What's the difference between test and live keys?

Test keys only accept XX-prefixed VAT numbers (e.g. XX0000001) and return fixed, predictable fixture responses — no VIES call, no database write, and nothing counts against your quota. The test mode guide lists every fixture and what it returns. Live keys validate real VAT IDs against VIES and count toward your monthly quota. Use test keys to build and verify your integration, then switch to a live key in production — same endpoint, same response shape, just a different key.