> ## Documentation Index
> Fetch the complete documentation index at: https://docs.oneclickapply.co/llms.txt
> Use this file to discover all available pages before exploring further.

# Introduction

> Understand general concepts, response codes, and authentication strategies.

## Base URL

The One Click Apply API is built on REST principles. We enforce HTTPS in every request to improve data security, integrity, and privacy. The API does not support HTTP.

All requests contain the following base URL:

```
https://api.oneclickapply.co/
```

## Authorization

To authenticate with the One Click Apply API, you need to add an `API-Key` header to your request. For example, if your API key is `oca_1jmGxFzT2RJBUY9wOOEC3Sg`, your header would look like this:

```
API-Key: oca_1jmGxFzT2RJBUY9wOOEC3Sg
```

## Response Codes

One Click Apply uses standard HTTP codes to indicate the success or failure of your requests.

In general, `2xx` HTTP codes correspond to success, `4xx` codes are for user-related failures, and `5xx` codes are for infrastructure issues.

| Status | Description                        |
| ------ | ---------------------------------- |
| `200`  | Successful request.                |
| `403`  | API key does not exist.            |
| `429`  | Out of API credits.                |
| `400`  | Check that parameters are correct. |
| `5xx`  | Internal server error.             |