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POST /v1/verify/w8ben-giin free No key needed Verify

Verify US IRS Form W-8BEN / W-8BEN-E foreign tax status inputs

Verify US IRS Form W-8BEN / W-8BEN-E foreign tax status inputs, validate 19-character FATCA GIIN Modulo 31 checksums, and evaluate US Bilateral Income Tax Treaty withholding rates

POST

Request

  • x-api-key: <your-api-key>
  • Content-Type: application/json

Response 

{
  "formAudit": {
    "formType": "standard",
    "isFormValid": true,
    "issues": [
      "<issue>"
    ],
    "warnings": []
  },
  "giinDetails": {
    "categoryCode": "ABC-123",
    "categoryName": "Ada Lovelace",
    "countryName": "Poland",
    "countryNumericCode": "ABC-123",
    "fatcaId": "fat_684777",
    "fiType": "standard",
    "isValid": true,
    "normalized": "PL61109010140000071219812874",
    "reason": "checksum ok"
  },
  "isValid": true,
  "status": "success",
  "tokensDeducted": 2,
  "treatyEligibility": {
    "countryOfTaxResidence": "PL",
    "isTreatyEligible": true,
    "treatyCountryName": "Poland",
    "withholdingRates": {
      "standardStatutoryRate": 4,
      "treatyDividendRate": 4,
      "treatyInterestRate": 4,
      "treatyRoyaltyRate": 4
    }
  }
}
The response below is a mock built from the endpoint's own schema — no request leaves this page. Edit the body, press Send, read the shape you will get back. Run it for real in the dashboard →

What it does

Verify US IRS Form W-8BEN / W-8BEN-E foreign tax status inputs, validate 19-character FATCA GIIN Modulo 31 checksums, and evaluate US Bilateral Income Tax Treaty withholding rates

Request

Body

FieldTypeRequiredDescription
chapter3StatusstringnoEntity classification (e.g., "Corporation", "Partnership", "Disregarded Entity")
chapter4StatusstringnoFATCA classification (e.g., "REPORTING_MODEL_1_FFI", "ACTIVE_NFFE", "PASSIVE_NFFE")
countryOfCitizenshipstringnoISO 2-letter or 3-letter country code of tax residence
dateOfBirthstringnoDate of birth YYYY-MM-DD (W-8BEN individual)
formTypeenumyesTarget tax form or standalone GIIN verification One of: W-8BEN, W-8BEN-E, GIIN_ONLY.
fullNamestringnoBeneficial owner name (W-8BEN individual)
giinstringno19-character FATCA GIIN identifier (e.g., "980000.00000.ME.250")
organizationNamestringnoOrganization name (W-8BEN-E entity)

Example request

{
  "chapter3Status": "success",
  "chapter4Status": "success",
  "countryOfCitizenship": "PL",
  "dateOfBirth": "2026-09-08",
  "formType": "W-8BEN",
  "fullName": "Ada Lovelace",
  "giin": "<giin>",
  "organizationName": "Ada Lovelace"
}

Response

FieldTypeDescription
formAuditobject
  ↳ formTypestring
  ↳ isFormValidboolean
  ↳ issuesarray<string>
  ↳ warningsarray<string>
giinDetailsobject
  ↳ categoryCodestring
  ↳ categoryNamestring
  ↳ countryNamestring
  ↳ countryNumericCodestring
  ↳ fatcaIdstring
  ↳ fiTypestring
  ↳ isValidboolean
  ↳ normalizedstring
  ↳ reasonstring
isValidboolean
statusstring
tokensDeductedinteger
treatyEligibilityobject
  ↳ countryOfTaxResidencestring
  ↳ isTreatyEligibleboolean
  ↳ treatyCountryNamestring
  ↳ withholdingRatesobject

Example response

{
  "formAudit": {
    "formType": "standard",
    "isFormValid": true,
    "issues": [
      "<issue>"
    ],
    "warnings": []
  },
  "giinDetails": {
    "categoryCode": "ABC-123",
    "categoryName": "Ada Lovelace",
    "countryName": "Poland",
    "countryNumericCode": "ABC-123",
    "fatcaId": "fat_684777",
    "fiType": "standard",
    "isValid": true,
    "normalized": "PL61109010140000071219812874",
    "reason": "checksum ok"
  },
  "isValid": true,
  "status": "success",
  "tokensDeducted": 2,
  "treatyEligibility": {
    "countryOfTaxResidence": "PL",
    "isTreatyEligible": true,
    "treatyCountryName": "Poland",
    "withholdingRates": {
      "standardStatutoryRate": 4,
      "treatyDividendRate": 4,
      "treatyInterestRate": 4,
      "treatyRoyaltyRate": 4
    }
  }
}

Errors

Every error answers with the same envelope: an HTTP status, a machine code and a sentence.

