GSTR-3B hard locking in 2026: what you can still edit, and how GSTR-1A fixes the rest

Until mid-2025, GSTR-3B was where you fixed things. GSTR-1 could be rough; the summary return was where the CA netted off a missed credit note, corrected a wrong-head entry and paid the right amount. That door is closed. The liability you pay is the liability your GSTR-1 says, and the only pre-filing correction route is GSTR-1A.

This post is for the person who files: which tables are locked, which are not, how GSTR-1A actually behaves, and a month-end sequence that keeps the lock from costing money.

Timeline of the lock

The June 2025 advisory and its background are summarised by ClearTax and analysed by Lakshmikumaran and Sridharan. The Table 3.2 sequence, which is messier, ends with the advisory dated 5 December 2025 reported by Taxmann: values in Table 3.2 are non-editable from the November 2025 tax period, and changes go through GSTR-1A for the same period or GSTR-1 for a later one. The three-year bar is covered in Taxmann’s note on the 29 October 2025 advisory.

What is locked and what is not

Two points on Table 4. First, it is editable, and that is a fact, not a loophole. GSTN has not issued an advisory making it non-editable. Several articles through the first half of 2026 described a July 2026 lock as a settled date; it was not, and the more careful write-ups, such as this CompuTax note, list it as anticipated rather than confirmed. If GSTN locks it, you will see a dated advisory on the portal first.

Second, editable does not mean free. Under Rule 88D, inserted by Notification 38/2023 of 4 August 2023, the system issues a DRC-01C intimation when ITC claimed in GSTR-3B exceeds GSTR-2B by more than a prescribed margin. You have seven days to pay the excess through DRC-03 or explain it in Part B, and until you respond your next GSTR-1 or IFF is blocked under Rule 59(6) (ClearTax on Rule 88D). The Council discussed 20 per cent and 25 lakh rupees as the trigger; the figures have not been notified, so treat any excess over 2B as visible. Our GSTR-2B reconciliation guide covers how to compute the safe claim.

How GSTR-1A actually behaves

GSTR-1A was introduced by Notification 12/2024 as a proviso to Rule 59(1). The GST portal’s FAQ, reproduced by TaxGuru, sets out the mechanics. The rules that bite:

• Window. It opens from the later of the GSTR-1 due date (the 11th for monthly filers) or the date you actually filed GSTR-1, and it closes when GSTR-3B is filed. File GSTR-1 on the 5th and GSTR-1A is still unavailable until the 11th.
• Once. One GSTR-1A per tax period, even if GSTR-3B is still pending. Get everything into it.
• Current period only. It amends or adds records for the same tax period. Earlier months are amended in a later GSTR-1, within the time limit.
• No GSTIN change. The recipient’s GSTIN cannot be amended in GSTR-1A.
• No nil filing, and no half-saved state. A GSTR-1A with saved but unfiled records blocks GSTR-3B until you file it, delete the records or reset.
• Instant on your side, delayed on the customer’s. Values in GSTR-1A flow into your GSTR-3B immediately. The customer sees the added or amended invoice in IMS, but the ITC lands in their GSTR-2B of the following month, because 2B for a period is generated on the 14th and is static (GST portal GSTR-2B FAQ).

That last point generates most of the phone calls. If you add a B2B invoice through GSTR-1A on the 18th, you pay the tax on the 20th, and your customer’s ITC arrives a month later. Tell them before they find out.

The month-end sequence

Because liability is fixed by GSTR-1 and ITC is fixed, in practice, by IMS and 2B, the order of work matters more than it used to.

1. By the 8th: close the sales register. Reconcile the books against e-invoices and e-way bills. Every difference you find now is a GSTR-1A you will not need later.
2. By the 11th: file GSTR-1. Treat it as the tax return it now is. A reviewer, not only a preparer, signs it off.
3. 11th to 13th: act on IMS. Accept, reject or keep pending every record. Anything left untouched is deemed accepted when 2B is generated. Our IMS guide covers which action to take on each type of record.
4. 14th: GSTR-2B is generated. Reconcile it against the purchase register. Compute ITC per 2B, ITC per books, the safe claim and the amount on hold.
5. 14th to 19th: GSTR-1A if needed. Missed invoice, missed credit note, wrong value, wrong rate or wrong place of supply in your own GSTR-1: fix it here, once, before 3B.
6. 20th (22nd or 24th under QRMP): file GSTR-3B. By now it is a review of auto-populated numbers and an ITC figure you have already justified line by line.

Errors that now cost money

Missed credit note. Before the lock you reduced the liability in 3B and reported the note next month. Now the liability stands as filed. Add the note in GSTR-1A before 3B or pay the gross amount and wait a month for the adjustment to flow.

Duplicate invoice in GSTR-1. Uploaded twice from two Excel files, it is taxed twice. Caught before 3B, GSTR-1A removes it. Caught after, the cash is out until an amendment in the next GSTR-1 and, for larger amounts, a refund claim.

Wrong GSTIN. GSTR-1A cannot fix it. You pay in full this month, amend the GSTIN next month, and the correct customer gets ITC the month after that. Expect them to hold your payment in the meantime.

Wrong head. An inter-State supply reported as intra-State means CGST and SGST paid where IGST was due. With Table 3.2 locked you cannot shift it in 3B; you pay the right head and claim the wrong head back. Lakshmikumaran’s note describes exactly this cash-flow trap.

GSTR-1 versus GSTR-3B gap. Rule 88C, inserted by Notification 26/2022, produces a DRC-01B intimation when tax paid in 3B falls short of GSTR-1 liability, with seven days to pay or explain and recovery under Section 79 after that. With the lock, this should be impossible unless you have paid less than the system computed, so a DRC-01B now usually indicates a filing done outside the normal flow.

Late GSTR-1. It delays GSTR-1A and 3B together. Interest under Section 50(1) runs at 18 per cent a year on tax paid late, plus the late fee for each return.

Anything more than three years old. Since the November 2025 period the portal blocks returns whose due date is more than three years past. There is no late fix.

None of these errors is new. What is new is that each one now costs the cash difference, the interest and a month of the customer’s goodwill, where it used to cost a line in the working papers.

What to automate

• A GSTR-1 pre-filing check that reconciles the sales register against e-invoices, e-way bills and last month’s amendments before the 11th, so GSTR-1A is rarely needed.
• IMS action proposals generated from the purchase register before the 14th, with deemed-acceptance risk flagged.
• GSTR-2B reconciliation and a safe-to-claim figure, with the Rule 88D excess computed before you file.
• A GSTR-1A draft built automatically from the differences found in steps 1 and 4, ready for one review and one filing.
• A client-facing note that explains, in plain language, why a GSTR-1A addition reaches the customer’s 2B next month.

This is the compliance sequence our accounting automation service sets up for CA firms, and you can see the reconciliation and safe-claim steps on sample data in the accounting demo. The lock did not add work. It moved the work to earlier in the month, where it is cheaper to do.