must be IGST. The Place of Supply is the buyer's state, yes, but the tax type is determined by the supplier-vs-buyer state comparison. If your invoicing tool doesn't enforce this, you'll keep tripping on it.
2. B2CS rows that should be B2CL
Threshold trap. B2CL applies to inter-state sales to unregistered buyers where invoice value exceeds ₹2.5 lakh. Invoice value — not taxable value. A ₹2.2L taxable + 18% GST = ₹2.6L invoice flips category from B2CS to B2CL.
Most teams classify based on taxable value out of habit. The portal compares against invoice value and rejects the row.
₹2,10,000 taxable + ₹37,800 IGST = ₹2,47,800 invoice → B2CS (under ₹2.5L)
3. HSN summary that doesn't tie to invoice totals
Your HSN summary table needs to match — to the rupee — the sum of all invoices broken down by HSN code. If you've manually edited an invoice after building the summary, or if a credit note was issued mid-month, the totals drift.
Fix: Regenerate the HSN summary from invoice data, not from a saved Excel sheet. And always do it as the last step before generating JSON.
4. Missing RCM flag on reverse-charge invoices
Reverse Charge invoices — typically from GTAs, advocates, or for imported services — must be marked with the RCM flag in B2B section. The portal validates whether the supplier has filed corresponding RCM in their GSTR-1. If your flag is missing, your buyer can't claim ITC, and they will call.
5. Exempt items at 5% (or worse, at 18%)
Common with stock masters that were set up without HSN linkage. Someone adds a new SKU, picks 18% out of habit, and ships invoices for an exempt item at full tax. The customer overpays. When you reconcile next quarter, you'll be issuing credit notes for six months of invoices.
Fix: Lock stock items to HSN codes. If the HSN maps to 0%, the line item should auto-set to 0% — and any manual override should require a second approval.
6. Invoice numbers with special characters or over 16 chars
The CGST Rules cap invoice numbers at 16 characters, alphanumeric only with hyphen and slash allowed. We see firms using prefixes like SHA/TEX/INV/2026-27/0001 — that's 24 characters and the portal will reject it on upload.
7. Cancelled invoices left in the B2B section
When an invoice is cancelled within the same month, it should be moved to the "Amendments" or simply removed before filing — not left in the B2B section with a strike-through. The portal sees it as a duplicate and flags it.
The fix for all seven isn't to be more careful at filing time — it's to enforce the rules at invoice-entry time. By the time you're staring at the offline utility on the 10th, you've already lost.
This is exactly the design principle behind how Quillix handles GSTR-1: every check happens at the point of sales entry, not at filing. Wrong place-of-supply triggers a warning before the invoice saves. Threshold flips are auto-detected. HSN summary regenerates from current data every time you open the GSTR-1 view.
The deadline is still the 11th of every month. We can't change that. But we can make sure that by the time you log in on the 10th, the JSON is already valid.
