Subchapter 191.1

Adding Feature Flag Code

references/adding-feature-flag-code.mdMarkdown109 KBView on GitHub

Adding feature flag code - Docs

Once you’ve created your feature flag in PostHog, the next step is to add your code:

Web

Adding Feature Flag Code · Feature Flags Node.js · PostHog/skills · Skills Docs

if (posthog.isFeatureEnabled('flag-key') ) {
    // Do something differently for this user
    // Optional: fetch the payload
    const matchedFlagPayload = posthog.getFeatureFlagPayload('flag-key')
}

if (posthog.getFeatureFlag('flag-key')  == 'variant-key') { // replace 'variant-key' with the key of your variant
    // Do something differently for this user
    // Optional: fetch the payload
    const matchedFlagPayload = posthog.getFeatureFlagPayload('flag-key')
}

posthog.onFeatureFlags(function (flags, flagVariants, { errorsLoading }) {
    // feature flags are guaranteed to be available at this point
    if (posthog.isFeatureEnabled('flag-key')) {
        // do something
    }
})

// set properties for a group
posthog.setGroupPropertiesForFlags({'company': {'property1': 'value', property2: 'value2'}})
// reset properties for a given group:
posthog.resetGroupPropertiesForFlags('company')
// reset properties for all groups:
posthog.resetGroupPropertiesForFlags()

posthog.init('<ph_project_token>', {
  api_host: 'https://us.i.posthog.com',
  defaults: '2026-01-30'
  feature_flag_request_timeout_ms: 3000 // Time in milliseconds. Default is 3000 (3 seconds).
})

function handleFeatureFlag(client, flagKey, distinctId) {
  try {
    const isEnabled = client.isFeatureEnabled(flagKey, distinctId);
    console.log(`Feature flag '${flagKey}' for user '${distinctId}' is ${isEnabled ? 'enabled' : 'disabled'}`);
    return isEnabled;
  } catch (error) {
    console.error(`Error fetching feature flag '${flagKey}': ${error.message}`);
    // Optionally, you can return a default value or throw the error
    // return false; // Default to disabled
    throw error;
  }
}
// Usage example
try {
  const flagEnabled = handleFeatureFlag(client, 'new-feature', 'user-123');
  if (flagEnabled) {
    // Implement new feature logic
  } else {
    // Implement old feature logic
  }
} catch (error) {
  // Handle the error at a higher level
  console.error('Feature flag check failed, using default behavior');
  // Implement fallback logic
}

Hook	Description
useFeatureFlagEnabled	Returns a boolean indicating whether the feature flag is enabled. This sends a $feature_flag_called event.
useFeatureFlagVariantKey	Returns the variant key of the feature flag. This sends a $feature_flag_called event.
useActiveFeatureFlags	Returns an array of active feature flags. This does not send a $feature_flag_called event.
useFeatureFlagPayload	Returns the payload of the feature flag. This does not send a $feature_flag_called event. Always use this with useFeatureFlagEnabled or useFeatureFlagVariantKey.

import { useFeatureFlagEnabled } from '@posthog/react'
function App() {
  const showWelcomeMessage = useFeatureFlagEnabled('flag-key')
  const payload = useFeatureFlagPayload('flag-key')
  return (
    <div className="App">
      {
        showWelcomeMessage ? (
          <div>
            <h1>Welcome!</h1>
            <p>Thanks for trying out our feature flags.</p>
          </div>
        ) : (
          <div>
            <h2>No welcome message</h2>
            <p>Because the feature flag evaluated to false.</p>
          </div>
        )
      }
    </div>
  );
}
export default App;

import { useFeatureFlagVariantKey } from '@posthog/react'
function App() {
  const variantKey = useFeatureFlagVariantKey('show-welcome-message')
  let welcomeMessage = ''
  if (variantKey === 'variant-a') {
    welcomeMessage = 'Welcome to the Alpha!'
  } else if (variantKey === 'variant-b') {
    welcomeMessage = 'Welcome to the Beta!'
  }
  return (
    <div className="App">
      {
        welcomeMessage ? (
          <div>
            <h1>{welcomeMessage}</h1>
            <p>Thanks for trying out our feature flags.</p>
          </div>
        ) : (
          <div>
            <h2>No welcome message</h2>
            <p>Because the feature flag evaluated to false.</p>
          </div>
        )
      }
    </div>
  );
}
export default App;

import { useFeatureFlagPayload } from '@posthog/react'
function App() {
  const variant = useFeatureFlagEnabled('show-welcome-message')
  const payload = useFeatureFlagPayload('show-welcome-message')
    return (
                <>
                {
                    variant ? (
                        <div className="welcome-message">
                            <h2>{payload?.welcomeTitle}</h2>
                            <p>{payload?.welcomeMessage}</p>
                        </div>
                    ) : <div>
                        <h2>No custom welcome message</h2>
                        <p>Because the feature flag evaluated to false.</p>
                    </div>
                }
        </>
    )
}

import { PostHogFeature } from '@posthog/react'
function App() {
    return (
        <PostHogFeature flag='show-welcome-message' match={true}>
            <div>
                <h1>Hello</h1>
                <p>Thanks for trying out our feature flags.</p>
            </div>
        </PostHogFeature>
    )
}

import { PostHogFeature } from '@posthog/react'
function App() {
    return (
        <PostHogFeature flag='show-welcome-message' match={true}>
           {(payload) => {
                return (
                    <div>
                        <h1>{payload.welcomeMessage}</h1>
                        <p>Thanks for trying out our feature flags.</p>
                    </div>
                )
           }}
        </PostHogFeature>
    )
}

