GST Compliance in Software: The Gotchas Nobody Warns You About

If you are building billing software for Indian businesses, GST compliance is not optional. The basic concept sounds simple, but the implementation details will test your patience. ## The Gotchas ### 1. GSTIN validation is not just a regex The 15-character GSTIN has a checksum digit. A regex without checksum validation accepts invalid GSTINs. The first two characters encode the state code — cross-check against the business's state. ### 2. Same product, different GST rates GST rates are by HSN code, not product name. A ceramic tile might be 18% or 28% depending on its value per square meter. Your system needs per-product GST rates. ### 3. Reverse charge mechanism Certain purchases require the buyer to pay GST. You need a reverse charge flag that shifts tax liability. ### 4. Credit notes and tax periods A credit note against a past invoice applies GST adjustment to the credit note's period, not the original invoice's. ### 5. E-invoicing thresholds keep changing As of 2026, e-invoicing is mandatory above Rs 5 crore turnover. Build the integration now. ## Our Approach In CoreAxis ePOS, every line item carries its own GST rate and HSN code. GSTIN checksums are validated on entry. CGST+SGST vs IGST is determined automatically by buyer state. GSTR-1 compatible data and GSTR-3B summaries are generated automatically.