> For clean Markdown of any page, append .md to the page URL.
> For a complete documentation index, see https://docs.getunleash.io/concepts/impression-data/llms.txt.
> For AI client integration (Claude Code, Cursor, etc.), connect to the MCP server at https://docs.getunleash.io/_mcp/server.

# Impression data

> Unleash can provide you with impression data about the flags in your application. Impression data contains information about a specific feature flag activation check.

v4.7

Requires [SDK compatibility](/sdks#feature-compatibility-in-backend-sdks).

Unleash can provide you with **impression data** about the flags in your application. Impression data contains information about a specific feature flag activation check: The client SDK will emit an **impression event** when it calls `isEnabled` or `getVariant`. Some front-end SDKs emit impression events only when a flag is enabled.

> **Warning**
>
> **Frontend SDKs and disabled flags**
>
> Frontend SDKs receive only enabled flags from [Unleash Edge](/unleash-edge) or the [Frontend API](/concepts/front-end-api), so by default they don't emit impression events for disabled flags. Some frontend SDKs have a configuration option that turns on impression events for all flags; check your SDK's documentation.

Impression data was designed to make it easier for you to **collect analytics data**, **perform A/B tests**, and **enrich experiments** in your applications. It contains information about the feature flag and the related [Unleash Context](/concepts/unleash-context).

Impression data is **opt-in on a per-flag basis**. Unleash will not emit impression events for flags not marked as such. Once you've turned impression data on for a flag, you can start listening for impression events in your client SDK.

## Impression event data

There's two types of impression events you can listen for:

* [`isEnabled` events](#example-isenabled-event)
* [`getVariant` events](#example-getvariant-event)

The `getVariant` event contains all the information found in an `isEnabled` event in addition to extra data that's only relevant to `getVariant` calls.

This table describes all the properties on the impression events:

| Property name | Description                                                                   | Event type               |
| ------------- | ----------------------------------------------------------------------------- | ------------------------ |
| `eventType`   | The type of the event: `isEnabled` or `getVariant`                            | All                      |
| `eventId`     | A globally unique id (GUID) assigned to this event.                           | All                      |
| `context`     | A representation of the current [Unleash Context](/concepts/unleash-context). | All                      |
| `enabled`     | Whether the flag was enabled or not at when the client made the request.      | All                      |
| `featureName` | The name of the feature flag.                                                 | All                      |
| `variant`     | The name of the active variant                                                | `getVariant` events only |

### Example `isEnabled` event

```js
{
  eventType: 'isEnabled',
  eventId: '84b41a43-5ba0-47d8-b21f-a60a319607b0',
  context: {
    sessionId: 54085233,
    appName: 'my-webapp',
    environment: 'default'
  },
  enabled: true,
  featureName: 'my-feature-flag',
}
```

### Example `getVariant` event

```js
{
  eventType: 'getVariant',
  eventId: '84b41a43-5ba0-47d8-b21f-a60a319607b0',
  context: {
    sessionId: 54085233,
    appName: 'my-webapp',
    environment: 'default'
  },
  enabled: true,
  featureName: 'my-feature-flag',
  variant: 'variantA'
}
```

## Enabling impression data

Impression data is strictly an **opt-in** feature and must be enabled on a **per-flag basis**. You can enable and disable it both when you create a flag and when you edit a flag.

You can enable impression data via the impression data flag in the admin UI's flag creation form. You can also go via the API, using the `impressionData` option.

