Last updated: 8 May 2026. GSTR-1 for April 2026 is due on Monday, 11 May 2026 for monthly filers. The errors that matter most at this point are not late filing — they are returns filed on time with bad data. An inactive buyer GSTIN denies the recipient their ITC. A missed inter-state B2C invoice above ₹2.5 lakh creates a downstream notice. A Table 12 HSN summary that does not tie up with the invoice tables now triggers a hard block on submission. With GSTR-1A introduced in July 2024 and GSTR-3B liability fields hard-locked from GSTR-1 since the July 2025 tax period, mistakes caught after submission can still be fixed in the same period — but only before GSTR-3B is filed.
Applicability Note: This guide is based on Section 37 of the CGST Act, 2017, the relevant CGST Rules (59(6), 88C, 88D), and Notification No. 12/2024-Central Tax dated 10 July 2024 introducing Form GSTR-1A. The position stated is as of 8 May 2026 and applies to the GSTR-1 return for the April 2026 tax period. Always verify the current position on gst.gov.in or with a GST professional before acting.
Who Should Care?
- Monthly GSTR-1 filers (turnover above ₹5 crore) preparing the April 2026 return
- QRMP filers using the IFF for April invoices ahead of the quarterly GSTR-1 due 13 July
- Accountants and in-house finance teams running the pre-submission check
- Anyone who has received a DRC-01B or DRC-01C intimation in the last quarter
1. Wrong or Inactive Buyer GSTIN in Table 4
Table 4 captures B2B outward supplies — buyer GSTIN, invoice number, taxable value, tax rate, tax amount. An incorrect or inactive GSTIN does not fail the return at submission, but it denies the buyer their input tax credit because the invoice does not flow into their GSTR-2B — Section 16(2)(aa) of the CGST Act conditions ITC eligibility on the supplier's reporting in GSTR-1. The buyer then reverses it in their GSTR-3B, raises a debit note against the supplier, and a clean invoice turns into a working capital problem.
Run the buyer GSTIN list through the portal's search facility before submission. The key check is the registration status — "Active" is the only acceptable state. A "Cancelled" or "Suspended" GSTIN on a B2B invoice means the supply has to be re-classified as B2C and the tax is the supplier's, not flow-through.
2. Inter-State B2C Above ₹2.5 Lakh Reported in the Wrong Table
Inter-state B2C supplies above ₹2.5 lakh per invoice go in Table 5 with state-wise reporting. Inter-state B2C below ₹2.5 lakh and all intra-state B2C go to Table 7 in consolidated form. Mixing these up does not show up at submission — but the state-wise consumption funds get wrong allocations, and the supplier looks under-reported when the buyer's state runs an audit.
The single most common error here is treating a high-value retail invoice as Table 7 because the buyer is unregistered. Above ₹2.5 lakh inter-state, even an unregistered buyer's invoice is a Table 5 invoice with the buyer's state recorded.
3. Table 12 HSN Summary That Does Not Reconcile
Table 12 is the HSN-wise summary of outward supplies. Since the May 2025 return period, Table 12 has been bifurcated into B2B and B2C sub-tables — the HSN can be selected only through the portal's dropdown (manual entry is disabled), and the HSN-wise totals must reconcile with the sum of invoices in Tables 4, 5, 6 and 7 for that rate and HSN. In FY 2026-27, a sub-table mismatch is a hard block on submission, not a warning.
| Aggregate Annual Turnover (preceding FY) | HSN Digits Required in Table 12 |
|---|---|
| Up to ₹5 crore | 4-digit HSN |
| Above ₹5 crore | 6-digit HSN |
B2B HSN reporting is mandatory in Table 12. B2C HSN is optional for taxpayers with turnover up to ₹5 crore. A B2C-only seller below the threshold who leaves Table 12B blank still has to enter "0" — leaving it empty triggers a portal validation error.
4. Cancelled Documents Skipped in Table 13
Table 13 captures the document series for the period — first and last serial number issued, number of documents cancelled, net issued — across tax invoices, credit notes, debit notes, and revised invoices. Two recurring errors break submission. First, leaving the cancelled count blank when a series has cancellations: the system reads zero cancelled and the next-period reconciliation flags a count mismatch. Second, mixing up debit notes and credit notes when entering the series, which carries forward into the GSTR-9 reconciliation as a value mismatch.
5. Place-of-Supply Tag That Flips IGST to CGST+SGST
The place of supply tag on each invoice is what tells the system whether to charge IGST or CGST+SGST. A supplier in Maharashtra raising an invoice on a Karnataka-registered buyer is making an inter-state supply, taxable to IGST. The same supplier raising an invoice on a Karnataka buyer for goods delivered to a project site in Maharashtra is making an intra-state supply, taxable to CGST+SGST. Place of supply for goods is the location where the movement of goods terminates for delivery; place of supply for services is more nuanced and follows Section 12 or Section 13 of the IGST Act — Section 12 covers supplies where both parties are in India; Section 13 covers cases where one party is outside India.
An IGST invoice raised on what should have been a CGST+SGST supply still carries tax — but the buyer cannot claim ITC because the tax type is wrong. The supplier's GSTR-1 has to be amended via GSTR-1A or in the next period.
