Adding feature flag code

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

Boolean feature flags

Web
if (posthog.isFeatureEnabled('flag-key') ) {
    // Do something differently for this user

    // Optional: fetch the payload
    const matchedFlagPayload = posthog.getFeatureFlagPayload('flag-key')
}

Multivariate feature flags

Web
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')
}

Ensuring flags are loaded before usage

Every time a user loads a page, we send a request in the background to fetch the feature flags that apply to that user. We store those flags in your chosen persistence option (local storage by default).

This means that for most pages, the feature flags are available immediately — except for the first time a user visits.

To handle this, you can use the onFeatureFlags callback to wait for the feature flag request to finish:

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

Callback parameters

The onFeatureFlags callback receives the following parameters:

flags: string[]: An object containing the feature flags that apply to the user.
flagVariants: Record<string, string | boolean>: An object containing the variants that apply to the user.
{ errorsLoading }: { errorsLoading?: boolean }: An object containing a boolean indicating if an error occurred during the request to load the feature flags. This is true if the request timed out or if there was an error. It will be false or undefined if the request was successful.

You won't usually need to use these, but they are useful if you want to be extra careful about feature flags not being loaded yet because of a network error and/or a network timeout (see feature_flag_request_timeout_ms).

Reloading feature flags

Feature flag values are cached. If something has changed with your user and you'd like to refetch their flag values, call:

Web
posthog.reloadFeatureFlags()

Overriding server properties

Sometimes, you might want to evaluate feature flags using properties that haven't been ingested yet, or were set incorrectly earlier. You can do so by setting properties the flag depends on with these calls:

Web
posthog.setPersonPropertiesForFlags({'property1': 'value', property2: 'value2'})

Note: These are set for the entire session. Successive calls are additive: all properties you set are combined together and sent for flag evaluation.

Whenever you set these properties, we also trigger a reload of feature flags to ensure we have the latest values. You can disable this by passing in the optional parameter for reloading:

Web
posthog.setPersonPropertiesForFlags({'property1': 'value', property2: 'value2'}, false)

At any point, you can reset these properties by calling resetPersonPropertiesForFlags:

Web
posthog.resetPersonPropertiesForFlags()

The same holds for group properties:

Web
// 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()

Note: You don't need to add the group names here, since these properties are automatically attached to the current group (set via posthog.group()). When you change the group, these properties are reset.

Automatic overrides

Whenever you call posthog.identify with person properties, we automatically add these properties to flag evaluation calls to help determine the correct flag values. The same is true for when you call posthog.group().

Default overridden properties

By default, we always override some properties based on the user IP address.

The list of properties that this overrides:

$geoip_city_name
$geoip_country_name
$geoip_country_code
$geoip_continent_name
$geoip_continent_code
$geoip_postal_code
$geoip_time_zone

This enables any geolocation-based flags to work without manually setting these properties.

Request timeout

You can configure the feature_flag_request_timeout_ms parameter when initializing your PostHog client to set a flag request timeout. This helps prevent your code from being blocked in the case when PostHog's servers are too slow to respond. By default, this is set at 3 seconds.

JavaScript
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).
})

Feature flag error handling

When using the PostHog SDK, it's important to handle potential errors that may occur during feature flag operations. Here's an example of how to wrap PostHog SDK methods in an error handler:

JavaScript
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
}

There are two ways to implement feature flags in React:

Using hooks.
Using the <PostHogFeature> component.

Method 1: Using hooks

PostHog provides several hooks to make it easy to use feature flags in your React app.

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`.

Example 1: Using a boolean feature flag

React
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;

Example 2: Using a multivariate feature flag

React
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;

Example 3: Using a flag payload

Payload hook

The useFeatureFlagPayload hook does not send a $feature_flag_called event, which is required for the experiment to be tracked. To ensure the exposure event is sent, you should always use the useFeatureFlagPayload hook with either the useFeatureFlagEnabled or useFeatureFlagVariantKey hook.

React
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>
                }
        </>
    )
}

Method 2: Using the PostHogFeature component

The PostHogFeature component simplifies code by handling feature flag related logic.

It also automatically captures metrics, like how many times a user interacts with this feature.

Note: You still need the PostHogProvider at the top level for this to work.

Here is an example:

React
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>
    )
}

The match on the component can be either true, or the variant key, to match on a specific variant.
If you also want to show a default message, you can pass these in the fallback attribute.

If you wish to customise logic around when the component is considered visible, you can pass in visibilityObserverOptions to the feature. These take the same options as the IntersectionObserver API. By default, we use a threshold of 0.1.

Payloads

If your flag has a payload, you can pass a function to children whose first argument is the payload. For example:

React
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>
    )
}

Request timeout

JavaScript
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).
}
)

Error handling

When using the PostHog SDK, it's important to handle potential errors that may occur during feature flag operations. Here's an example of how to wrap PostHog SDK methods in an error handler:

JavaScript
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
}

There are two steps to implement feature flags in Node:

Step 1: Evaluate flags once

Call client.evaluateFlags() once for the user, then read values from the returned snapshot.

