IMS and GSTR-3B locking: what decides your ITC now, and what has actually been notified

A quiet but significant shift has happened in GST compliance, and many businesses are still handling it the old way.

GSTR-3B is no longer a form you adjust on the way out.

The old routine was: post the month's entries, file GSTR-1, then look at the GSTR-3B figures, fix whatever needed fixing, and file. That flexibility is gone.

# What is locked, and what is not

This is where most of the confusion sits, so the table comes first:

The distinction matters because if you assume "the portal will decide ITC for me", you stop doing your own reconciliation — and when the real locking arrives, you will not be ready for it.

# IMS — where your ITC is now decided

The Invoice Management System is a dashboard on the GST portal. Every invoice and credit note your suppliers upload lands there, and you take one of three actions on each:

GSTR-2B used to build itself and you simply downloaded it. You are now a participant in how it is built.

# The cost of doing nothing

Taking no action in IMS is itself a decision — and usually the most expensive one.

It hurts in both directions:

1. Credit you were entitled to never appears in GSTR-2B — you have paid extra tax in cash
2. Invoices that should have been rejected flow into your credit — and later count against you in scrutiny

# GSTR-1A — the only correction window

Once Table 3 is locked, the obvious question is how to fix a wrong liability figure.

GSTR-1A, introduced by Notification 12/2024-Central Tax dated 10 July 2024.

The window is narrow and specific:

GSTR-1 filed
      ↓
  ⬅ GSTR-1A window is here ⬅
      ↓
GSTR-3B filed
      ↓
  window closed

You can amend the same period's GSTR-1 after filing it but before filing GSTR-3B, and the amendment flows into the locked liability figure.

Miss that window and the correction moves to the next GSTR-1, affecting the following period's liability rather than the month in which the error occurred.

# The monthly routine

1. Open IMS — before GSTR-3B, every time
2. Take an action on every invoice — accept, reject or pending, with nothing left untouched
3. Generate GSTR-2B and reconcile it with the books
4. File GSTR-1
5. Check the liability figure — if it is wrong, use GSTR-1A now, in this window
6. File GSTR-3B

Step 5 is the one most often skipped, and it is the most expensive to skip.

# A conversation worth having with suppliers

IMS has created a new dependency: your ITC now depends on your supplier's timing.

If a supplier uploads late, the invoice never appears in your IMS for that period, and the credit moves to the following month — even though you did everything correctly.

# What to do today

1. Open IMS once — if you never have, there are probably a large number of untouched invoices sitting there
2. Ask your accountant whether they act on IMS or only download GSTR-2B
3. Add GSTR-1A to the monthly checklist — it is absent from most checklists today
4. Send your larger suppliers a filing timeline
5. Watch for the ITC locking notification — from the GSTN advisory, not from a blog

Written by

Prabhakar Kumar

Chartered Accountant (ICAI, Nov 2019)

Founder of VittSphere Technologies. Practicing CA serving 200+ MSME clients across Pune. 86% win-rate at AO and CIT(A) level tax appeals. Writes on Indian taxation, capital gains, and personal finance.