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domain-availability-checker.underscoredone.com check
https://domain-availability-checker.underscoredone.com/check
answers 402
Give this API up to 10 domain names and it tells you whether each has a registration record at the official registry. It uses RDAP, the structured replacement for WHOIS, so there is no free-text parsing. Each domain comes back as 'registered', 'unregistered', or 'indeterminate' when the registry has no RDAP for that ending. 'Unregistered' means no record was found, which usually means available -
We sent a real
POST request to this endpoint and it
answered 402 with payment requirements we could parse.
- Price
- $0.01
- Network
- base
- Pay to
- 0xE9740820225B3918b4ddd1292C7cA4Ca0e2C2F08
- Spec dialect
- v2 (declares x402Version 2)
- Transport
- PAYMENT-REQUIRED header
- HTTP method
- POST
- Response latency
- 172 ms
- Payment options
- 2 usable of 2
- Last checked
- 2026-09-06T01:09:45.775Z (31d ago)
- Category
- identity
Re-check it yourself:
curl "https://api.the402.dev/validate?url=https%3A%2F%2Fdomain-availability-checker.underscoredone.com%2Fcheck"