posthog.init('<ph_project_token>', {
  api_host: 'https://us.i.posthog.com',
  defaults: '2026-01-30'
  feature_flag_request_timeout_ms: 3000 // Time in milliseconds. Default is 3000 (3 seconds).
}
)

function handleFeatureFlag(client, flagKey, distinctId) {
    try {
        const isEnabled = client.isFeatureEnabled(flagKey, distinctId);
        console.log(`Feature flag '${flagKey}' for user '${distinctId}' is ${isEnabled ? 'enabled' : 'disabled'}`);
        return isEnabled;
    } catch (error) {
        console.error(`Error fetching feature flag '${flagKey}': ${error.message}`);
        // Optionally, you can return a default value or throw the error
        // return false; // Default to disabled
        throw error;
    }
}
// Usage example
try {
    const flagEnabled = handleFeatureFlag(client, 'new-feature', 'user-123');
    if (flagEnabled) {
        // Implement new feature logic
    } else {
        // Implement old feature logic
    }
} catch (error) {
    // Handle the error at a higher level
    console.error('Feature flag check failed, using default behavior');
    // Implement fallback logic
}

const isFeatureFlagEnabled = await client.isFeatureEnabled('flag-key', 'distinct_id_of_your_user')
if (isFeatureFlagEnabled) {
    // Your code if the flag is enabled
    // Optional: fetch the payload
    const matchedFlagPayload = await client.getFeatureFlagPayload('flag-key', 'distinct_id_of_your_user', isFeatureFlagEnabled)
}

const enabledVariant = await client.getFeatureFlag('flag-key', 'distinct_id_of_your_user')
if (enabledVariant === 'variant-key') {  // replace 'variant-key' with the key of your variant
    // Do something differently for this user
    // Optional: fetch the payload
    const matchedFlagPayload = await client.getFeatureFlagPayload('flag-key', 'distinct_id_of_your_user', enabledVariant)
}

client.capture({
    distinctId: 'distinct_id_of_your_user',
    event: 'event_name',
    properties: {
        '$feature/feature-flag-key': 'variant-key' // replace feature-flag-key with your flag key. Replace 'variant-key' with the key of your variant
    },
})

client.capture({
    distinctId: 'distinct_id_of_your_user',
    event: 'event_name',
    sendFeatureFlags: {
        onlyEvaluateLocally: true,
        personProperties: { plan: 'premium' },
        groupProperties: { org: { tier: 'enterprise' } }
    }
})

await client.getFeatureFlag(
    'flag-key',
    'distinct_id_of_the_user',
    {
        personProperties: {
            'property_name': 'value'
        },
        groups: {
            "your_group_type": "your_group_id",
            "another_group_type": "your_group_id",
        },
        groupProperties: {
            'your_group_type': {
                'group_property_name': 'value'
            },
            'another_group_type': {
                'group_property_name': 'value'
            }
        },
    }
)

const client = new PostHog('<ph_project_token>', {
        api_host: 'https://us.i.posthog.com',
        feature_flag_request_timeout_ms: 3000 // Time in milliseconds. Default is 3000 (3 seconds).
    }
)

async function handleFeatureFlag(client, flagKey, distinctId) {
    try {
        const isEnabled = await client.isFeatureEnabled(flagKey, distinctId);
        console.log(`Feature flag '${flagKey}' for user '${distinctId}' is ${isEnabled ? 'enabled' : 'disabled'}`);
        return isEnabled;
    } catch (error) {
        console.error(`Error fetching feature flag '${flagKey}': ${error.message}`);
        // Optionally, you can return a default value or throw the error
        // return false; // Default to disabled
        throw error;
    }
}
// Usage example
try {
    const flagEnabled = await handleFeatureFlag(client, 'new-feature', 'user-123');
    if (flagEnabled) {
        // Implement new feature logic
    } else {
        // Implement old feature logic
    }
} catch (error) {
    // Handle the error at a higher level
    console.error('Feature flag check failed, using default behavior');
    // Implement fallback logic
}

is_my_flag_enabled = posthog.feature_enabled('flag-key', 'distinct_id_of_your_user')
if is_my_flag_enabled:
    # Do something differently for this user
    # Optional: fetch the payload
    matched_flag_payload = posthog.get_feature_flag_payload('flag-key', 'distinct_id_of_your_user')

enabled_variant = posthog.get_feature_flag('flag-key', 'distinct_id_of_your_user')
if enabled_variant == 'variant-key': # replace 'variant-key' with the key of your variant
    # Do something differently for this user
    # Optional: fetch the payload
    matched_flag_payload = posthog.get_feature_flag_payload('flag-key', 'distinct_id_of_your_user')

posthog.capture(
    "event_name",
    distinct_id="distinct_id_of_the_user",
    properties={
        "$feature/feature-flag-key": "variant-key"  # replace feature-flag-key with your flag key. Replace 'variant-key' with the key of your variant
    },
)

posthog.capture(
    distinct_id="distinct_id_of_the_user",
    event='event_name',
    send_feature_flags={
        'only_evaluate_locally': True,
        'person_properties': {'plan': 'premium'},
        'group_properties': {'org': {'tier': 'enterprise'}}
    }
)

posthog.get_feature_flag(
    'flag-key',
    'distinct_id_of_the_user',
    person_properties={'property_name': 'value'},
    groups={
        'your_group_type': 'your_group_id',
        'another_group_type': 'your_group_id'},
    group_properties={
        'your_group_type': {'group_property_name': 'value'},
        'another_group_type': {'group_property_name': 'value'}
    },
)

def handle_feature_flag(client, flag_key, distinct_id):
    try:
        is_enabled = client.is_feature_enabled(flag_key, distinct_id)
        print(f"Feature flag '{flag_key}' for user '{distinct_id}' is {'enabled' if is_enabled else 'disabled'}")
        return is_enabled
    except Exception as e:
        print(f"Error fetching feature flag '{flag_key}': {str(e)}")
        raise e
# Usage example
try:
    flag_enabled = handle_feature_flag(client, 'new-feature', 'user-123')
    if flag_enabled:
        # Implement new feature logic
    else:
        # Implement old feature logic
except Exception as e:
    # Handle the error at a higher level