Boolean feature flags

Node.js
const flags = await client.evaluateFlags('distinct_id_of_your_user')

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

Multivariate feature flags

Node.js
const flags = await client.evaluateFlags('distinct_id_of_your_user')
const enabledVariant = flags.getFlag('flag-key')

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 = flags.getFlagPayload('flag-key')
}

flags.getFlag() returns the variant string for multivariate flags, true for enabled boolean flags, false for disabled flags, and undefined when the flag wasn't returned by the evaluation.

Note: client.isFeatureEnabled(), client.getFeatureFlag(), client.getFeatureFlagPayload(), and capture({ sendFeatureFlags: true }) still work during the migration period, but they're deprecated. Prefer evaluateFlags() for new code.

Step 2: Include feature flag information when capturing events

If you want use your feature flag to breakdown or filter events in your insights, you'll need to include feature flag information in those events. This ensures that the feature flag value is attributed correctly to the event.

Note: This step is only required for events captured using our server-side SDKs or API.

There are two methods you can use to include feature flag information in your events:

Method 1: Pass the evaluated flags snapshot to `capture()`

Pass the same flags object that you used for branching. This attaches the exact flag values from that evaluation and doesn't make another /flags request.

Node.js
const flags = await client.evaluateFlags('distinct_id_of_your_user')

if (flags.isEnabled('flag-key')) {
    // Do something differently for this user
}

client.capture({
    distinctId: 'distinct_id_of_your_user',
    event: 'event_name',
    flags,
})

By default, this attaches every flag in the snapshot using $feature/<flag-key> properties and $active_feature_flags.

To reduce event property bloat, pass a filtered snapshot:

Node.js
// Attach only flags accessed with isEnabled() or getFlag() before this call
client.capture({
    distinctId: 'distinct_id_of_your_user',
    event: 'event_name',
    flags: flags.onlyAccessed(),
})

// Attach only specific flags
client.capture({
    distinctId: 'distinct_id_of_your_user',
    event: 'event_name',
    flags: flags.only(['checkout-flow', 'new-dashboard']),
})

onlyAccessed() is order-dependent. If you call it before accessing any flags with isEnabled() or getFlag(), no feature flag properties are attached.

Method 2: Include the `$feature/feature_flag_name` property manually

In the event properties, include $feature/feature_flag_name: variant_key:

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

Evaluating only specific flags

By default, evaluateFlags() evaluates every flag for the user. If you only need a few flags, pass flagKeys to request only those flags:

Node.js
const flags = await client.evaluateFlags('distinct_id_of_your_user', {
    flagKeys: ['checkout-flow', 'new-dashboard'],
})

Sending `$feature_flag_called` events

Capturing $feature_flag_called events enables PostHog to know when a flag was accessed by a user and provide analytics and insights on the flag. With evaluateFlags(), the SDK sends this event when you call flags.isEnabled() or flags.getFlag() for a flag.

The SDK deduplicates these events per (distinct_id, flag, value) in a local cache. If you reinitialize the PostHog client, the cache resets and $feature_flag_called events may be sent again. PostHog handles duplicates, so duplicate $feature_flag_called events don't affect your analytics.

flags.getFlagPayload() doesn't send $feature_flag_called events and doesn't count as an access for onlyAccessed().

Advanced: Overriding server properties

Sometimes, you may want to evaluate feature flags using person properties, groups, or group properties that haven't been ingested yet, or were set incorrectly earlier.

You can provide properties to evaluate the flag with by using the person properties, groups, and group properties arguments. PostHog will then use these values to evaluate the flag, instead of any properties currently stored on your PostHog server.

For example:

Node.js
const flags = await client.evaluateFlags('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',
        },
    },
})

if (flags.isEnabled('flag-key')) {
    // Do something differently for this user
}

Overriding GeoIP properties

By default, a user's GeoIP properties are set using the IP address they use to capture events on the frontend. You may want to override the these properties when evaluating feature flags. A common reason to do this is when you're not using PostHog on your frontend, so the user has no GeoIP properties.

You can override GeoIP properties by including them in the person_properties parameter when evaluating feature flags. This is useful when you're evaluating flags on your backend and want to use the client's location instead of your server's location.

The following GeoIP properties can be overridden:

$geoip_country_code
$geoip_country_name
$geoip_city_name
$geoip_city_confidence
$geoip_continent_code
$geoip_continent_name
$geoip_latitude
$geoip_longitude
$geoip_postal_code
$geoip_subdivision_1_code
$geoip_subdivision_1_name
$geoip_subdivision_2_code
$geoip_subdivision_2_name
$geoip_subdivision_3_code
$geoip_subdivision_3_name
$geoip_time_zone

Simply include any of these properties in the person_properties parameter alongside your other person properties when calling feature flags.

Request timeout

You can configure the feature_flag_request_timeout_ms parameter when initializing your PostHog client to set a flag request timeout. This helps prevent your code from being blocked if PostHog's servers are too slow to respond. By default, this is set to 3 seconds.

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

There are two steps to implement feature flags in Python:

Step 1: Evaluate flags once

Call posthog.evaluate_flags() once for the user, then read values from the returned snapshot.

Boolean feature flags

Python
flags = posthog.evaluate_flags("distinct_id_of_your_user")

if flags.is_enabled("flag-key"):
    # Do something differently for this user
    # Optional: fetch the payload
    matched_flag_payload = flags.get_flag_payload("flag-key")

Multivariate feature flags

Python
flags = posthog.evaluate_flags("distinct_id_of_your_user")

enabled_variant = flags.get_flag("flag-key")

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 = flags.get_flag_payload("flag-key")

flags.get_flag() returns the variant string for multivariate flags, True for enabled boolean flags, False for disabled flags, and None when the flag wasn't returned by the evaluation.

