GST E-Invoicing API: How to Integrate Your Billing Software
Who must generate e-invoices, how IRN generation works, access through a GSP or directly, the API flow, validations, cancellation rules and common integration errors.
E-invoicing under GST does not mean generating invoices on a government site. Your billing software creates the invoice as before, then reports it to an Invoice Registration Portal, which validates it and returns a unique Invoice Reference Number and a signed QR code. Without those, the invoice is not valid for a business that is covered.
Who is covered
E-invoicing applies to GST-registered businesses whose aggregate annual turnover exceeded ₹5 crore in any financial year from 2017-18 onwards. Once covered, a business stays covered. It applies to business-to-business supplies, exports and supplies to SEZs, including credit and debit notes. Certain classes are exempt, including banks and financial institutions, insurers, goods transport agencies, passenger transport services and SEZ units.
How it works
The exchange is a single API call per document, wrapped in authentication.
Ways to get API access
There are several IRPs, and several ways to reach them.
Integration details
A few technical points cause most of the effort.
Clean the customer and item masters before go-live. The API will reject what the print format used to hide.
Cancellation and amendment
An e-invoice can be cancelled on the IRP only within 24 hours, and only in full. It cannot be edited on the IRP. After 24 hours, corrections are made through credit or debit notes, which are themselves reported, or through amendments in GSTR-1. Taxpayers with aggregate turnover of ₹10 crore and above cannot report documents older than 30 days, so late reporting is blocked by the system.
What to build around the API
The call itself is small. The value is in the surrounding workflow.
This article explains what to build, not legal advice. Rules change; confirm against the current official text before relying on it.