$enabledVariant = PostHog::getFeatureFlag('flag-key', 'distinct_id_of_your_user')
if ($enabledVariant === 'variant-key') { # replace 'variant-key' with the key of your variant
    # Do something differently for this user
}

PostHog::capture([
  'distinctId' => 'distinct_id_of_your_user',
  'event' => 'event_name',
  'properties' => [
    '$feature/feature-flag-key' => 'variant-key' // replace feature-flag-key with your flag key. Replace 'variant-key' with the key of your variant
  ]
]);

PostHog::getFeatureFlag(
    'flag-key',
    'distinct_id_of_the_user',
    [
        'your_group_type' => 'your_group_id',
        'another_group_type' => 'your_group_id'
    ], // groups
    ['property_name' => 'value'], // person properties
    [
        'your_group_type' => ['group_property_name' => 'value'],
        'another_group_type' => ['group_property_name' => 'value']
    ], // group properties
    false, // onlyEvaluateLocally, Optional. Defaults to false.
    true // sendFeatureFlagEvents
);

function handleFeatureFlag($client, $flagKey, $distinctId) {
    try {
        $isEnabled = $client->isFeatureEnabled($flagKey, $distinctId);
        echo "Feature flag '$flagKey' for user '$distinctId' is " . ($isEnabled ? 'enabled' : 'disabled') . "\n";
        return $isEnabled;
    } catch (Exception $e) {
        echo "Error fetching feature flag '$flagKey': " . $e->getMessage() . "\n";
        // Optionally, you can return a default value or throw the error
        // return false; // Default to disabled
        throw $e;
    }
}
// Usage example
try {
    $flagEnabled = handleFeatureFlag($client, 'new-feature', 'user-123');
    if ($flagEnabled) {
        // Implement new feature logic
    } else {
        // Implement old feature logic
    }
} catch (Exception $e) {
    // Handle the error at a higher level
    echo 'Feature flag check failed, using default behavior';
    // Implement fallback logic
}

is_my_flag_enabled = posthog.is_feature_enabled('flag-key', 'distinct_id_of_your_user')
if is_my_flag_enabled
    # Do something differently for this user
    # Optional: fetch the payload
    matched_flag_payload = posthog.get_feature_flag_payload('flag-key', 'distinct_id_of_your_user')
end

enabled_variant = posthog.get_feature_flag('flag-key', 'distinct_id_of_your_user')
if enabled_variant == 'variant-key' # replace 'variant-key' with the key of your variant
    # Do something differently for this user
    # Optional: fetch the payload
    matched_flag_payload = posthog.get_feature_flag_payload('variant-key', 'distinct_id_of_your_user')
end

posthog.capture({
    distinct_id: 'distinct_id_of_your_user',
    event: 'event_name',
    properties: {
        '$feature/feature-flag-key': 'variant-key', # replace feature-flag-key with your flag key. Replace 'variant-key' with the key of your variant
    }
})

posthog.capture({
    distinct_id: 'distinct_id_of_your_user',
    event: 'event_name',
    send_feature_flags: {
        only_evaluate_locally: true,
        person_properties: { plan: 'premium' },
        group_properties: { org: { tier: 'enterprise' } }
    }
})

posthog.get_feature_flag(
    'flag-key',
    'distinct_id_of_the_user',
    person_properties: {
        'property_name': 'value'
    },
    groups: {
        'your_group_type': 'your_group_id',
        'another_group_type': 'your_group_id',
    },
    group_properties: {
        'your_group_type': {
            'group_property_name': 'value'
        }
        'another_group_type': {
            'group_property_name': 'value'
        }
    },
)

def handle_feature_flag(client, flag_key, distinct_id)
    begin
        is_enabled = client.is_feature_enabled(flag_key, distinct_id)
        puts "Feature flag '#{flag_key}' for user '#{distinct_id}' is #{is_enabled ? 'enabled' : 'disabled'}"
        return is_enabled
    rescue => e
        puts "Error fetching feature flag '#{flag_key}': #{e.message}"
        # Optionally, you can return a default value or throw the error
        # return false # Default to disabled
        raise e
    end
end
# Usage example
try
    flag_enabled = handle_feature_flag(client, 'new-feature', 'user-123')
    if flag_enabled
        # Implement new feature logic
    else
        # Implement old feature logic
    end
rescue => e
    # Handle the error at a higher level
    puts 'Feature flag check failed, using default behavior'
    # Implement fallback logic
end

isMyFlagEnabled, err := client.IsFeatureEnabled(posthog.FeatureFlagPayload{
    Key:        "flag-key",
    DistinctId: "distinct_id_of_your_user",
})
if err != nil {
    // Handle error (e.g. capture error and fallback to default behavior)
}
if isMyFlagEnabled == true {
    // Do something differently for this user
}

enabledVariant, err := client.GetFeatureFlag(posthog.FeatureFlagPayload{
    Key:        "flag-key",
    DistinctId: "distinct_id_of_your_user",
})
if err != nil {
    // Handle error (e.g. capture error and fallback to default behavior)
}
if enabledVariant == "variant-key" { // replace 'variant-key' with the key of your variant
    // Do something differently for this user
}

