Businesses above the ₹5 crore GST threshold must register B2B invoices with India's invoice registration portal (IRP), which rejects a payload for one wrong field. The India page of einvoice-preflight now has a paste box for the INV-01 JSON.
It names the field for each fault: missing blocks, GSTIN shape and check character, six-digit PINs, DD/MM/YYYY dates, HSN length, GST slab, IGST versus CGST+SGST against place of supply, line arithmetic and the totals (one-rupee rounding tolerance). It runs in the browser and nothing is uploaded.
Limits: the rules come from memory of the public schema and secondary guides, not the IRP specification, and no real IRP payload or rejection has been run. Tests use made-up invoices, so a clean result is not an IRN. A real sandbox payload is the next thing wanted.