For a decade, HSN codes on a GST invoice have been treated by many small businesses as decorative — four or six digits copied from a competitor’s invoice or a random number from a search engine, with no consequence. The 2024-25 enforcement cycle changed that. The GSTN data lake, the E-Way Bill interface, and the E-Invoicing system now cross-reference HSN codes across GSTR-1, GSTR-3B, E-Way Bills, and the buyer’s claims of input tax credit. Mismatches are no longer paper errors. They are automated triggers.

This is what we are seeing in notices received by our clients in the last 12 months.

The current rule, restated

Under Notification 12/2017 (Central Tax) as amended, the HSN code requirement on a tax invoice is:

Turnover up to ₹5 crore in the preceding FY: 4-digit HSN code mandatory.
Turnover above ₹5 crore: 6-digit HSN code mandatory.

For services, the SAC (Services Accounting Code) follows the same rule.

The code must be correct for the goods or services actually supplied, not for an adjacent category. The description on the invoice must align with the code. A “consulting” line item coded under SAC 997331 (financial consulting) instead of SAC 998311 (management consulting) is a flag.

What the 2024-25 enforcement wave looks like

The department has been issuing three categories of notice to small and medium taxpayers:

HSN mismatch in GSTR-1 vs GSTR-3B. Your GSTR-1 shows ₹10 lakh in supplies of HSN 8471 (laptops) at 18%. Your GSTR-3B shows ₹10 lakh in supplies but split across three different HSN codes at 18% and 28%. The mismatch is auto-detected and triggers a Section 61 notice (scrutiny without audit).
HSN mismatch across E-Way Bills. Your E-Way Bill for a consignment says HSN 3923 (plastic articles for packing). Your GSTR-1 for the same invoice says HSN 4819 (cartons). The goods are the same; the codes are not. Auto-flag.
HSN-SAC mismatch between supplier and buyer. You are the seller. You invoice under SAC 998311 (management consulting). The buyer claims ITC under SAC 997332 (tax consulting) — different rate, different service category, different ITC eligibility. The buyer’s claim raises a red flag on the buyer’s side, which then pulls your GSTR-1 into the reconciliation.

In all three cases, the notice goes to the assessee. The response is FORM GST ASMT-10 within the stipulated period. The demand, if the mismatch is real, can include 18% of the differential tax + interest under Section 50 + penalty under Section 122 (₹10,000 or the tax amount, whichever is higher).

The bogus ITC chain

The more serious side of the 2024-25 enforcement is the bogus ITC chain detection. The department has mapped several thousand suspected shell entities that exist on paper to issue invoices without actual supply, allowing buyers to claim ITC they have not paid for.

If your supplier is in such a chain — and you may not know — your ITC claim becomes invalid retroactively. Section 16(2)(aa) read with the second proviso to Section 16(2) of the CGST Act requires you to verify that your supplier has filed GSTR-1, paid the tax, and reflected the supply in their returns. The 2024-25 system flags ITC claims where the supplier’s GSTR-1 does not match the buyer’s GSTR-2B (the auto-generated purchase statement). The buyer loses the credit and the cash.

How to get HSN right

The practical workflow for a small business is:

Identify the actual good or service. “Marketing services” is too vague. “Search engine optimisation services” maps to a specific SAC. “Search engine marketing services” maps to a different one. The line item text should match the SAC’s official description in the CBIC SAC table.
Use the HSN-SAC master for verification. The official HSN master is published on the CBIC website and updated quarterly. Do not use a 2018 spreadsheet. The codes have been reorganised several times.
Train the invoicing team. The CA cannot be the only person who knows the code. Whoever raises the invoice must look up the code, not copy from a past invoice that may have been wrong to begin with.
Run a monthly reconciliation. Compare GSTR-1 line items to GSTR-3B, to E-Way Bills, to the actual invoices raised. A 30-minute monthly check saves several hours of departmental correspondence per mismatch.
For services, the description matters more than the SAC. The department’s first line of attack is the description. “Professional services” is generic and will be challenged. “Tax compliance services for partnership firms” is specific and defensible.

The high-risk categories in 2024-25

Three categories are getting disproportionate enforcement attention:

Hire-of-equipment and machinery under SAC 997332 / 9987. Frequent re-classification between 12% and 18% GST. The “equipment” vs “tool” vs “machinery” distinction is heavily contested.
Software and IT services under SAC 998314 / 997331. The line between “software” (18%) and “service” (18%) is a fiction — the rate is the same — but the ITC eligibility for some government buyers differs.
Restaurant and food services under SAC 9963. The composition scheme for restaurants at 5% is being audited heavily because the alleged mix of restaurant (5%) and outdoor catering (18%) on the same kitchen has been a widespread practice.

If your business is in any of these three, the next 12 months are not the time to be casual about HSN/SAC.

What to do if you get a notice

If you receive a notice under Section 61 (HSN mismatch in GSTR-1 vs GSTR-3B), respond within 15 working days. The response is:

Identify the specific invoices with the mismatch.
Rectify the error in the next GSTR-1 amendment window. The amendment window under Section 39(9) is now time-bound to the next November’s annual return or the next two years, whichever is earlier.
Pay any differential tax with interest under Section 50.
Submit a response in FORM GST ASMT-10 with the rectification proof.

If the mismatch is genuine and you cannot justify it, pay. Penalties compound. The department is willing to compound at the lower end if the rectification is voluntary and the differential tax is paid with interest.

If you receive a Section 74 notice (bogus ITC, non-genuine supplier), do not respond without a CA. The burden of proof is on you. You need to demonstrate actual receipt of goods, payment trail, and physical possession. We have walked clients through this with success — but only because the underlying paper trail was clean.

HSN codes are not a back-office formality. They are the foundation on which the entire GST reconciliation system is built. Get them right at the invoice level and the rest of compliance follows. Get them wrong and you will spend the next quarter’s CA fees on rectification.

If you want a one-time review of your HSN/SAC master against your actual invoicing pattern, share your last quarter’s GSTR-1 and your SAC/HSN list on WhatsApp — we will identify mismatches in 48 hours.