GST Integration for SaaS: A Complete Developer Guide
Every Indian SaaS founder figures out GST the hard way. Here's the developer's guide that covers what actually matters in your billing system.
Key takeaways
- For B2B (customer provides GSTIN), you don't charge GST on cross-border subscriptions but customers self-account under reverse charge.
- For domestic B2B, you do charge GST and your customer claims ITC.
- For B2C, you charge GST at the applicable rate.
- E-invoicing (IRN generation) is mandatory above a turnover threshold; assume you'll cross it.
- GSTIN validation should be real-time at billing time.
What every billing system needs
GSTIN capture and validation
When a B2B customer signs up, capture GSTIN. Validate against the GSTN registry (paid APIs from KnowYourGST, Masters India, etc.). Re-validate at renewal.
Tax treatment by customer type
- Domestic B2B (Indian GSTIN): Charge GST. Issue tax invoice.
- Domestic B2C (no GSTIN): Charge GST. Issue invoice.
- Cross-border B2B (foreign customer): Zero-rated. Reverse charge applies on their side.
- Cross-border B2C (foreign individual): Treat based on place of supply rules.
Place of supply
For digital services, place of supply usually matches customer location. Get it wrong and you owe back taxes.
Tax invoice generation
Invoices must contain: GSTIN of supplier, customer GSTIN (if applicable), invoice number, date, HSN/SAC code, taxable value, GST rate, tax amount, place of supply.
E-invoicing (IRN)
Above ₹5 crore turnover, mandatory IRN generation. Send invoice to GST Network's IRP; get back IRN + QR code; include on invoice. Integrate with a GSP or use ClearTax/Tally APIs.
Returns and credit notes
Refund a customer? Issue a credit note. Adjust GST. Update returns.
GSTR-1 / GSTR-3B impact
Your billing system feeds your GST returns. Build the export early, don't scramble at month-end.
What to outsource
GSP (GST Suvidha Provider) APIs for IRN generation. Don't build direct GSTN integration unless you're a Goods and Services Tax Suvidha Provider yourself.
CA firm or compliance software (Cleartax, Tally, Zoho) for actual return filing. Your job is to feed them clean data.
Common pitfalls
Computing GST on the wrong base. Discounts before tax, not after.
Place of supply errors. Most common for cross-border edge cases.
Late e-invoice generation. Penalty risk.
Wrong HSN/SAC code. Software services is typically 9983 or 998314, confirm with your CA.
What we recommend
Build GST into your billing system from day one if you're targeting Indian customers. Use a GSP for IRN. Use a CA-friendly format for returns. Don't treat GST as an afterthought, it gets harder to retrofit.
FAQs
What if our customer doesn't have a GSTIN? Treat as B2C.
Software exports to US/EU? Usually zero-rated under "export of services."
Reverse charge? Buyer accounts for the GST. You don't charge.
Do we need a CA? Yes, even for small SaaS.