Note: posthog.feature_enabled(), posthog.get_feature_flag(), posthog.get_feature_flag_payload(), and posthog.capture(send_feature_flags=True) still work during the migration period, but they're deprecated. Prefer posthog.evaluate_flags() for new code.

Step 2: Include feature flag information when capturing events

Note: This step is only required for events captured using our server-side SDKs or API.

There are two methods you can use to include feature flag information in your events:

Method 1: Pass the evaluated flags snapshot to `capture()`

Pass the same flags object that you used for branching. This attaches the exact flag values from that evaluation and doesn't make another /flags request.

Python
flags = posthog.evaluate_flags("distinct_id_of_your_user")

if flags.is_enabled("flag-key"):
    # Do something differently for this user
    pass

posthog.capture(
    "event_name",
    distinct_id="distinct_id_of_your_user",
    flags=flags,
)

By default, this attaches every flag in the snapshot using $feature/<flag-key> properties and $active_feature_flags.

To reduce event property bloat, pass a filtered snapshot:

Python
# Attach only flags accessed with is_enabled() or get_flag() before this call
posthog.capture(
    "event_name",
    distinct_id="distinct_id_of_your_user",
    flags=flags.only_accessed(),
)

# Attach only specific flags
posthog.capture(
    "event_name",
    distinct_id="distinct_id_of_your_user",
    flags=flags.only(["checkout-flow", "new-dashboard"]),
)

only_accessed() is order-dependent. If you call it before accessing any flags with is_enabled() or get_flag(), no feature flag properties are attached.

Method 2: Include the `$feature/feature_flag_name` property manually

In the event properties, include $feature/feature_flag_name: variant_key:

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

Evaluating only specific flags

By default, posthog.evaluate_flags() evaluates every flag for the user. If you only need a few flags, pass flag_keys to request only those flags:

Python
flags = posthog.evaluate_flags(
    "distinct_id_of_your_user",
    flag_keys=["checkout-flow", "new-dashboard"],
)

Sending `$feature_flag_called` events

Capturing $feature_flag_called events enables PostHog to know when a flag was accessed by a user and provide analytics and insights on the flag. With posthog.evaluate_flags(), the SDK sends this event when you call flags.is_enabled() or flags.get_flag() for a flag.

flags.get_flag_payload() doesn't send $feature_flag_called events and doesn't count as an access for only_accessed().

Advanced: Overriding server properties

Sometimes, you may want to evaluate feature flags using person properties, groups, or group properties that haven't been ingested yet, or were set incorrectly earlier.

For example:

Python
flags = posthog.evaluate_flags(
    "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"},
    },
)

if flags.is_enabled("flag-key"):
    # Do something differently for this user

Overriding GeoIP properties

The following GeoIP properties can be overridden:

$geoip_country_code
$geoip_country_name
$geoip_city_name
$geoip_city_confidence
$geoip_continent_code
$geoip_continent_name
$geoip_latitude
$geoip_longitude
$geoip_postal_code
$geoip_subdivision_1_code
$geoip_subdivision_1_name
$geoip_subdivision_2_code
$geoip_subdivision_2_name
$geoip_subdivision_3_code
$geoip_subdivision_3_name
$geoip_time_zone

Simply include any of these properties in the person_properties parameter alongside your other person properties when calling feature flags.

Request timeout

You can configure the feature_flags_request_timeout_seconds parameter when initializing your PostHog client to set a flag request timeout. This helps prevent your code from being blocked if PostHog's servers are too slow to respond. By default, this is set to 3 seconds.

Python
posthog = Posthog(
    "<ph_project_token>",
    host="https://us.i.posthog.com",
    feature_flags_request_timeout_seconds=3,  # Time in seconds. Defaults to 3.
)

There are two steps to implement feature flags in PHP:

Step 1: Evaluate flags once

Call PostHog::evaluateFlags() once for the user, then read values from the returned snapshot.

Boolean feature flags

PHP
$flags = PostHog::evaluateFlags('distinct_id_of_your_user');

if ($flags->isEnabled('flag-key')) {
    // Do something differently for this user
    // Optional: fetch the payload
    $matchedFlagPayload = $flags->getFlagPayload('flag-key');
}

Multivariate feature flags

PHP
$flags = PostHog::evaluateFlags('distinct_id_of_your_user');

$enabledVariant = $flags->getFlag('flag-key');

if ($enabledVariant === 'variant-key') { // replace 'variant-key' with the key of your variant
    // Do something differently for this user
    // Optional: fetch the payload
    $matchedFlagPayload = $flags->getFlagPayload('flag-key');
}

$flags->getFlag() returns the variant string for multivariate flags, true for enabled boolean flags, false for disabled flags, and null when the flag wasn't returned by the evaluation.

Note: PostHog::isFeatureEnabled(), PostHog::getFeatureFlag(), PostHog::getFeatureFlagPayload(), and capture(['send_feature_flags' => true]) still work during the migration period, but they're deprecated. Prefer evaluateFlags() for new code.