client.Enqueue(posthog.Capture{
  DistinctId: "distinct_id_of_your_user",
  Event:      "event_name",
  Properties: posthog.NewProperties().
    Set("$feature/feature-flag-key", "variant-key"), // replace feature-flag-key with your flag key. Replace 'variant-key' with the key of your variant
})

client.Enqueue(posthog.Capture{
  DistinctId: "distinct_id_of_your_user",
  Event:      "event_name",
  SendFeatureFlags: posthog.SendFeatureFlagsOptions{
    OnlyEvaluateLocally: true,
    PersonProperties: map[string]interface{}{
      "plan": "premium",
    },
    GroupProperties: map[string]map[string]interface{}{
      "org": {
        "tier": "enterprise",
      },
    },
  },
})

sendFeatureFlags := false
isMyFlagEnabled, err := client.IsFeatureEnabled(posthog.FeatureFlagPayload{
    Key:                    "flag-key",
    DistinctId:             "distinct_id_of_your_user",
    SendFeatureFlagEvents:  &sendFeatureFlags,
})

enabledVariant, err := client.GetFeatureFlag(
    FeatureFlagPayload{
        Key:        "flag-key",
        DistinctId: "distinct_id_of_the_user",
        Groups: posthog.NewGroups().
            Set("your_group_type", "your_group_id").
            Set("another_group_type", "your_group_id"),
        PersonProperties: posthog.NewProperties().
            Set("property_name", "value"),
        GroupProperties: map[string]map[string]interface{}{
            "your_group_type": {
                "group_property_name": "value",
            },
            "another_group_type": {
                "group_property_name": "value",
            },
        },
    },
)

client, _ := posthog.NewWithConfig(
   os.Getenv("<ph_project_token>"),
   posthog.Config{
      PersonalApiKey:            "your personal API key", // Optional, but much more performant.  If this token is not supplied, then fetching feature flag values will be slower.
      Endpoint:                  "https://us.i.posthog.com",
      FeatureFlagRequestTimeout: 3 // Time in seconds. Default is 3.
   },
)

func handleFeatureFlag(client *posthog.Client, flagKey string, distinctId string) {
    flag, err := client.GetFeatureFlag(posthog.FeatureFlagPayload{
        Key:        flagKey,
        DistinctId: distinctId,
    })
    if err != nil {
        // Handle the error appropriately
        log.Printf("Error fetching feature flag: %v", err)
        return
    }
    // Use the flag value as needed
    fmt.Printf("Feature flag '%s' for user '%s': %s\n", flagKey, distinctId, flag)
}

import { useFeatureFlag } from 'posthog-react-native'
const MyComponent = () => {
    const booleanFlag = useFeatureFlag('key-for-your-boolean-flag')
    if (booleanFlag === undefined) {
        // the response is undefined if the flags are being loaded
        return null
    }
    // Optional use the 'useFeatureFlagWithPayload' hook for fetching the feature flag payload
    return booleanFlag ? <Text>Testing feature 😄</Text> : <Text>Not Testing feature 😢</Text>
}

import { useFeatureFlag } from 'posthog-react-native'
const MyComponent = () => {
    const multiVariantFeature = useFeatureFlag('key-for-your-multivariate-flag')
    if (multiVariantFeature === undefined) {
        // the response is undefined if the flags are being loaded
        return null
    } else if (multiVariantFeature === 'variant-name') { // replace 'variant-name' with the name of your variant
      // Do something
    }
    // Optional use the 'useFeatureFlagWithPayload' hook for fetching the feature flag payload
    return <div/>
}

// Defaults to undefined if not loaded yet or if there was a problem loading
posthog.isFeatureEnabled('key-for-your-boolean-flag')
// Defaults to undefined if not loaded yet or if there was a problem loading
posthog.getFeatureFlag('key-for-your-boolean-flag')
// Multivariant feature flags are returned as a string
posthog.getFeatureFlag('key-for-your-multivariate-flag')
// Optional fetch the payload returns 'JsonType' or undefined if not loaded yet or if there was a problem loading
posthog.getFeatureFlagPayload('key-for-your-multivariate-flag')

export const posthog = new PostHog('<ph_project_token>', {
  // usually 'https://us.i.posthog.com' or 'https://eu.i.posthog.com'
  host: 'https://us.i.posthog.com',
  featureFlagsRequestTimeoutMs: 10000 // Time in milliseconds. Default is 10000 (10 seconds).
})

function handleFeatureFlag(client, flagKey, distinctId) {
    try {
        const isEnabled = client.isFeatureEnabled(flagKey, distinctId);
        console.log(`Feature flag '${flagKey}' for user '${distinctId}' is ${isEnabled ? 'enabled' : 'disabled'}`);
        return isEnabled;
    } catch (error) {
        console.error(`Error fetching feature flag '${flagKey}': ${error.message}`);
        // Optionally, you can return a default value or throw the error
        // return false; // Default to disabled
        throw error;
    }
}
// Usage example
try {
    const flagEnabled = handleFeatureFlag(client, 'new-feature', 'user-123');
    if (flagEnabled) {
        // Implement new feature logic
    } else {
        // Implement old feature logic
    }
} catch (error) {
    // Handle the error at a higher level
    console.error('Feature flag check failed, using default behavior');
    // Implement fallback logic
}

