Validate Polish NIP (Tax Identification Number) (POST nip)
Validate Polish NIP (Tax Identification Number) and REGON (Business Registry Number) mathematical Modulo 11 checksums
Request
- x-api-key: <your-api-key>
- Content-Type: application/json
Response
{
"status": "success",
"nip": {
"input": "8993033031",
"normalized": "8993033031",
"valid": true,
"checksum": "modulo 11 ok",
"formatted": "899-303-30-31"
},
"regon": null,
"tokensDeducted": 1
}What it does
Validates the structure and Modulo 11 checksum of a Polish NIP (tax identification number) and, when given, a REGON (business registry number, 9 or 14 digits). This is the arithmetic check — whether the number can exist. Whether it is registered, and to whom, is what the VAT endpoint (VIES) and the CEIDG check answer.
When to use it
A Polish B2B form the moment the field loses focus; deduplicating companies in a CRM by normalised NIP.
Request
Body
| Field | Type | Required | Description |
|---|---|---|---|
identifier | string | no | Auto-detected fallback identifier (will auto-detect NIP or REGON based on length) |
nip | string | no | Polish NIP taxpayer identification number (e.g. "5252604877" or "525-260-48-77") |
regon | string | no | Polish REGON business entity number (9 or 14 digits) |
Example request
{
"nip": "8993033031"
}Response
| Field | Type | Description |
|---|---|---|
isValid | boolean | |
nipResult | object | |
↳ isValid | boolean | |
↳ normalized | string | |
↳ rawInput | string | |
↳ reason | string | |
regonResult | object | |
↳ isValid | boolean | |
↳ normalized | string | |
↳ rawInput | string | |
↳ reason | string | |
↳ type | string | |
status | string | |
tokensDeducted | integer |
Example response
{
"status": "success",
"nip": {
"input": "8993033031",
"normalized": "8993033031",
"valid": true,
"checksum": "modulo 11 ok",
"formatted": "899-303-30-31"
},
"regon": null,
"tokensDeducted": 1
}Errors
Every error answers with the same envelope: an HTTP status, a machine code and a sentence.
400 | The body or a parameter failed validation. The details array names the field. |
401 | No key, a revoked key or a wrong header. Send x-api-key. |
402 | The wallet cannot cover this call. Top up or wait for the monthly grant. |
404 | The resource does not exist, or it is not yours. Both answer identically and cost nothing. |
429 | Over the per-minute quota of your tier. Back off and retry. |
5xx | Upstream refused or timed out. The call is not billed. |
Code
curl -X POST 'https://app.castomapi.com/v1/verify/nip' \
-H 'x-api-key: <your-api-key>' \
-H 'Content-Type: application/json' \
-d '{"nip": "8993033031"}'const res = await fetch('https://app.castomapi.com/v1/verify/nip', {
method: 'POST',
headers: { 'x-api-key': '<your-api-key>', 'Content-Type': 'application/json' },
body: JSON.stringify({"nip": "8993033031"})
});
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/nip', {
method: 'POST',
headers: { 'x-api-key': '<your-api-key>', 'Content-Type': 'application/json' },
body: JSON.stringify({"nip": "8993033031"})
});
const data = await res.json();
console.log(data);import requests
res = requests.post(
'https://app.castomapi.com/v1/verify/nip',
headers={'x-api-key': '<your-api-key>'},
json={"nip": "8993033031"},
timeout=30,
)
print(res.json())<?php
$ch = curl_init('https://app.castomapi.com/v1/verify/nip');
curl_setopt_array($ch, [
CURLOPT_CUSTOMREQUEST => 'POST',
CURLOPT_RETURNTRANSFER => true,
CURLOPT_HTTPHEADER => ['x-api-key: <your-api-key>', 'Content-Type: application/json'],
CURLOPT_POSTFIELDS => json_encode(['nip' => '8993033031']),
]);
$data = json_decode(curl_exec($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 key | 10 |
| Free | 60 |
| Pro | 300 |
| Enterprise | 1200 |
Questions
What does Validate Polish NIP (Tax Identification Number) (POST nip) 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