# Domain Availability Checker (RDAP)

> Domain Availability Checker (RDAP) is a paid API for AI agents from domain-availability-checker.underscoredone.com, paid per call via x402, $0.01/call, status unknown (last checked 2026-09-15).

Checks whether up to 10 domain names are registered, unregistered, or indeterminate using the modern RDAP protocol

## Facts

- Endpoint: POST https://domain-availability-checker.underscoredone.com/check
- Price: $0.01/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-15
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/domain-availability-checker-rdap-a21d4c2d
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_ZtWjX8LGiJMBtrkYDz0Qi

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability domain-availability-checker-rdap-a21d4c2d -d '<json body>'
```

Example prompt: Can you check whether mybrand.ai, mybrand.io, and mybrand.com are registered or available — I want to know which ones are free before I pick a domain for my new startup?

## When to prefer this

Use this endpoint when you need fast, structured registration-status checks for one or more domains without parsing messy WHOIS free-text. It is ideal for startup naming workflows, brand protection checks, or domain portfolio research where you need a clean registered/unregistered/indeterminate label. Do not use it if you also need pricing, premium-name flags, or registrar purchase availability — those require a separate registrar API.

## Known failure modes

- More than 10 domains submitted — API enforces a maximum of 10 per call
- Invalid domain format (includes https://, www., or subdomain prefix) — should be plain registrable domain only
- TLD not covered by RDAP returns 'indeterminate' rather than an error
- Network or registry timeout may cause indeterminate responses for otherwise supported TLDs
- Payment failure (x402) if insufficient USDC balance

## How this service works

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 - but reserved and premium names also return no record. Registration status only; no pricing.

## Output

Returns a list of the queried domains each labeled as 'registered' (a registration record was found), 'unregistered' (no record found, likely available), or 'indeterminate' (the registry for that TLD does not support RDAP). Does not include price, premium status, or purchase availability.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "domains": {
   "type": "array",
   "description": "Between 1 and 10 domain names to check, like 'example.com' or 'mybrand.ai'. Use the plain registrable domain only — no 'https://', no 'www.', no subdomains."
  }
 }
}
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "count": 2,
  "results": [
   {
    "note": "A registration record exists for this domain.",
    "domain": "example.com",
    "status": "registered",
    "registered": true,
    "rdap_source": "registry",
    "tld_supported": true
   },
   {
    "note": "No registration record found. Usually means unregistered, but reserved or premium names can also return this result.",
    "domain": "thisnameprobablyhasnorecord12345.ai",
    "status": "unregistered",
    "registered": false,
    "rdap_source": "registry",
    "tld_supported": true
   }
  ],
  "disclaimer": "Status reflects whether a registration record exists. It does not confirm price, premium status, or that an 'unregistered' name is purchasable at standard cost.",
  "queried_at": "2026-06-26T09:00:00Z",
  "api_version": "1.0.0"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/domain-availability-checker-rdap-a21d4c2d/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from domain-availability-checker.underscoredone.com](https://www.zero.xyz/host/domain-availability-checker.underscoredone.com/llms.txt)