// set properties for a group
posthog.setGroupPropertiesForFlags({'company': {'property1': 'value', property2: 'value2'}})
// reset properties for all groups:
posthog.resetGroupPropertiesForFlags()

import com.posthog.PostHog
if (PostHog.isFeatureEnabled("flag-key")) {
    // Do something differently for this user
    // Optional: fetch the payload
    val matchedFlagPayload = PostHog.getFeatureFlagPayload("flag-key")
}

import com.posthog.PostHog
if (PostHog.getFeatureFlag("flag-key") == "variant-key") { // replace 'variant-key' with the key of your variant
    // Do something differently for this user
    // Optional: fetch the payload
    val matchedFlagPayload = PostHog.getFeatureFlagPayload("flag-key")
}

import com.posthog.PostHog
import com.posthog.android.PostHogAndroidConfig
import com.posthog.PostHogOnFeatureFlags
// During SDK initialization
val config = PostHogAndroidConfig(apiKey = "<ph_project_token>").apply {
    onFeatureFlags = PostHogOnFeatureFlags {
        if (PostHog.isFeatureEnabled("flag-key")) {
            // do something
        }
    }
}
// And/Or manually the SDK is initialized
PostHog.reloadFeatureFlags {
    if (PostHog.isFeatureEnabled("flag-key")) {
        // do something
    }
}

if (PostHogSDK.shared.isFeatureEnabled("flag-key")) {
    // Do something differently for this user
    // Optional: fetch the payload
    let matchedFlagPayload = PostHogSDK.shared.getFeatureFlagPayload("flag-key")
}

if (PostHogSDK.shared.getFeatureFlag("flag-key") as? String == "variant-key") { // replace "variant-key" with the key of your variant
    // Do something differently for this user
    // Optional: fetch the payload
    let matchedFlagPayload = PostHogSDK.shared.getFeatureFlagPayload("flag-key")
}

class AppDelegate: NSObject, UIApplicationDelegate {
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
        // register for `didReceiveFeatureFlags` notification before SDK initialization
        NotificationCenter.default.addObserver(
            self,
            selector: #selector(receiveFeatureFlags),
            name: PostHogSDK.didReceiveFeatureFlags,
            object: nil
        )
        let POSTHOG_API_KEY = "<ph_project_token>"
        // usually 'https://us.i.posthog.com' or 'https://eu.i.posthog.com'
        let POSTHOG_HOST = "https://us.i.posthog.com"
        let config = PostHogConfig(apiKey: POSTHOG_API_KEY, host: POSTHOG_HOST)
        PostHogSDK.shared.setup(config)
        return true
    }
    // The "receiveFeatureFlags" method will be called when the SDK receives the feature flags from the server.
    @objc func receiveFeatureFlags() {
        print("receiveFeatureFlags called")
    }
}

// Reload feature flags and check if a specific feature is enabled
PostHogSDK.shared.reloadFeatureFlags {
    if PostHogSDK.shared.isFeatureEnabled("flag-key") {
        // do something
    }
}

if (await Posthog().isFeatureEnabled('flag-key')) {
  // Do something differently for this user
  // Optional: fetch the payload
  final matchedFlagPayload = await Posthog().getFeatureFlagPayload('flag-key');
}

if (await Posthog().getFeatureFlag('flag-key') == 'variant-key') { // replace 'variant-key' with the key of your variant
  // Do something differently for this user
  // Optional: fetch the payload
  final matchedFlagPayload = await Posthog().getFeatureFlagPayload('flag-key');
}

final config = PostHogConfig('<ph_project_token>');
config.host = 'https://us.i.posthog.com';
config.onFeatureFlags = () async {
  if (await Posthog().isFeatureEnabled('flag-key')) {
    // do something
  }
};
await Posthog().setup(config);

if (posthog.isFeatureEnabled("distinct_id_of_your_user", "flag-key")) {
    // Do something differently for this user
    // Optional: fetch the payload
    Object matchedFlagPayload = posthog.getFeatureFlagPayload("distinct_id_of_your_user", "flag-key");
}

if ("variant-key".equals(posthog.getFeatureFlag("distinct_id_of_your_user", "flag-key"))) { // replace 'variant-key' with the key of your variant
    // Do something differently for this user
    // Optional: fetch the payload
    Object matchedFlagPayload = posthog.getFeatureFlagPayload("distinct_id_of_your_user", "flag-key");
}

import com.posthog.server.PostHogFeatureFlagOptions;
posthog.getFeatureFlag(
    "distinct_id_of_the_user",
    "flag-key",
    PostHogFeatureFlagOptions
        .builder()
        .defaultValue(false)
        .group("your_group_type", "your_group_id")
        .group("another_group_type", "your_group_id")
        .groupProperty("your_group_type", "group_property_name", "value")
        .groupProperty("another_group_type", "group_property_name", "value")
        .personProperty("property_name", "value")
        .build());

let is_enabled = client.is_feature_enabled(
    "flag-key".to_string(),
    "distinct_id_of_your_user".to_string(),
    None, // groups
    None, // person_properties
    None, // group_properties
).await.unwrap();
if is_enabled {
    // Do something differently for this user
}

use posthog_rs::FlagValue;
match client.get_feature_flag(
    "flag-key".to_string(),
    "distinct_id_of_your_user".to_string(),
    None, // groups
    None, // person_properties
    None, // group_properties
).await.unwrap() {
    Some(FlagValue::String(variant)) => {
        if variant == "variant-key" {
            // Do something for this variant
        }
    }
    Some(FlagValue::Boolean(enabled)) => {
        // Handle boolean flag
    }
    None => {
        // Flag not found or disabled
    }
}

