> For the complete documentation index, see [llms.txt](https://docs.hackle.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.hackle.io/en/development-guide/http-api/api-feature-flag.md).

# Feature Flag

{% hint style="info" %}
The HTTP API can only be used in a server environment.
{% endhint %}

You can use the HTTP API to check the Feature Flag Decision result for a user.

Authentication is performed using the SDK Key.

* Add the SDK Key to the HTTP header as shown below.
* You can find the SDK Key in the Hackle Dashboard under [SDK Integration Info](https://dashboard.hackle.io/config/sdk-setting).

```http
X-HACKLE-API-KEY: {YOUR_SERVER_SDK_KEY}
```

### Request

```http
POST https://api.hackle.io/v1/feature-flag
```

#### Request Body

| Field      | Type        | Required | Description             |
| ---------- | ----------- | -------- | ----------------------- |
| featureKey | number      | O        | Feature Key to evaluate |
| user       | User object | O        | User to evaluate        |

**User Object**

| Field       | Type   | Required | Description                                      |
| ----------- | ------ | -------- | ------------------------------------------------ |
| userId      | string | X        | User ID (Hackle ID can be used)                  |
| deviceId    | string | X        | Device ID (Hackle ID can be used)                |
| identifiers | object | X        | Custom identifier key(string)-value(string) pair |
| properties  | object | X        | User Property key(string)-value(string)          |

#### Example

```shell
curl -i -X POST \
  'https://api.hackle.io/v1/feature-flag' \
  -H 'X-HACKLE-API-KEY: YOUR_SDK_KEY_HERE' \
  -H 'Content-Type: application/json' \
  -d '{
    "featureKey": 42,
    "user": {
      "userId": "19285035",
      "deviceId": "ae2182e0",
      "identifiers": {
        "sessionId": "16930492542",
        "myCustomId": "ff01eae2"
      },
      "properties": {
        "grade": "GOLD",
        "age": 30,
        "isLogin": true
      }
    }
  }'
```

### Response

#### 200 OK

Returns a 200 response and the decision result for a successful request.

**Response Body**

| Field      | Type    | Description                                   |
| ---------- | ------- | --------------------------------------------- |
| isOn       | boolean | Decision result (true/false)                  |
| reason     | string  | Decision reason                               |
| parameters | object  | Parameters configured for the decision result |

**Example**

```json
{
  "isOn": "true",
  "reason": "DEFAULT_RULE",
  "parameters": {
    "buttonText": "Purchase",
    "buttonSize": 16
  }
}
```

#### 400 Bad Request

Returns a 400 status for invalid request information.

#### 401 Unauthorized

Returns a 401 status when the `X-HACKLE-API-KEY` header value is missing or the SDK Key is invalid.


---

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## Querying This Documentation
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Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://docs.hackle.io/en/development-guide/http-api/api-feature-flag.md?ask=<question>&goal=<endgoal>
```

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