If you sell B2B in the EU, you'll eventually need to validate a customer's VAT number. There are two very different levels of "validation" here and it's worth knowing which one you actually need:
Format validation — does the string match the country's VAT number pattern (length, structure)? This catches typos instantly, works offline, and is free forever.
VIES lookup — is this VAT number actually registered with that EU country's tax authority right now? This requires a live call to the European Commission's VIES service, can be slow (VIES has outages more often than you'd like), and is the only way to know if the number is real vs just correctly-formatted.
For most signup flows, format validation is enough to catch mistakes, with an optional VIES check before you actually apply a reverse-charge VAT exemption (since that's the part with tax liability implications).
POST /v1/validate/vat
{ "countryCode": "IE", "vatNumber": "6388047V", "checkExistence": true }
That's the shape I settled on for Validate — format check always runs, VIES lookup is opt-in per request so you're not waiting on a flaky government API when you don't need to. Sibling APIs on the same account: QR API and Currency API.