let (flags, _) = client.get_feature_flags(
    "distinct_id_of_your_user".to_string(),
    None, None, None
).await.unwrap();
let mut event = Event::new("event_name", "distinct_id_of_your_user");
for (key, value) in flags {
    let prop_key = format!("$feature/{}", key);
    match value {
        FlagValue::Boolean(b) => event.insert_prop(&prop_key, b).unwrap(),
        FlagValue::String(s) => event.insert_prop(&prop_key, s).unwrap(),
    };
}
client.capture(event).unwrap();

let (flags, payloads) = client.get_feature_flags(
    "distinct_id_of_your_user".to_string(),
    None, // groups
    None, // person_properties
    None, // group_properties
).await.unwrap();
for (key, value) in flags {
    println!("Flag {}: {:?}", key, value);
}

let payload = client.get_feature_flag_payload(
    "flag-key".to_string(),
    "distinct_id_of_your_user".to_string()
).await.unwrap();
if let Some(data) = payload {
    println!("Payload: {}", data);
}

use std::collections::HashMap;
use serde_json::json;
let mut person_props = HashMap::new();
person_props.insert("plan".to_string(), json!("enterprise"));
person_props.insert("country".to_string(), json!("US"));
let flag = client.get_feature_flag(
    "premium-feature".to_string(),
    "distinct_id_of_your_user".to_string(),
    None, // groups
    Some(person_props),
    None, // group_properties
).await.unwrap();

use std::collections::HashMap;
use serde_json::json;
let mut groups = HashMap::new();
groups.insert("company".to_string(), "company-123".to_string());
let mut group_props = HashMap::new();
let mut company_props = HashMap::new();
company_props.insert("size".to_string(), json!(500));
group_props.insert("company".to_string(), company_props);
let flag = client.get_feature_flag(
    "b2b-feature".to_string(),
    "distinct_id_of_your_user".to_string(),
    Some(groups),
    None, // person_properties
    Some(group_props),
).await.unwrap();

match client.get_feature_flag(
    "flag-key".to_string(),
    "distinct_id_of_your_user".to_string(),
    None, None, None
).await {
    Ok(Some(value)) => {
        // Use the flag value
        println!("Flag value: {:?}", value);
    }
    Ok(None) => {
        // Flag not found or disabled
        println!("Flag not found");
    }
    Err(e) => {
        // Handle the error appropriately
        eprintln!("Error fetching feature flag: {}", e);
        // Fall back to default behavior
    }
}

iex> PostHog.FeatureFlags.check("example-feature-flag-3", "user123")
{:error, %PostHog.UnexpectedResponseError{message: "Feature flag example-feature-flag-3 was not found in the response", response: ...}}

var flag = await posthog.GetFeatureFlagAsync(
    "flag-key",
    "distinct_id_of_your_user"
);
// replace "variant-key" with the key of your variant
if (flag is { VariantKey: "variant-key"} ) {
    // Do something differently for this user
    // Optional: fetch the payload
    var matchedPayload = flag.Payload;
}

posthog.Capture(
    "distinct_id_of_your_user",
    "event_name",
    properties: new() {
        // replace feature-flag-key with your flag key.
        // Replace "variant-key" with the key of your variant
        ["$feature/feature-flag-key"] = "variant-key"
    }
);

var isMyFlagEnabled = await posthog.IsFeatureEnabledAsync(
    "flag-key",
    "distinct_id_of_your_user",
    options: new FeatureFlagOptions
    {
        SendFeatureFlagEvents = true
    }
);
// will not send `$feature_flag_called` events

// Overriding Person Properties
var personFlag = await posthog.GetFeatureFlagAsync(
    "flag-key",
    "distinct_id_of_the_user",
    personProperties: new() {["property_name"] = "value"});
// Overriding Group Properties
var groupFlag = await posthog.GetFeatureFlagAsync(
    "flag-key",
    "distinct_id_of_the_user",
    options: new FeatureFlagOptions
    {
        Groups = [
            new Group("your_group_type", "your_group_id")
            {
                ["group_property_name"] = "your group value"
            },
            new Group(
                "another_group_type",
                "another_group_id")
                {
                    ["group_property_name"] = "another group value"
                }
        ]
    });
// Overriding both Person and Group Properties
var bothFlag = await posthog.GetFeatureFlagAsync(
    "flag-key",
    "distinct_id_of_the_user",
    options: new FeatureFlagOptions
    {
        PersonProperties = new() { ["property_name"] = "value" },
        Groups = [
            new Group("your_group_type", "your_group_id")
            {
                ["group_property_name"] = "your group value"
            },
            new Group(
                "another_group_type",
                "another_group_id")
                {
                    ["group_property_name"] = "another group value"
                }
        ]
    });

# Basic request (flags only)
curl -v -L --header "Content-Type: application/json" -d '  {
    "api_key": "<ph_project_token>",
    "distinct_id": "distinct_id_of_your_user",
    "groups" : {
        "group_type": "group_id"
    }
}' "https://us.i.posthog.com/flags?v=2"
# With configuration (flags + PostHog config)
curl -v -L --header "Content-Type: application/json" -d '  {
    "api_key": "<ph_project_token>",
    "distinct_id": "distinct_id_of_your_user",
    "groups" : {
        "group_type": "group_id"
    }
}' "https://us.i.posthog.com/flags?v=2&config=true"

import requests
import json
# Basic request (flags only)
url = "https://us.i.posthog.com/flags?v=2"
headers = {
    "Content-Type": "application/json"
}
payload = {
    "api_key": "<ph_project_token>",
    "distinct_id": "user distinct id",
    "groups": {
        "group_type": "group_id"
    }
}
response = requests.post(url, headers=headers, data=json.dumps(payload))
print(response.json())
# With configuration (flags + PostHog config)
url_with_config = "https://us.i.posthog.com/flags?v=2&config=true"
response_with_config = requests.post(url_with_config, headers=headers, data=json.dumps(payload))
print(response_with_config.json())