6. Filing GSTR-3B Before Using the GSTR-1A Fix Window
Form GSTR-1A was introduced via Notification No. 12/2024-Central Tax dated 10 July 2024, effective from the July 2024 tax period. It is an optional form that allows additions, amendments, or rectifications in the same period's GSTR-1, but only before the GSTR-3B for that period is filed. Once GSTR-3B is filed, the GSTR-1A window for that period is closed and the only fix is an amendment in the next period's GSTR-1 (Tables 9, 10, 11) or, where needed, GSTR-1A for that next period.
| Mistake Caught | Fix Window |
|---|---|
| Before GSTR-1 submission | Edit in GSTR-1 directly |
| After GSTR-1 submitted, before GSTR-3B filed (same period) | File GSTR-1A for that period |
| After GSTR-3B filed (any later catch) | Amendment table in next period's GSTR-1, or next period's GSTR-1A |
The practical implication is sequencing. Many taxpayers used to file GSTR-1 and GSTR-3B back to back on the same day. Holding back GSTR-3B for 24 to 48 hours after GSTR-1, while the recipient confirms invoice flow into their GSTR-2B, opens the GSTR-1A window for last-minute fixes that do not need to wait a month.
7. Unresolved DRC-01B or DRC-01C That Blocks the Next Filing
Two intimation rules sit on top of the return calendar:
- Rule 88C — where outward liability in GSTR-1 exceeds the liability declared in GSTR-3B by more than the prescribed limit, the system issues Form DRC-01B asking either to pay the difference plus interest via DRC-03 or to explain the variance, both within 7 days.
- Rule 88D — where ITC auto-populated in GSTR-2B exceeds the ITC availed in GSTR-3B (or vice versa) beyond limits, Form DRC-01C is issued with the same 7-day reply window.
Rule 59(6) blocks the filing of GSTR-1 or IFF for the next tax period until the DRC-01C reply is filed. So a DRC-01C ignored from the March 2026 cycle does not just sit in the inbox — it stops the May 2026 filing from going through. The fix is either DRC-03 payment or a portal reply explaining the difference within the 7-day window.
Bonus: The 3-Year Lock You Can't Miss
Sections 37(5), 39(11), 44(2), and 52(15) of the CGST Act — inserted by the Finance Act 2023 and effective from 1 October 2023 under Notification No. 28/2023-CT dated 31 July 2023 — prevent filing of any GST return after three years from its original due date. The GST portal began enforcing this block from the September 2025 tax period, so this is not a new rule — it has been operational for months. A return for April 2022 with an 11 May 2022 due date has been permanently locked since May 2025. Going forward, there is no second chance beyond three years and the amendment route closes too.
Key Takeaways
- The expensive GSTR-1 errors are not about being late — they are about filing on time with bad data: inactive GSTINs, missed Table 5 invoices, mismatched Table 12 HSN summaries, wrong place of supply.
- Table 12 has been bifurcated into B2B and B2C sub-tables since the May 2025 return period — B2B HSN is mandatory, HSN dropdown is the only entry path, and sub-table totals must reconcile to the invoice tables. In FY 2026-27, a mismatch is a hard block on submission.
- GSTR-1A lets you fix the same period after submission but only until GSTR-3B is filed. Hold GSTR-3B for at least a day after GSTR-1 to keep the window open.
- Rule 88C and Rule 88D intimations carry a 7-day reply window. An unresolved DRC-01C blocks the next GSTR-1 under Rule 59(6).
- The 3-year filing lock — Finance Act 2023, effective 1 October 2023 (Notification No. 28/2023-CT), portal-enforced from September 2025 — means returns more than three years past their due date cannot be filed. The amendment route closes too.
Frequently Asked Questions
What is the GSTR-1 due date for April 2026?
For monthly filers, GSTR-1 for April 2026 is due on 11 May 2026. For QRMP filers using the IFF for the first two months of a quarter, the IFF for April is due on 13 May 2026; the quarterly GSTR-1 for the April–June quarter is due on 13 July 2026.
Can I correct a wrong invoice in GSTR-1 after submission?
Yes. If GSTR-3B for the same period has not been filed, file GSTR-1A in that same period to add, amend, or remove the invoice. If GSTR-3B has been filed, the correction goes in the amendment tables (9, 10, 11) of the next period's GSTR-1.
What happens if I do not reply to a DRC-01C intimation within 7 days?
Under Rule 59(6), the portal blocks the next period's GSTR-1 or IFF filing until the reply is filed. Beyond the 7-day window, the unrebutted excess credit can also be recovered as if it were a confirmed demand.
Is HSN reporting in Table 12 mandatory for B2C supplies in FY 2026-27?
For taxpayers with aggregate annual turnover up to ₹5 crore, B2C HSN reporting in Table 12B remains optional, but the field cannot be left blank — enter "0" to pass portal validation. B2B HSN in Table 12A is mandatory for everyone, with 4-digit HSN for AATO up to ₹5 crore and 6-digit HSN above ₹5 crore.
Does GSTR-1A change the GSTR-1 due date?
No. GSTR-1A is an optional add-on to GSTR-1 in the same tax period and does not extend any due date. The original GSTR-1 still has to be filed by the 11th (monthly) or 13th (quarterly), and any GSTR-1A filing must be completed before GSTR-3B for that same period.
Disclaimer: This article is for informational purposes only and does not constitute professional tax advice. GST rules are subject to frequent changes through notifications and circulars. Please consult a qualified tax professional or verify the current provisions on the official GST portal (gst.gov.in) before making any compliance decisions.
Have a specific question about a DRC-01C intimation, a Table 12 mismatch, or a GSTR-1A correction? Our GST experts can help → gstconsultancy.com