Step 2: Include feature flag information when capturing events

Note: This step is only required for events captured using our server-side SDKs or API.

There are two methods you can use to include feature flag information in your events:

Method 1: Pass the evaluated flags snapshot to `capture()`

Pass the same flags object that you used for branching. This attaches the exact flag values from that evaluation and doesn't make another /flags request.

PHP
$flags = PostHog::evaluateFlags('distinct_id_of_your_user');

if ($flags->isEnabled('flag-key')) {
    // Do something differently for this user
}

PostHog::capture([
    'distinctId' => 'distinct_id_of_your_user',
    'event' => 'event_name',
    'flags' => $flags,
]);

By default, this attaches every flag in the snapshot using $feature/<flag-key> properties and $active_feature_flags.

To reduce event property bloat, pass a filtered snapshot:

PHP
// Attach only flags accessed with isEnabled() or getFlag() before this call
PostHog::capture([
    'distinctId' => 'distinct_id_of_your_user',
    'event' => 'event_name',
    'flags' => $flags->onlyAccessed(),
]);

// Attach only specific flags
PostHog::capture([
    'distinctId' => 'distinct_id_of_your_user',
    'event' => 'event_name',
    'flags' => $flags->only(['checkout-flow', 'new-dashboard']),
]);

onlyAccessed() is order-dependent. If you call it before accessing any flags with isEnabled() or getFlag(), no feature flag properties are attached.

Method 2: Include the `$feature/feature_flag_name` property manually

In the event properties, include $feature/feature_flag_name: variant_key:

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

Evaluating only specific flags

By default, evaluateFlags() evaluates every flag for the user. If you only need a few flags, pass flagKeys to request only those flags:

PHP
$flags = PostHog::evaluateFlags(
    distinctId: 'distinct_id_of_your_user',
    flagKeys: ['checkout-flow', 'new-dashboard'],
);

Sending `$feature_flag_called` events

Capturing $feature_flag_called events enables PostHog to know when a flag was accessed by a user and provide analytics and insights on the flag. With evaluateFlags(), the SDK sends this event when you call $flags->isEnabled() or $flags->getFlag() for a flag.

$flags->getFlagPayload() doesn't send $feature_flag_called events and doesn't count as an access for onlyAccessed().

Advanced: Overriding server properties

Sometimes, you may want to evaluate feature flags using person properties, groups, or group properties that haven't been ingested yet, or were set incorrectly earlier.

For example:

PHP
$flags = PostHog::evaluateFlags(
    distinctId: 'distinct_id_of_the_user',
    groups: [
        'your_group_type' => 'your_group_id',
        'another_group_type' => 'your_group_id',
    ],
    personProperties: ['property_name' => 'value'],
    groupProperties: [
        'your_group_type' => ['group_property_name' => 'value'],
        'another_group_type' => ['group_property_name' => 'value'],
    ],
);

if ($flags->isEnabled('flag-key')) {
    // Do something differently for this user
}

Overriding GeoIP properties

The following GeoIP properties can be overridden:

$geoip_country_code
$geoip_country_name
$geoip_city_name
$geoip_city_confidence
$geoip_continent_code
$geoip_continent_name
$geoip_latitude
$geoip_longitude
$geoip_postal_code
$geoip_subdivision_1_code
$geoip_subdivision_1_name
$geoip_subdivision_2_code
$geoip_subdivision_2_name
$geoip_subdivision_3_code
$geoip_subdivision_3_name
$geoip_time_zone

Simply include any of these properties in the person_properties parameter alongside your other person properties when calling feature flags.

Request timeout

You can configure the feature_flag_request_timeout_ms parameter when initializing your PostHog client to set a flag request timeout. This helps prevent your code from being blocked if PostHog's servers are too slow to respond. By default, this is set to 3 seconds.

PHP
PostHog::init("<ph_project_token>",
    [
        'host' => 'https://us.i.posthog.com',
        'feature_flag_request_timeout_ms' => 3000, // Time in milliseconds. Defaults to 3000 (3 seconds).
    ]
);

There are two steps to implement feature flags in Ruby:

Step 1: Evaluate flags once

Call posthog.evaluate_flags() once for the user, then read values from the returned snapshot.

Boolean feature flags

Ruby
flags = posthog.evaluate_flags('distinct_id_of_your_user')

if flags.enabled?('flag-key')
    # Do something differently for this user
    # Optional: fetch the payload
    matched_flag_payload = flags.get_flag_payload('flag-key')
end

Multivariate feature flags

Ruby
flags = posthog.evaluate_flags('distinct_id_of_your_user')

enabled_variant = flags.get_flag('flag-key')

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 = flags.get_flag_payload('flag-key')
end

flags.get_flag() returns the variant string for multivariate flags, true for enabled boolean flags, false for disabled flags, and nil when the flag wasn't returned by the evaluation.

Note: posthog.is_feature_enabled(), posthog.get_feature_flag(), posthog.get_feature_flag_payload(), and capture(send_feature_flags: true) still work during the migration period, but they're deprecated. Prefer evaluate_flags() for new code.

Step 2: Include feature flag information when capturing events

Note: This step is only required for events captured using our server-side SDKs or API.

There are two methods you can use to include feature flag information in your events:

Method 1: Pass the evaluated flags snapshot to `capture()`

Pass the same flags object that you used for branching. This attaches the exact flag values from that evaluation and doesn't make another /flags request.