### Request

POST [https://app.unleash-instance.example.com/api/admin/projects/\{projectId}/features](https://app.unleash-instance.example.com/api/admin/projects/\{projectId}/features)

```curl
curl -X POST https://app.unleash-instance.example.com/api/admin/projects/projectId/features \
     -H "Authorization: <apiKey>" \
     -H "Content-Type: application/json" \
     -d '{
  "name": "disable-comments"
}'
```

```python
import requests

url = "https://app.unleash-instance.example.com/api/admin/projects/projectId/features"

payload = { "name": "disable-comments" }
headers = {
    "Authorization": "<apiKey>",
    "Content-Type": "application/json"
}

response = requests.post(url, json=payload, headers=headers)

print(response.json())
```

```javascript
const url = 'https://app.unleash-instance.example.com/api/admin/projects/projectId/features';
const options = {
  method: 'POST',
  headers: {Authorization: '<apiKey>', 'Content-Type': 'application/json'},
  body: '{"name":"disable-comments"}'
};

try {
  const response = await fetch(url, options);
  const data = await response.json();
  console.log(data);
} catch (error) {
  console.error(error);
}
```

```go
package main

import (
	"fmt"
	"strings"
	"net/http"
	"io"
)

func main() {

	url := "https://app.unleash-instance.example.com/api/admin/projects/projectId/features"

	payload := strings.NewReader("{\n  \"name\": \"disable-comments\"\n}")

	req, _ := http.NewRequest("POST", url, payload)

	req.Header.Add("Authorization", "<apiKey>")
	req.Header.Add("Content-Type", "application/json")

	res, _ := http.DefaultClient.Do(req)

	defer res.Body.Close()
	body, _ := io.ReadAll(res.Body)

	fmt.Println(res)
	fmt.Println(string(body))

}
```

```ruby
require 'uri'
require 'net/http'

url = URI("https://app.unleash-instance.example.com/api/admin/projects/projectId/features")

http = Net::HTTP.new(url.host, url.port)
http.use_ssl = true

request = Net::HTTP::Post.new(url)
request["Authorization"] = '<apiKey>'
request["Content-Type"] = 'application/json'
request.body = "{\n  \"name\": \"disable-comments\"\n}"

response = http.request(request)
puts response.read_body
```

```java
import com.mashape.unirest.http.HttpResponse;
import com.mashape.unirest.http.Unirest;

HttpResponse<String> response = Unirest.post("https://app.unleash-instance.example.com/api/admin/projects/projectId/features")
  .header("Authorization", "<apiKey>")
  .header("Content-Type", "application/json")
  .body("{\n  \"name\": \"disable-comments\"\n}")
  .asString();
```

```php
<?php
require_once('vendor/autoload.php');

$client = new \GuzzleHttp\Client();

$response = $client->request('POST', 'https://app.unleash-instance.example.com/api/admin/projects/projectId/features', [
  'body' => '{
  "name": "disable-comments"
}',
  'headers' => [
    'Authorization' => '<apiKey>',
    'Content-Type' => 'application/json',
  ],
]);

echo $response->getBody();
```

```csharp
using RestSharp;

var client = new RestClient("https://app.unleash-instance.example.com/api/admin/projects/projectId/features");
var request = new RestRequest(Method.POST);
request.AddHeader("Authorization", "<apiKey>");
request.AddHeader("Content-Type", "application/json");
request.AddParameter("application/json", "{\n  \"name\": \"disable-comments\"\n}", ParameterType.RequestBody);
IRestResponse response = client.Execute(request);
```

```swift
import Foundation

let headers = [
  "Authorization": "<apiKey>",
  "Content-Type": "application/json"
]
let parameters = ["name": "disable-comments"] as [String : Any]

let postData = JSONSerialization.data(withJSONObject: parameters, options: [])

let request = NSMutableURLRequest(url: NSURL(string: "https://app.unleash-instance.example.com/api/admin/projects/projectId/features")! as URL,
                                        cachePolicy: .useProtocolCachePolicy,
                                    timeoutInterval: 10.0)
request.httpMethod = "POST"
request.allHTTPHeaderFields = headers
request.httpBody = postData as Data

let session = URLSession.shared
let dataTask = session.dataTask(with: request as URLRequest, completionHandler: { (data, response, error) -> Void in
  if (error != nil) {
    print(error as Any)
  } else {
    let httpResponse = response as? HTTPURLResponse
    print(httpResponse)
  }
})

dataTask.resume()
```

For more detailed instructions, see [the section on enabling impression data in the how-to guide for capturing impression data](/guides/how-to-capture-impression-data).

![A feature flag creation form. At the end of the form is a heading that says "Impression data", a short paragraph that describes the feature, and a flag to opt in or out of it.](/_fern-img/5ccfda7d6d240a5c8a50ee62a4c37413f22e165737323b6026a9ca8e88296fc9.webp)

## Example setup

The exact setup will vary depending on your [client SDK](/sdks). The below example configures the [Unleash Proxy client](/sdks/javascript-browser) to listen for impression events and log them to the console. If "my-feature-flag" is configured to emit impression data, then it will trigger an impression event as soon as Unleash is ready.

```js
const unleash = new UnleashClient({
  url: 'https://eu.unleash-hosted.com/hosted/proxy',
  clientKey: 'your-proxy-key',
  appName: 'my-webapp',
});

unleash.start();

unleash.on('ready', () => {
  unleash.isEnabled('my-feature-flag');
});

unleash.on('impression', (event) => {
  // Capture the event here and pass it to internal data lake or analytics provider
  console.log(event);
});
```

## Understand why a flag was enabled

Unleash SDKs do not expose an evaluation reason API: `isEnabled` and `getVariant` return the result of the evaluation, but not which strategy or constraint produced it. If you need to understand why a flag evaluated the way it did, you have the following options:

* Use the [Playground](/concepts/playground) to simulate a context and see detailed evaluation results, including which strategies and constraints matched. This is the best tool for debugging targeting rules.
* Use impression data, as described on this page, to track that a flag was evaluated, with which context, and with what result.

> Unleash can provide you with impression data about the flags in your application. Impression data contains information about a specific feature flag activation check.