400The body or a parameter failed validation. The details array names the field.
401No key, a revoked key or a wrong header. Send x-api-key.
402The wallet cannot cover this call. Top up or wait for the monthly grant.
404The resource does not exist, or it is not yours. Both answer identically and cost nothing.
429Over the per-minute quota of your tier. Back off and retry.
5xxUpstream refused or timed out. The call is not billed.

Code

curl -X POST 'https://app.castomapi.com/v1/verify/w8ben-giin' \
  -H 'x-api-key: <your-api-key>' \
  -H 'Content-Type: application/json' \
  -d '{"chapter3Status": "success", "chapter4Status": "success", "countryOfCitizenship": "PL", "dateOfBirth": "2026-09-08", "formType": "W-8BEN", "fullName": "Ada Lovelace", "giin": "<giin>", "organizationName": "Ada Lovelace"}'
const res = await fetch('https://app.castomapi.com/v1/verify/w8ben-giin', {
  method: 'POST',
  headers: { 'x-api-key': '<your-api-key>', 'Content-Type': 'application/json' },
  body: JSON.stringify({"chapter3Status": "success", "chapter4Status": "success", "countryOfCitizenship": "PL", "dateOfBirth": "2026-09-08", "formType": "W-8BEN", "fullName": "Ada Lovelace", "giin": "<giin>", "organizationName": "Ada Lovelace"})
});
const data = await res.json();
console.log(data);
import { request } from 'node:https';

// Node 18+ has fetch built in
const res = await fetch('https://app.castomapi.com/v1/verify/w8ben-giin', {
  method: 'POST',
  headers: { 'x-api-key': '<your-api-key>', 'Content-Type': 'application/json' },
  body: JSON.stringify({"chapter3Status": "success", "chapter4Status": "success", "countryOfCitizenship": "PL", "dateOfBirth": "2026-09-08", "formType": "W-8BEN", "fullName": "Ada Lovelace", "giin": "<giin>", "organizationName": "Ada Lovelace"})
});
const data = await res.json();
console.log(data);
import requests

res = requests.post(
    'https://app.castomapi.com/v1/verify/w8ben-giin',
    headers={'x-api-key': '<your-api-key>'},
    json={"chapter3Status": "success", "chapter4Status": "success", "countryOfCitizenship": "PL", "dateOfBirth": "2026-09-08", "formType": "W-8BEN", "fullName": "Ada Lovelace", "giin": "<giin>", "organizationName": "Ada Lovelace"},
    timeout=30,
)
print(res.json())
<?php
$ch = curl_init('https://app.castomapi.com/v1/verify/w8ben-giin');
curl_setopt_array($ch, [
    CURLOPT_CUSTOMREQUEST => 'POST',
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_HTTPHEADER => ['x-api-key: <your-api-key>', 'Content-Type: application/json'],
    CURLOPT_POSTFIELDS => json_encode(['chapter3Status' => 'success', 'chapter4Status' => 'success', 'countryOfCitizenship' => 'PL', 'dateOfBirth' => '2026-09-08', 'formType' => 'W-8BEN', 'fullName' => 'Ada Lovelace', 'giin' => '<giin>', 'organizationName' => 'Ada Lovelace']),
]);
$data = json_decode(curl_exec($ch), true);
print_r($data);

Cost and limits

This operation needs no API key and costs nothing: called without a key it answers 200 and deducts zero tokens, up to 10 requests a minute per IP address.

Rate limits per minute

No key10
Free60
Pro300
Enterprise1200

Docs → Pricing

Questions

What does Verify US IRS Form W-8BEN / W-8BEN-E foreign tax status inputs cost?

This operation needs no API key and costs nothing: called without a key it answers 200 and deducts zero tokens, up to 10 requests a minute per IP address.

How do I authenticate?

Send your key in the x-api-key header. One key covers every metered endpoint; there is nothing to enable per endpoint.

Can I send an Authorization: Bearer token instead?

Not for a metered endpoint — those take the x-api-key header and nothing else. Bearer tokens belong to the account side: the dashboard and account APIs accept a JWT session, and castomapi-auth issues those through a standard OAuth2 handshake, with authorization code, client credentials and refresh grants. The auth service page covers it.

What are the limits?

10 requests a minute per IP with no key at all, 60 a minute on the free tier, 300 on Pro, 1200 on Enterprise. Above the no-key tier the quota is per account, across every endpoint.

OpenAPI: https://app.castomapi.com/docs/json