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

# API standards

Lago API endpoints follow common RESTful standards.

## Authentication

Authentication is required for most endpoints, via an API key provided in the `Authorization` header as:

```http theme={"dark"}
Authorization: Bearer <your_api_key>
```

## HTTP status codes

HTTP status codes communicate the outcome of each API request. For example, a successful request returns `200 OK`, while a malformed request returns `400 Bad Request`. More information about error codes can be found in the [Errors](./errors) section.

## Request and response body

Unless otherwise noted, both requests and responses use a JSON body, and the `Content-Type` header is expected to be set to `application/json`.

## Query parameters

Query parameters allow you to filter, search, and paginate API results. The Lago API supports several types of query parameters, described below.

### Simple parameters

Simple query parameters are sent as key-value pairs. These parameters are used for pagination or to filter results by a single value:

```http List subscriptions theme={"dark"}
GET /api/v1/subscriptions?page=1&per_page=20&external_customer_id=123
```

In this case, the parameter will be described in the API reference as:

<ParamField query="external_customer_id" type="string">
  The customer external unique identifier (provided by your own application)

  Example: `"5eb02857-a71e-4ea2-bcf9-57d3a41bc6ba"`
</ParamField>

### Array parameters

Array parameters are sent as multiple entries of the same key, always ending with square brackets (`[]`). These parameters are used to filter results by multiple values:

```http List applied coupons theme={"dark"}
GET /api/v1/applied_coupons?coupon_code[]=10_OFF&coupon_code[]=20_OFF
```

In this case, the parameter will be described in the API reference as:

<ParamField query="coupon_code[]" type="enum<string>[]">
  The coupon codes to filter the applied coupons by.
</ParamField>