Ruby
flags = posthog.evaluate_flags('distinct_id_of_your_user')

if flags.enabled?('flag-key')
    # Do something differently for this user
end

posthog.capture({
    distinct_id: 'distinct_id_of_your_user',
    event: 'event_name',
    flags: flags,
})

By default, this attaches every flag in the snapshot using $feature/<flag-key> properties and $active_feature_flags.

To reduce event property bloat, pass a filtered snapshot:

Ruby
# Attach only flags accessed with enabled?() or get_flag() before this call
posthog.capture({
    distinct_id: 'distinct_id_of_your_user',
    event: 'event_name',
    flags: flags.only_accessed,
})

# Attach only specific flags
posthog.capture({
    distinct_id: 'distinct_id_of_your_user',
    event: 'event_name',
    flags: flags.only(['checkout-flow', 'new-dashboard']),
})

only_accessed is order-dependent. If you call it before accessing any flags with enabled?() or get_flag(), no feature flag properties are attached.

Method 2: Include the `$feature/feature_flag_name` property manually

In the event properties, include $feature/feature_flag_name: variant_key:

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

Evaluating only specific flags

By default, evaluate_flags() evaluates every flag for the user. If you only need a few flags, pass flag_keys to request only those flags:

Ruby
flags = posthog.evaluate_flags(
    'distinct_id_of_your_user',
    flag_keys: ['checkout-flow', 'new-dashboard'],
)

Sending `$feature_flag_called` events

Capturing $feature_flag_called events enables PostHog to know when a flag was accessed by a user and provide analytics and insights on the flag. With evaluate_flags(), the SDK sends this event when you call flags.enabled?() or flags.get_flag() for a flag.

flags.get_flag_payload() doesn't send $feature_flag_called events and doesn't count as an access for only_accessed.

Advanced: Overriding server properties

Sometimes, you may want to evaluate feature flags using person properties, groups, or group properties that haven't been ingested yet, or were set incorrectly earlier.

For example:

Ruby
flags = posthog.evaluate_flags(
    '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'
        },
    },
)

if flags.enabled?('flag-key')
    # Do something differently for this user
end

Overriding GeoIP properties

The following GeoIP properties can be overridden:

$geoip_country_code
$geoip_country_name
$geoip_city_name
$geoip_city_confidence
$geoip_continent_code
$geoip_continent_name
$geoip_latitude
$geoip_longitude
$geoip_postal_code
$geoip_subdivision_1_code
$geoip_subdivision_1_name
$geoip_subdivision_2_code
$geoip_subdivision_2_name
$geoip_subdivision_3_code
$geoip_subdivision_3_name
$geoip_time_zone

Simply include any of these properties in the person_properties parameter alongside your other person properties when calling feature flags.

Request timeout

You can configure the feature_flag_request_timeout_seconds parameter when initializing your PostHog client to set a flag request timeout. This helps prevent your code from being blocked if PostHog's servers are too slow to respond. By default, this is set to 3 seconds.

Ruby
posthog = PostHog::Client.new({
    # rest of your configuration...
    feature_flag_request_timeout_seconds: 3 # Time in seconds. Defaults to 3.
})

There are two steps to implement feature flags in Go:

Step 1: Evaluate flags once

Call client.EvaluateFlags() once for the user, then read values from the returned snapshot.

Boolean feature flags

Go
flags, err := client.EvaluateFlags(posthog.EvaluateFlagsPayload{
    DistinctId: "distinct_id_of_your_user",
})
if err != nil {
    // Handle error (e.g. capture error and fallback to default behavior)
}

if flags.IsEnabled("flag-key") {
    // Do something differently for this user
    // Optional: fetch the payload
    matchedFlagPayload := flags.GetFlagPayload("flag-key")
}

Multivariate feature flags

Go
flags, err := client.EvaluateFlags(posthog.EvaluateFlagsPayload{
    DistinctId: "distinct_id_of_your_user",
})
if err != nil {
    // Handle error (e.g. capture error and fallback to default behavior)
}

enabledVariant := flags.GetFlag("flag-key")

if enabledVariant == "variant-key" { // replace "variant-key" with the key of your variant
    // Do something differently for this user
    // Optional: fetch the payload
    matchedFlagPayload := flags.GetFlagPayload("flag-key")
}

flags.GetFlag() returns the variant string for multivariate flags, true for enabled boolean flags, false for disabled flags, and nil when the flag wasn't returned by the evaluation.

Note: client.IsFeatureEnabled(), client.GetFeatureFlag(), client.GetFeatureFlagPayload(), and Capture.SendFeatureFlags still work during the migration period, but they're deprecated. Prefer EvaluateFlags() for new code.

Step 2: Include feature flag information when capturing events

Note: This step is only required for events captured using our server-side SDKs or API.

There are two methods you can use to include feature flag information in your events:

Method 1: Pass the evaluated flags snapshot to `Capture`

Pass the same flags object that you used for branching. This attaches the exact flag values from that evaluation and doesn't make another /flags request.

