Your GSTR-3B is decided before you open it

Ask an accountant what GSTR-3B used to be and you will hear some version of the same answer: the place where the month got fixed. A sales invoice went into GSTR-1 with the wrong value? Adjust the liability in 3B and carry on. A supplier's bill looked doubtful? Leave the credit out of Table 4 and sort it out with them later. The return was where judgement happened.

That return no longer exists. Over the last two years GSTN has moved every decision that used to be made inside GSTR-3B to somewhere earlier in the month. The return is still filed on the 20th. It just reports what you already decided, or, more often, what you did not stop the portal from deciding for you.

The liability half is locked

The GST portal has pre-filled the liability tables of GSTR-3B from GSTR-1 for years. The change is that you can no longer type over it.

GSTN's advisory of 7 June 2025 made the auto-populated liability non-editable from the July 2025 tax period. Corrections now go through GSTR-1A, "which can be filed for the same tax period before filing of GSTR 3B". A second advisory on 5 December 2025 did the same to Table 3.2, the inter-state supplies to unregistered persons, composition dealers and UIN holders, from the November 2025 period.

The practical meaning is blunt. Whatever your sales register said on the 11th is your tax, unless you file a GSTR-1A before you file 3B. The return is a mirror of the sales ledger, and a mirror cannot be edited.

The credit half is decided in IMS, and silence counts

The input side has moved too, into the Invoice Management System. Every invoice your suppliers save in their GSTR-1 lands on your IMS dashboard, where you can accept it, reject it or keep it pending.

The rule that catches people out is in GSTN's own FAQs on IMS: records with "No Action" status "will be deemed accepted at the time of GSTR-2B generation". The draft GSTR-2B is generated on the 14th. So every invoice you have not looked at by then is in your credit, including the supplier's duplicate, the bill for goods that came back damaged, and the invoice raised to your GSTIN by mistake.

You can still act after the 14th, but the FAQ is specific about what that costs: any change made after the draft is generated makes it "mandatory to re-compute GSTR 2B" before filing. And once GSTR-3B is filed, every action you took is frozen.

Pending is the useful option most teams never use. A pending invoice stays out of GSTR-2B and out of the return, and waits on the dashboard until you decide, up to the Section 16(4) time limit. That is exactly what you want for the bill that is right in principle but wrong in the detail.

Two things you may have read that are not true yet

Search "GST changes October 2026" and you will find two claims repeated with total confidence.

"Table 4 ITC is hard-locked from July 2026." Plenty of posts describe this as Phase 2 of the lock. We could not find a GSTN advisory that says it, and neither could at least one of the sites tracking it, which says plainly that no advisory has been issued. The ICAI index of GSTN advisories shows 2026 advisories on GSTR-3B interest and on confirming the liability breakup. None of them locks Table 4.

"IMS becomes mandatory this month." Different blogs give different months. The design GSTN published has not changed: you are never forced to press a button, because not pressing one is treated as acceptance. That is not a lighter obligation. It is a heavier one, because doing nothing is still a decision. It is just a decision nobody reviewed.

Neither correction makes the month easier. Table 4 being editable today does not help much when the number in it comes from a GSTR-2B you already let the portal build.

Where the month is actually decided

Put the dates side by side and the shape of the job changes. For a monthly filer, GSTR-1 is due on the 11th and GSTR-3B on the 20th. Everything that matters happens in the nine days between them.

Here is the routine we would want any team running its own books to follow:

1. Close the sales side before you file GSTR-1. Review the draft GSTR-1 against the sales ledger, not against memory. GSTN's December advisory recommends exactly that review, because it is now the only cheap moment to catch an error.
2. Open IMS the day after the 11th, not the day before the 20th. Monthly suppliers file their GSTR-1 by the 11th as well, so most of what will arrive is already there. Reject what is wrong, mark pending what is incomplete, and accept the rest on purpose.
3. Recompute GSTR-2B if you acted after the 14th. It is mandatory, and it is the step that gets skipped when the review starts late.
4. Use GSTR-1A for your own errors, before 3B. After that the correction moves to a later period's GSTR-1, and the tax for this month is already paid on the wrong number.
5. File GSTR-3B and treat it as the read-out. If something surprises you at this stage, the surprise came from steps one to four.

This is a bookkeeping problem, not a filing problem

None of this is solved on the portal. The portal shows you the numbers. It cannot tell you that invoice 214 was cancelled and reissued as 215, or that a supplier billed you twice for the same delivery. Only your own books can.

That is why the lock changes the job of the accounting system more than it changes the job of whoever files the return. If the sales register is right on the 11th, GSTR-1 is right and the locked liability is right. If your purchase ledger already holds every bill you have actually received, checking IMS turns into a match: what the suppliers filed against what you have, with the differences shown. The month-end scramble does not disappear. It moves to the first half of the month, where it belongs.

We built Abibin Accounts around that order of work. GSTR-1 and GSTR-3B come out of the vouchers you have already posted, with portal-ready JSON and GSTR-2B reconciliation, so the return reads from the books rather than the other way round. And if your team is still running GST from spreadsheets and a filing deadline, our consultancy practice can help you set up the monthly routine before the next return is due.

The 20th is not when the return gets done any more. It is when you find out how the rest of the month went.