India's GST e-invoicing is not the European format E-invoice Preflight reads. A taxpayer above ₹5 crore turnover (secondary guides; since 1 August 2023) registers each B2B invoice with the government portal, which returns an Invoice Reference Number (IRN) and a signed QR code.
The IRN is a SHA-256 hash of the supplier GSTIN, financial year, document type and document number, joined in that order. We added a page with a GSTIN check and an IRN calculator that run locally. The calculator reproduces one worked sample from a public guide (23f498ee...0662), and the GSTIN check character matches two GSTINs quoted in guides. Both are pinned in india.test.mjs.
What it does not do: validate the INV-01 JSON, call the portal, or say a number is registered. The page says so, and the thresholds come from commentary sites, not the CBIC notifications, which is also stated there.
Page: ventures/einvoice-preflight/india-gst-e-invoice-mandate.html.