Go
flags, err := client.EvaluateFlags(posthog.EvaluateFlagsPayload{
    DistinctId: "distinct_id_of_your_user",
})
if err != nil {
    // Handle error
}

if flags.IsEnabled("flag-key") {
    // Do something differently for this user
}

client.Enqueue(posthog.Capture{
    DistinctId: "distinct_id_of_your_user",
    Event:      "event_name",
    Flags:      flags,
})

By default, this attaches every flag in the snapshot using $feature/<flag-key> properties and $active_feature_flags.

To reduce event property bloat, pass a filtered snapshot:

Go
// Attach only flags accessed with IsEnabled() or GetFlag() before this call
client.Enqueue(posthog.Capture{
    DistinctId: "distinct_id_of_your_user",
    Event:      "event_name",
    Flags:      flags.OnlyAccessed(),
})

// Attach only specific flags
client.Enqueue(posthog.Capture{
    DistinctId: "distinct_id_of_your_user",
    Event:      "event_name",
    Flags:      flags.Only([]string{"checkout-flow", "new-dashboard"}),
})

OnlyAccessed() is order-dependent. If you call it before accessing any flags with IsEnabled() or GetFlag(), no feature flag properties are attached.

Method 2: Include the `$feature/feature_flag_name` property manually

In the event properties, include $feature/feature_flag_name: variant_key:

Go
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
})

Evaluating only specific flags

By default, EvaluateFlags() evaluates every flag for the user. If you only need a few flags, pass FlagKeys to request only those flags:

Go
flags, err := client.EvaluateFlags(posthog.EvaluateFlagsPayload{
    DistinctId: "distinct_id_of_your_user",
    FlagKeys:   []string{"checkout-flow", "new-dashboard"},
})

Sending `$feature_flag_called` events

Capturing $feature_flag_called events enables PostHog to know when a flag was accessed by a user and provide analytics and insights on the flag. With EvaluateFlags(), the SDK sends this event when you call flags.IsEnabled() or flags.GetFlag() for a flag.

flags.GetFlagPayload() doesn't send $feature_flag_called events and doesn't count as an access for OnlyAccessed().

Advanced: Overriding server properties

Sometimes, you may want to evaluate feature flags using person properties, groups, or group properties that haven't been ingested yet, or were set incorrectly earlier.

For example:

Go
flags, err := client.EvaluateFlags(posthog.EvaluateFlagsPayload{
    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]posthog.Properties{
        "your_group_type": posthog.NewProperties().
            Set("group_property_name", "value"),
        "another_group_type": posthog.NewProperties().
            Set("group_property_name", "value"),
    },
})
if err != nil {
    // Handle error
}

if flags.IsEnabled("flag-key") {
    // Do something differently for this user
}

Overriding GeoIP properties

The following GeoIP properties can be overridden:

$geoip_country_code
$geoip_country_name
$geoip_city_name
$geoip_city_confidence
$geoip_continent_code
$geoip_continent_name
$geoip_latitude
$geoip_longitude
$geoip_postal_code
$geoip_subdivision_1_code
$geoip_subdivision_1_name
$geoip_subdivision_2_code
$geoip_subdivision_2_name
$geoip_subdivision_3_code
$geoip_subdivision_3_name
$geoip_time_zone

Simply include any of these properties in the person_properties parameter alongside your other person properties when calling feature flags.

Request timeout

You can configure the FeatureFlagRequestTimeout parameter when initializing your PostHog client to set a flag request timeout. This helps prevent your code from being blocked if PostHog's servers are too slow to respond. By default, this is set to 3 seconds.

Go
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. Defaults to 3.
    },
)

There are two ways to implement feature flags in React Native:

Using hooks.
Loading the flag directly.

Method 1: Using hooks

Example 1: Boolean feature flags

React Native
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>
}

Example 2: Multivariate feature flags

React Native
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/>
}

Method 2: Loading the flag directly

React Native
// 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')

Ensuring flags are loaded before usage

Every time a user opens the app, we send a request in the background to fetch the feature flags that apply to that user. We store those flags in the storage.

This means that for most screens, the feature flags are available immediately — except for the first time a user visits.

To handle this, you can use the onFeatureFlags callback to wait for the feature flag request to finish:

React Native
posthog.onFeatureFlags((flags) => {
  // feature flags are guaranteed to be available at this point
  if (posthog.isFeatureEnabled('flag-key')) {
    // do something
  }
})

Reloading flags

PostHog loads feature flags when instantiated and refreshes whenever methods are called that affect the flag.

If want to manually trigger a refresh, you can call reloadFeatureFlagsAsync():

React Native
posthog.reloadFeatureFlagsAsync().then((refreshedFlags) => console.log(refreshedFlags))

Or when you want to trigger the reload, but don't care about the result:

React Native
posthog.reloadFeatureFlags()

Feature flag caching

The React Native SDK caches feature flag values in AsyncStorage. Cached values persist indefinitely with no TTL until updated by a successful API call. This enables offline support and reduces latency, but means inactive users may see stale flag values from their last session.

For example, if a user last opened your app when a flag was false, that value remains cached even after you roll it out to 100%. When they reopen the app, the SDK returns the cached false first, then fetches the fresh true value from the API.

To ensure fresh flag values:

React Native
// Force refresh on app start
await posthog.reloadFeatureFlagsAsync()

Or clear cached values for inactive users:

React Native
if (lastActiveDate < migrationDate) {
  posthog.reset() // Clears all cached data
}

Request timeout

You can configure the featureFlagsRequestTimeoutMs parameter when initializing your PostHog client to set a flag request timeout. This helps prevent your code from being blocked in the case when PostHog's servers are too slow to respond. By default, this is set at 10 seconds.

