GST from first principles: a tax that runs on matched invoices

A value-added tax is only as honest as its invoices: how India's Goods and Services Tax matches them, where fraud gets in, and what to build on top.

Thesis: India’s Goods and Services Tax (GST) is a credit system run as a tax. A business may deduct the tax its suppliers charged only when their invoices match on one national portal, so most of what GST does, and most of what goes wrong, happens in the matching.

Start from zero

A tax on every sale cascades. If a mill, a garment maker and a shop each pay 10% on their full price, each stage pays tax on the tax inside its costs.

A Value Added Tax (VAT) fixes this. Each seller charges tax on its full price, then deducts the tax it paid on inputs, its Input Tax Credit (ITC). Each stage pays only on the value it added.

The deduction is the weak point: a claim on the state, backed by a supplier’s word that it charged and paid the tax. For it to be safe, five things must be true:

Every seller is known, with one identity across states.
The seller reports every business-to-business (B2B) invoice, so the buyer’s claim can be checked.
Credit flows only after the seller reports and pays.
Tax follows the consumer. The state where goods are used gets the revenue, wherever they were made.
Goods match paper. An invoice with no truck behind it is the cheapest way to manufacture credit.

India built a primitive for each.

The primitives

One identity, one portal. Each business gets a 15-character GST Identification Number (GSTIN) per state, built on its Permanent Account Number (PAN). The Goods and Services Tax Network (GSTN), a not-for-profit company, runs one portal for the Centre and every state. On 30 June 2026 it had 1.67 crore active taxpayers (a crore is 10 million; a lakh is 100,000), 76.9% of them sole proprietors. Applicants who will pass on no more than ₹2.5 lakh of credit a month are now registered automatically: the cap is itself a fraud fence.

Returns that check each other. Returns are numbered forms called GST Returns (GSTR).

GSTR-1: the seller’s invoice-level list of sales, due on the 11th of the next month for monthly filers.
GSTR-2B: the credit statement the portal builds for each buyer on the 14th from its suppliers’ GSTR-1s. Since 2022, only invoices that appear here earn credit. In the Invoice Management System (IMS), the buyer first accepts, rejects or keeps each invoice pending; silence counts as acceptance.
GSTR-3B: the summary return and payment, due on the 20th. Since the July 2025 tax period, the liability filled in from GSTR-1 cannot be edited here; corrections go through GSTR-1A.

Electronic invoices (e-invoices). A business whose turnover passed ₹5 crore in any year since 2017-18 must send each B2B invoice to an Invoice Registration Portal (IRP), run by the National Informatics Centre (NIC) or, since 2023, approved private firms such as ClearTax and IRIS. The IRP returns an Invoice Reference Number (IRN) and a signed Quick Response (QR) code, without which the invoice is invalid, and passes the data into the seller’s GSTR-1 and the e-way bill system.

Electronic way bills (e-way bills). Moving goods worth more than ₹50,000 needs one: Part A carries the invoice, Part B the vehicle, valid one day per 200 km. India generated 155.96 crore of them in 2025-26.

Integrated GST (IGST). A sale within a state carries Central GST (CGST) and State GST (SGST) in equal halves. A sale across states carries IGST at the combined rate, collected by the Centre. Buyers must use IGST credit first; when it pays a buyer’s SGST, the Centre transfers that amount to the buyer’s state. Of ₹23.3 lakh crore paid through the portal in 2025-26, ₹12.3 lakh crore was IGST.

Rates. The 56th GST Council meeting replaced four slabs with two, plus a demerit rate, from 22 September 2025. Tobacco and pan masala followed on 1 February 2026, as their compensation cess gave way to excise duty and a health cess.

Rate	Examples
Nil	Ultra-high temperature (UHT) milk, paneer, Indian breads; individual life and health insurance exempt
5% (merit)	Soap, toothpaste, packaged namkeen, tractors
18% (standard)	Air conditioners, all televisions, small cars, cement, bidis
40% (demerit)	Sugared and caffeinated drinks, pan masala, cigarettes, larger cars, motorcycles above 350 cubic centimetres (cc), yachts

The 57th meeting on 8 October 2026 changed no slabs.