import fetch from "node-fetch";
async function sendFlagsRequest() {
    const headers = {
        "Content-Type": "application/json",
    };
    const payload = {
        api_key: "<ph_project_token>",
        distinct_id: "user distinct id",
        groups: {
            group_type: "group_id",
        },
    };
    // Basic request (flags only)
    const url = "https://us.i.posthog.com/flags?v=2";
    const response = await fetch(url, {
        method: "POST",
        headers: headers,
        body: JSON.stringify(payload),
    });
    const data = await response.json();
    console.log(data);
    // With configuration (flags + PostHog config)
    const urlWithConfig = "https://us.i.posthog.com/flags?v=2&config=true";
    const responseWithConfig = await fetch(urlWithConfig, {
        method: "POST",
        headers: headers,
        body: JSON.stringify(payload),
    });
    const dataWithConfig = await responseWithConfig.json();
    console.log(dataWithConfig);
}
sendFlagsRequest();

curl -v -L --header "Content-Type: application/json" -d '  {
    "api_key": "<ph_project_token>",
    "distinct_id": "distinct_id_of_your_user",
    "evaluation_contexts": ["production", "web"]
}' "https://us.i.posthog.com/flags?v=2"

import requests
import json
url = "https://us.i.posthog.com/flags?v=2"
headers = {
    "Content-Type": "application/json"
}
payload = {
    "api_key": "<ph_project_token>",
    "distinct_id": "user distinct id",
    "evaluation_contexts": ["production", "web"]
}
response = requests.post(url, headers=headers, data=json.dumps(payload))
print(response.json())

const response = await fetch("https://us.i.posthog.com/flags?v=2", {
    method: "POST",
    headers: {
        "Content-Type": "application/json",
    },
    body: JSON.stringify({
        api_key: "<ph_project_token>",
        distinct_id: "user-distinct-id",
        evaluation_contexts: ["production", "web"]
    }),
});
const data = await response.json();

// Browser fetch - Detected as CLIENT runtime
// Will receive: client-only flags + "all" flags
// Won't receive: server-only flags
const response = await fetch("https://us.i.posthog.com/flags?v=2", {
    method: "POST",
    headers: {
        "Content-Type": "application/json",
        // Browser automatically adds Origin, Referer, Sec-Fetch-* headers
    },
    body: JSON.stringify({
        api_key: "<ph_project_token>",
        distinct_id: "user-id"
    })
});

# Python requests - Detected as SERVER runtime
# Will receive: server-only flags + "all" flags
# Won't receive: client-only flags
import requests
response = requests.post(
    "https://us.i.posthog.com/flags?v=2",
    json={
        "api_key": "<ph_project_token>",
        "distinct_id": "user-id"
    }
    # python-requests/ in User-Agent indicates server-side
)

# curl - Detected as SERVER runtime
# Will receive: server-only flags + "all" flags
# Won't receive: client-only flags
curl -v -L --header "Content-Type: application/json" -d '{
    "api_key": "<ph_project_token>",
    "distinct_id": "user-id"
}' "https://us.i.posthog.com/flags?v=2"
# curl/ in User-Agent indicates server-side

// Node.js with custom User-Agent - Control runtime detection
const response = await fetch("https://us.i.posthog.com/flags?v=2", {
    method: "POST",
    headers: {
        "Content-Type": "application/json",
        "User-Agent": "posthog-node/3.0.0"  // Explicitly indicates server-side
    },
    body: JSON.stringify({
        api_key: "<ph_project_token>",
        distinct_id: "user-id"
    })
});

// Example: Production web client
const response = await fetch("https://us.i.posthog.com/flags?v=2", {
    method: "POST",
    headers: {
        "Content-Type": "application/json",
        // Browser headers will trigger client runtime detection
    },
    body: JSON.stringify({
        api_key: "<ph_project_token>",
        distinct_id: "user-id",
        evaluation_contexts: ["production", "web"]
    })
});
// This request will only receive flags that:
// 1. Have runtime set to "client" OR "all" (due to browser headers)
// AND
// 2. Have evaluation context tags matching "production" OR "web" (or no tags)
// Note: You can also use the legacy "evaluation_environments" parameter

{
  "flags": {
    "my-awesome-flag": {
      "key": "my-awesome-flag",
      "enabled": true,
      "reason": {
        "code": "condition_match",
        "condition_index": 0,
        "description": "Condition set 1 matched"
      },
      "metadata": {
        "id": 1,
        "version": 1,
        "payload": "{\"example\": \"json\", \"payload\": \"value\"}"
      }
    },
    "my-multivariate-flag" :{
      "key":"my-multivariate-flag",
      "enabled": true,
      "variant": "some-string-value",
      "reason": {
        "code": "condition_match",
        "condition_index": 1,
        "description": "Condition set 2 matched"
      },
      "metadata": {
        "id": 2,
        "version": 42,
      }
    },
    "flag-thats-not-on": {
      "key": "flag-thats-not-on",
      "enabled": false,
      "reason": {
        "code": "no_condition_match",
        "condition_index": 0,
        "description": "No condition sets matched"
      },
      "metadata": {
        "id": 3,
        "version": 1
      }
    }
  },
  "errorsWhileComputingFlags": false,
  "requestId": "550e8400-e29b-41d4-a716-446655440000"
}

