GST-Compliant ERP Development: Building Indian Tax Logic into Your System

The GST Data Model

Every transaction in a GST-compliant ERP must store: GSTIN of buyer and seller, HSN/SAC code of each line item, supply type (B2B, B2C, exports), place of supply (determines CGST+SGST vs IGST), taxable value, tax rate, and tax amounts split by component. This is more complex than simple VAT — the interstate vs intrastate distinction changes the entire tax calculation.

HSN Code Management

HSN (Harmonised System of Nomenclature) codes determine GST rate for each product. Your ERP must maintain an HSN code master with associated tax rates, handle HSN code changes when GST council revises rates (common), apply correct rates based on the effective date of rate change, and generate the 6-digit or 8-digit HSN code in invoices and returns based on turnover thresholds.

E-Invoice Integration (IRP)

Businesses above ₹5 crore annual turnover must generate e-invoices via the Invoice Registration Portal (IRP). Your ERP must: generate invoices in the IRP JSON format, call the IRP API to get an IRN (Invoice Reference Number) and QR code, embed the QR code in the PDF invoice, and handle IRP API failures gracefully (queue for retry).

GSTR Return Generation

A complete GST module generates GSTR-1 (outward supplies, monthly/quarterly), GSTR-3B (summary return, monthly), and GSTR-2B reconciliation (purchase register matched against supplier filings). Export these in the NIC offline tool format for upload to the GST portal, or integrate directly via the GST API if volume justifies the setup cost.