# Idna bidi check

`idna-bidi-check` · version 1.0.0 · Numbers & math · free, no key needed

Apply the RFC 5893 Bidi Rule to one label and name the first failing condition.

**Use when you need to: check the idna bidi rule · rfc 5893 label direction test · right to left label ending check.**

## Decide before calling

Read the [versioned contract](/v1/tools/idna-bidi-check/versions/1.0.0) and the supported scope below. Reuse `idna-bidi-check@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-bidi-check@1.0.0` for check the idna bidi rule. 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

- check the idna bidi rule
- rfc 5893 label direction test
- right to left label ending check
- does this label mix arabic and european digits
- bidi condition for a domain label
- when does the bidi rule apply
- בדיקת כיוון אותיות בתווית

## Not supported

- reject disallowed code points
- punycode encode the label
- apply one bidi rule to a whole multi-label domain

## Behavior

- RFC 5893 only. Disallowed characters, hyphens, NFC, and punycode are not judged.
- enforce when_rtl: if the label has no Bidi_Class R, AL, or AN, applicable is false, valid is true, and direction is none.
- enforce always: the six conditions run even when no RTL class is present. A first class other than L, R, or AL fails bidi_condition_1.
- direction is rtl when the first class is R or AL, ltr when it is L, and none otherwise. RTL then checks conditions 2, 3, and 4. LTR checks 5 then 6. The first failure wins.
- reason_index and cp identify the failing code point. first_rtl_index is the first R, AL, or AN, or null.
- Results are not a homograph, phishing, DNS, or authenticity verdict. No DNS query is performed.

## Input

- `label` (string, required): min length 1; max length 252
- `enforce` (one of "when_rtl", "always", required)

## Output

- `valid` (boolean, required)
- `applicable` (boolean, required)
- `direction` (one of "ltr", "rtl", "none", required)
- `reason` (one of null, "bidi_condition_1", "bidi_condition_2", "bidi_condition_3", "bidi_condition_4", "bidi_condition_5", "bidi_condition_6", required)
- `reason_index` (integer or null, required): min 0; max 251
- `cp` (integer or null, required): min 0; max 1114111
- `first_rtl_index` (integer or null, required): min 0; max 251
- `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",
  "enforce": "when_rtl"
}
```

Response:

```json
{
  "result": {
    "valid": true,
    "applicable": false,
    "direction": "none",
    "reason": null,
    "reason_index": null,
    "cp": null,
    "first_rtl_index": 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-bidi-check",
  "version": "1.0.0",
  "input": {
    "label": "a",
    "enforce": "when_rtl"
  }
}
```

### HTTP (no key)

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

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

### CLI

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

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

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