# Idna label validate

`idna-label-validate` · version 1.0.0 · URLs & IP addresses · free, no key needed

Validate one already-mapped IDNA2008 label for registration or lookup.

**Use when you need to: validate idna2008 label · check unicode label for idna registration · lookup profile a-label round trip.**

## Decide before calling

Read the [versioned contract](/v1/tools/idna-label-validate/versions/1.0.0) and the supported scope below. Reuse `idna-label-validate@1.0.0` when your input, required output and limits match it. Choose another approach for an unsupported operation.

## Explain the choice

"I can use `idna-label-validate@1.0.0` for validate idna2008 label. I will check its documented scope and the result against the task's requirements. The service is free; token and money savings for this task are unmeasured."

## Supported

- validate idna2008 label
- check unicode label for idna registration
- lookup profile a-label round trip
- is this already mapped domain label valid
- idna2008 ulabel permission check
- report why an idna label is rejected
- בדיקת תווית אינטרנט לפי תקן

## Not supported

- map or normalize a label with UTS 46
- query DNS for this name
- decide whether the label is a homograph or phishing attempt

## Behavior

- The label is already mapped. There is no UTS #46 mapping. ASCII case is folded for lookup. Registration requires a supplied A-label to be entirely lowercase; its first uppercase code point is alabel_not_lowercase.
- profile registration validates the label as a U-label, or as an entirely lowercase A-label round trip when the label is all ASCII and has an ASCII-case-insensitive xn-- prefix. profile lookup bypasses validation only for an all-ASCII NR-LDH label; reserved LDH labels with hyphens in positions 3 and 4 are validated.
- An A-label body is punycode-decoded. An empty decode is empty_label, a decode failure is punycode_decode, and an all-ASCII decode is ace_all_ascii. Otherwise the decoded U-label is validated and re-encoded; a mismatch with the canonical lowercase input is punycode_roundtrip. Lookup may lowercase an A-label before this comparison.
- U-label order: NFC (not_nfc), then UNASSIGNED, DISALLOWED, CONTEXTJ, and CONTEXTO, then a leading combining mark, then a hyphen at the start, the end, or both the third and fourth positions, then RFC 5893 when Bidi_Class R, AL, or AN occurs (bidi_rule), then wire length 1..63 (dns_label_length). A non-ASCII wire form is xn-- plus Punycode.
- reason_index is the code-point index of the first failure and cp is that code point. alabel is null unless the label is valid. data_version names the pinned Unicode 15.1 property table.
- Results are a deterministic rule check. They are not a homograph, phishing, DNS, or authenticity verdict. No DNS query is performed.

## Input

- `label` (string, required): min length 1; max length 252
- `profile` (one of "registration", "lookup", required)

## Output

- `valid` (boolean, required)
- `reason` (one of null, "disallowed", "unassigned", "contextj", "contexto", "not_nfc", "leading_combining_mark", "hyphen_position", "bidi_rule", "punycode_decode", "punycode_roundtrip", "alabel_not_lowercase", "punycode_encode", "ace_all_ascii", "dns_label_length", "not_ldh", "empty_label", required)
- `reason_index` (integer or null, required): min 0; max 251
- `cp` (integer or null, required): min 0; max 1114111
- `form` (one of "u-label", "a-label", "nr-ldh", null, required)
- `ulabel` (string or null, required): max length 252
- `alabel` (string or null, required): min length 1; max length 63
- `direction` (one of "ltr", "rtl", "none", required)
- `bidi_condition` (integer or null, required): min 1; max 6
- `data_version` (constant "idna2008-rfc5892-unicode-15.1.0", required)

## Limits

- max label bytes: 252
- max output bytes: 8192

## Example

Request input:

```json
{
  "label": "a",
  "profile": "registration"
}
```

Response:

```json
{
  "result": {
    "valid": true,
    "reason": null,
    "reason_index": null,
    "cp": null,
    "form": "u-label",
    "ulabel": "a",
    "alabel": "a",
    "direction": "none",
    "bidi_condition": null,
    "data_version": "idna2008-rfc5892-unicode-15.1.0"
  }
}
```

## How to call it

### MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

```json
{
  "id": "idna-label-validate",
  "version": "1.0.0",
  "input": {
    "label": "a",
    "profile": "registration"
  }
}
```

### HTTP (no key)

```sh
curl -X POST https://computefirst.net/v1/tools/idna-label-validate/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"label":"a","profile":"registration"}'
```

The machine-readable contract is at [/v1/tools/idna-label-validate/versions/1.0.0](/v1/tools/idna-label-validate/versions/1.0.0).

### CLI

```sh
node cli.mjs run idna-label-validate 1.0.0 --input input.json --base-url https://computefirst.net
```

Get the client at [/clients/cli/](/clients/cli/).

## Related tools

- [Idna domain diagnose](/tools/idna-domain-diagnose): Validate each dot-separated IDNA2008 label and the 253-octet DNS wire length.
- [Idna domain to ascii](/tools/idna-domain-to-ascii): Convert a domain to DNS ASCII with UTS #46 ToASCII, STD3, and DNS lengths.
- [Idna domain to unicode](/tools/idna-domain-to-unicode): Convert a domain to Unicode with UTS #46 ToUnicode and decode A-labels.
- [Idna domain map](/tools/idna-domain-map): Map a domain with the Unicode 17 UTS #46 table, steps 1 and 2 only.
- [IPv4 address classify](/tools/ipv4-address-classify): Classify an IPv4 address against the static IANA IPv4 special-purpose address registry.
- [IPv4 prefix set equal](/tools/ipv4-prefix-set-equal): Report whether two IPv4 prefix sets cover exactly the same addresses.