{
  "config": {
    "enable_collect_everything": true
  },
  "toolbarParams": {},
  "errorsWhileComputingFlags": false,
  "isAuthenticated": false,
  "requestId": "550e8400-e29b-41d4-a716-446655440000",
  "supportedCompression": [
    "gzip",
    "lz64"
  ],
  "flags": {
    "my-awesome-flag": {
      "key": "my-awesome-flag",
      "enabled": true,
      "reason": {
        "code": "condition_match",
        "condition_index": 0,
        "description": "Condition set 1 matched"
      },
      "metadata": {
        "id": 1,
        "version": 1,
        "payload": "{\"example\": \"json\", \"payload\": \"value\"}"
      }
    },
    "my-multivariate-flag" :{
      "key":"my-multivariate-flag",
      "enabled": true,
      "variant": "some-string-value",
      "reason": {
        "code": "condition_match",
        "condition_index": 1,
        "description": "Condition set 2 matched"
      },
      "metadata": {
        "id": 2,
        "version": 42,
      }
    },
    "flag-thats-not-on": {
      "key": "flag-thats-not-on",
      "enabled": false,
      "reason": {
        "code": "no_condition_match",
        "condition_index": 0,
        "description": "No condition sets matched"
      },
      "metadata": {
        "id": 3,
        "version": 1
      }
    }
  }
}

{
  "config": {
    "enable_collect_everything": true
  },
  "toolbarParams": {},
  "isAuthenticated": false,
  "supportedCompression": [
    "gzip",
    "lz64"
  ],
  "flags": {},
  "errorsWhileComputingFlags": false,
  "quotaLimited": ["feature_flags"],
  "requestId": "d4d89b14-9619-4627-adf2-01b761691c2e"
  // ... other fields, not relevant to feature flags
}

curl -v -L --header "Content-Type: application/json" -d '  {
    "api_key": "<ph_project_token>",
    "event": "your_event_name",
    "distinct_id": "distinct_id_of_your_user",
    "properties": {
      "$feature/feature-flag-key": "variant-key" # Replace feature-flag-key with your flag key. Replace 'variant-key' with the key of your variant
    }
}' https://us.i.posthog.com/i/v0/e/

import requests
import json
url = "https://us.i.posthog.com/i/v0/e/"
headers = {
    "Content-Type": "application/json"
}
payload = {
    "api_key": "<ph_project_token>",
    "event": "your_event_name",
    "distinct_id": "distinct_id_of_your_user,
    "properties": {
      "$feature/feature-flag-key": "variant-key" # Replace feature-flag-key with your flag key. Replace 'variant-key' with the key of your variant
    }
}
response = requests.post(url, headers=headers, data=json.dumps(payload))
print(response)

curl -v -L --header "Content-Type: application/json" -d '  {
    "api_key": "<ph_project_token>",
    "event": "$feature_flag_called",
    "distinct_id": "distinct_id_of_your_user",
    "properties": {
      "$feature_flag": "feature-flag-key",
      "$feature_flag_response": "variant-name"
    }
}' https://us.i.posthog.com/i/v0/e/

import requests
import json
url = "https://us.i.posthog.com/i/v0/e/"
headers = {
    "Content-Type": "application/json"
}
payload = {
    "api_key": "<ph_project_token>",
    "event": "feature_flag_called",
    "distinct_id": "distinct_id_of_your_user,
    "properties": {
      "$feature_flag": "feature-flag-key",
      "$feature_flag_response": "variant-name"
    }
}
response = requests.post(url, headers=headers, data=json.dumps(payload))
print(response)

curl -v -L --header "Content-Type: application/json" -d '  {
    "api_key": "<ph_project_token>",
    "distinct_id": "distinct_id_of_your_user",
    "groups" : { # Required only for group-based feature flags
      "group_type": "group_id" # Replace "group_type" with the name of your group type. Replace "group_id" with the id of your group.
    },
    "person_properties": {"<personProp1>": "<personVal1>"}, # Optional. Include any properties used to calculate the value of the feature flag.
    "group_properties": {"group type": {"<groupProp1>":"<groupVal1>"}} # Optional. Include any properties used to calculate the value of the feature flag.
}' https://us.i.posthog.com/flags?v=2

import requests
import json
url = "https://us.i.posthog.com/flags?v=2"
headers = {
    "Content-Type": "application/json"
}
payload = {
    "api_key": "<ph_project_token>",
    "distinct_id": "distinct_id_of_your_user",
    "groups" : { # Required only for group-based feature flags
      "group_type": "group_id" # Replace "group_type" with the name of your group type. Replace "group_id" with the id of your group.
    },
    "person_properties": {"<personProp1>": "<personVal1>"}, # Optional. Include any properties used to calculate the value of the feature flag.
    "group_properties": {"group type": {"<groupProp1>":"<groupVal1>"}} # Optional. Include any properties used to calculate the value of the feature flag.
}
response = requests.post(url, headers=headers, data=json.dumps(payload))
print(response.json())

curl -v -L \
--header "Content-Type: application/json" \
--header "HTTP_X_FORWARDED_FOR: the_client_ip_address_to_use " \
-d '  {
    "api_key": "<ph_project_token>",
    "distinct_id": "distinct_id_of_your_user"
}' https://us.i.posthog.com/flags?v=2

import requests
import json
url = "https://us.i.posthog.com/flags?v=2"
headers = {
    "Content-Type": "application/json",
    "HTTP_X_FORWARDED_FOR": "the_client_ip_address_to_use"
}
payload = {
    "api_key": "<ph_project_token>",
    "distinct_id": "distinct_id_of_your_user"
}
response = requests.post(url, headers=headers, data=json.dumps(payload))
print(response.json())