Follow one invoice

A manufacturer in Pune, Maharashtra, sells ₹1 lakh of goods at 18% to a retailer in Bengaluru, Karnataka, on 10 October:

The manufacturer’s software sends the invoice to an IRP and gets back an IRN and QR code. Tax: ₹18,000 of IGST, since the goods cross a state line.
The IRP fills Part A of the e-way bill; the transporter adds the truck. For roughly 850 km, it is valid five days.
The invoice lands in the manufacturer’s GSTR-1, filed by 11 November.
The retailer accepts it in IMS after checking the goods arrived.
On 14 November the retailer’s GSTR-2B shows ₹18,000 of IGST credit.
By 20 November the manufacturer files GSTR-3B, its liability locked to GSTR-1, and pays the ₹18,000 of IGST.
The retailer sells the goods in Bengaluru for ₹1.3 lakh plus 18%: ₹11,700 each of CGST and SGST. Its IGST credit pays the CGST and ₹6,300 of the SGST; it pays the last ₹5,400 in cash.
In settlement, the Centre moves ₹6,300 from its IGST account to Karnataka, which ends with the full ₹11,700 of SGST. Maharashtra collects nothing on goods consumed elsewhere.

Total tax: ₹18,000 plus ₹5,400, exactly 18% of the final price.

Where the money is

The base: about ₹2.04 lakh crore of gross GST in September 2026, up 14.7% on a year earlier. From July 2025 to June 2026, e-invoices carried 83.6% of the value reported in GSTR-1, so most B2B trade is now machine-readable.

The leak: in 2025-26 central tax officers detected ₹74,782 crore of fraudulent ITC across 30,162 cases, up from ₹58,773 crore in 15,283 cases the year before, with 358 arrests. That is more than a third of a month’s collections, in two shapes:

Fake invoices. A shell registers, issues invoices with no goods, files GSTR-1 so the credit reaches buyers’ GSTR-2B, then vanishes without paying. In 2025-26, 1,517 registrations made with forged PAN and Aadhaar details were linked to ₹9,940 crore of such credit.
Circular trading. Invoices loop among related firms, A to B to C to A, inflating turnover for bank loans or passing credit down a chain that ends at a missing trader.

Matching catches an invoice nobody reported. It cannot catch one that was reported but never backed by goods or payment. The 57th Council meeting proposed to narrow the main offence to “fraudulent availment of ITC without receipt of goods or services or without invoice or bill”, raise the prosecution threshold from ₹1 crore to ₹5 crore, remove arrest powers and drop notices under ₹10,000. Enforcement is moving from officers to data.

Compliance and reconciliation software sits on top. Clear, which runs ClearTax, reported ₹272 crore of revenue in the year to March 2025, under 0.5% of the ₹58,773 crore of fake credit found that year.

What is hard, and what I would build next

The hard parts: matching proves an invoice was reported, not that goods moved or tax was paid; buyers lose credit for suppliers’ failures; and most taxpayers are sole proprietors.

1. An IMS assistant for small buyers. Match every invoice in IMS against purchase records and goods received, pre-fill accept or reject with a reason, and flag gaps before GSTR-3B. The Council wants formal ways to correct mismatches from the April 2027 return; a claim right the first time needs none. The metric: credit claimed that survives unchanged.

2. A supplier score at the purchase order. Every GSTIN’s filing history is public on the portal. Score suppliers on filing streak, registration age and cancellations, and show it when the order is placed, not at month end: a credit bureau for tax compliance. The metric: credit reversed because a supplier never paid.

3. Refund-backed working capital. Exporters and firms with inverted duty wait for refunds; ₹1.68 lakh crore were processed online in 2025-26. The Council has proposed automatic 90% provisional sanction after a system risk check. A lender that underwrites a claimant’s refund history can advance cash the day the claim is filed. The metric: days from filing to cash.