React Native
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).
})

Error handling

When using the PostHog SDK, it's important to handle potential errors that may occur during feature flag operations. Here's an example of how to wrap PostHog SDK methods in an error handler:

React Native
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
}

Overriding server properties

React Native
posthog.setPersonPropertiesForFlags({'property1': 'value', property2: 'value2'})

Note that these are set for the entire session. Successive calls are additive: all properties you set are combined together and sent for flag evaluation.

Whenever you set these properties, we also trigger a reload of feature flags to ensure we have the latest values. You can disable this by passing in the optional parameter for reloading:

React Native
posthog.setPersonPropertiesForFlags({'property1': 'value', property2: 'value2'}, false)

At any point, you can reset these properties by calling resetPersonPropertiesForFlags:

React Native
posthog.resetPersonPropertiesForFlags()

The same holds for group properties:

React Native
// set properties for a group
posthog.setGroupPropertiesForFlags({'company': {'property1': 'value', property2: 'value2'}})

// reset properties for all groups:
posthog.resetGroupPropertiesForFlags()

Note: You don't need to add the group names here, since these properties are automatically attached to the current group (set via posthog.group()). When you change the group, these properties are reset.

Automatic overrides

Default overridden properties

By default, we always override some properties based on the user IP address.

The list of properties that this overrides:

$geoip_city_name
$geoip_country_name
$geoip_country_code
$geoip_continent_name
$geoip_continent_code
$geoip_postal_code
$geoip_time_zone

This enables any geolocation-based flags to work without manually setting these properties.

Boolean feature flags

Kotlin
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")
}

Multivariate feature flags

Kotlin
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")
}

Ensuring flags are loaded before usage

Every time a user opens the app, we send a request in the background to fetch the feature flags that apply to that user. We store those flags in the storage.

This means that for most screens, the feature flags are available immediately – except for the first time a user visits.

To handle this, you can use the onFeatureFlags callback to wait for the feature flag request to finish:

Kotlin
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
    }
}

Reloading feature flags

Feature flag values are cached. If something has changed with your user and you'd like to refetch their flag values, call:

Kotlin
import com.posthog.PostHog

PostHog.reloadFeatureFlags()

Boolean feature flags

Swift
if (PostHogSDK.shared.isFeatureEnabled("flag-key")) {
    // Do something differently for this user

    // Optional: fetch the payload
    let matchedFlagPayload = PostHogSDK.shared.getFeatureFlagPayload("flag-key")
}

Multivariate feature flags

Swift
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")
}

Reloading feature flags

Feature flag values are cached. If something has changed with your user and you'd like to refetch their flag values, call:

Swift
PostHogSDK.shared.reloadFeatureFlags()

Ensuring flags are loaded before usage

Every time a user opens the app, we send a request in the background to fetch the feature flags that apply to that user. We store those flags in the storage.

This means that for most screens, the feature flags are available immediately – except for the first time a user visits.

To handle this, you can use the didReceiveFeatureFlags notification to wait for the feature flag request to finish:

Swift
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_PROJECT_TOKEN = "<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(projectToken: POSTHOG_PROJECT_TOKEN, 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")
    }
}

Alternatively, you can use the completion block of the reloadFeatureFlags(_:) method. This allows you to execute logic immediately after the flags are reloaded:

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

Boolean feature flags

Dart
if (await Posthog().isFeatureEnabled('flag-key')) {
  // Do something differently for this user

  // Optional: fetch the payload
  final matchedFlagPayload = await Posthog().getFeatureFlagPayload('flag-key');
}

Multivariate feature flags

Dart
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');
}

Ensuring flags are loaded before usage

To use the onFeatureFlags callback, you must set up the SDK manually by disabling the com.posthog.posthog.AUTO_INIT mode.

Every time a user opens the app, we send a request in the background to fetch the feature flags that apply to that user. We store those flags in the storage.

This means that for most screens, the feature flags are available immediately – except for the first time a user visits.

To handle this, you can use the onFeatureFlags callback in your config to be notified when flags are loaded:

Dart
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);

Reloading feature flags

Feature flag values are cached. If something has changed with your user and you'd like to refetch their flag values, call:

Dart
await Posthog().reloadFeatureFlags();

There are two steps to implement feature flags in Java:

Step 1: Evaluate flags once

Call posthog.evaluateFlags() once for the user, then read values from the returned snapshot.

Boolean feature flags

Java
PostHogFeatureFlagEvaluations flags = posthog.evaluateFlags("distinct_id_of_your_user");

if (flags.isEnabled("flag-key")) {
    // Do something differently for this user

    // Optional: fetch the payload
    String matchedFlagPayload = flags.getFlagPayload("flag-key");
}

Multivariate feature flags

Java
PostHogFeatureFlagEvaluations flags = posthog.evaluateFlags("distinct_id_of_your_user");

Object flagValue = flags.getFlag("flag-key");
String enabledVariant = flagValue instanceof String ? (String) flagValue : null;

if ("variant-key".equals(enabledVariant)) { // replace "variant-key" with the key of your variant
    // Do something differently for this user

    // Optional: fetch the payload
    String matchedFlagPayload = flags.getFlagPayload("flag-key");
}

flags.getFlag() returns the variant string for multivariate flags, true for enabled boolean flags, false for disabled flags, and null when the flag wasn't returned by the evaluation.

