# Headers

Certain Tilia API endpoints can accept custom headers, which are detailed here.

## Idempotency Key

The `Idempotency-Key` header is used to eliminate duplicate processing of unique requests. The header's value must be unique for each request and the format must be UUID4.

#### Example


```
Idempotency-Key: fe2fb0da-95fb-4a6b-954f-6897f9cae952
```

#### Errors

HTTP 400 Bad Request - Idempotency key is not UUID4 or the endpoint requires the Idempotency-Key header and none was provided.


```json
{
    "payload": {
        "errors": "failed to handle request. idempotency key is not in a valid format. the request is invalid"
    }
}
```

HTTP 409 Conflict - Idempotency key is currently being used to process a request.


```json
{
    "payload": {
        "errors": "failed to handle request. idempotency key is in use. resource already exists"
    }
}
```

HTTP 422 Unprocessable Content - Idempotency key is associated with a request body that's different from a previous request using the same key.


```json
{
    "payload": {
        "errors": "failed to handle request. idempotency key has different request body. unprocessable"
    }
}
```