Note: posthog.isFeatureEnabled(), posthog.getFeatureFlag(), posthog.getFeatureFlagPayload(), and PostHogCaptureOptions.builder().appendFeatureFlags(true) still work during the migration period, but they're deprecated. Prefer evaluateFlags() for new code.

Step 2: Include feature flag information when capturing events

Note: This step is only required for events captured using our server-side SDKs or API.

There are two methods you can use to include feature flag information in your events:

Method 1: Pass the evaluated flags snapshot to `capture()`

Pass the same flags object that you used for branching. This attaches the exact flag values from that evaluation and doesn't make another /flags request.

Java
PostHogFeatureFlagEvaluations flags = posthog.evaluateFlags("distinct_id_of_your_user");

if (flags.isEnabled("flag-key")) {
    // Do something differently for this user
}

posthog.capture(
    "distinct_id_of_your_user",
    "event_name",
    PostHogCaptureOptions.builder()
        .flags(flags)
        .build()
);

By default, this attaches every flag in the snapshot using $feature/<flag-key> properties and $active_feature_flags.

To reduce event property bloat, pass a filtered snapshot:

Java
// Attach only flags accessed with isEnabled() or getFlag() before this call
posthog.capture(
    "distinct_id_of_your_user",
    "event_name",
    PostHogCaptureOptions.builder()
        .flags(flags.onlyAccessed())
        .build()
);

// Attach only specific flags
posthog.capture(
    "distinct_id_of_your_user",
    "event_name",
    PostHogCaptureOptions.builder()
        .flags(flags.only("checkout-flow", "new-dashboard"))
        .build()
);

onlyAccessed() is order-dependent. If you call it before accessing any flags with isEnabled() or getFlag(), no feature flag properties are attached.

Method 2: Include the `$feature/feature_flag_name` property manually

In the event properties, include $feature/feature_flag_name: variant_key:

Java
posthog.capture(
    "distinct_id_of_your_user",
    "event_name",
    PostHogCaptureOptions.builder()
        .property("$feature/feature-flag-key", "variant-key") // replace feature-flag-key with your flag key. Replace "variant-key" with the key of your variant
        .build()
);

Evaluating only specific flags

By default, evaluateFlags() evaluates every flag for the user. If you only need a few flags, pass flagKeys to request only those flags:

Java
import java.util.Arrays;

PostHogFeatureFlagEvaluations flags = posthog.evaluateFlags(
    "distinct_id_of_your_user",
    PostHogEvaluateFlagsOptions.builder()
        .flagKeys(Arrays.asList("checkout-flow", "new-dashboard"))
        .build()
);

Sending `$feature_flag_called` events

flags.getFlagPayload() doesn't send $feature_flag_called events and doesn't count as an access for onlyAccessed().

Advanced: Overriding server properties

Sometimes, you may want to evaluate feature flags using person properties, groups, or group properties that haven't been ingested yet, or were set incorrectly earlier.

For example:

Java
import com.posthog.server.PostHogEvaluateFlagsOptions;

PostHogFeatureFlagEvaluations flags = posthog.evaluateFlags(
    "distinct_id_of_the_user",
    PostHogEvaluateFlagsOptions.builder()
        .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()
);

if (flags.isEnabled("flag-key")) {
    // Do something differently for this user
}

Overriding GeoIP properties

The following GeoIP properties can be overridden:

$geoip_country_code
$geoip_country_name
$geoip_city_name
$geoip_city_confidence
$geoip_continent_code
$geoip_continent_name
$geoip_latitude
$geoip_longitude
$geoip_postal_code
$geoip_subdivision_1_code
$geoip_subdivision_1_name
$geoip_subdivision_2_code
$geoip_subdivision_2_name
$geoip_subdivision_3_code
$geoip_subdivision_3_name
$geoip_time_zone

Simply include any of these properties in the person_properties parameter alongside your other person properties when calling feature flags.

There are two steps to implement feature flags in Rust:

Step 1: Evaluate flags once

Call client.evaluate_flags() once for the user, then read values from the returned snapshot.

Boolean feature flags

Rust
use posthog_rs::EvaluateFlagsOptions;

let flags = client.evaluate_flags(
    "distinct_id_of_your_user",
    EvaluateFlagsOptions::default(),
).await.unwrap();

if flags.is_enabled("flag-key") {
    // Do something differently for this user
    // Optional: fetch the payload
    let matched_flag_payload = flags.get_flag_payload("flag-key");
}

Multivariate feature flags

Rust
use posthog_rs::{EvaluateFlagsOptions, FlagValue};

let flags = client.evaluate_flags(
    "distinct_id_of_your_user",
    EvaluateFlagsOptions::default(),
).await.unwrap();

match flags.get_flag("flag-key") {
    Some(FlagValue::String(variant)) if variant == "variant-key" => {
        // Do something differently for this user
        // Optional: fetch the payload
        let matched_flag_payload = flags.get_flag_payload("flag-key");
    }
    _ => {}
}

flags.get_flag() returns Some(FlagValue::String(...)) for multivariate flags, Some(FlagValue::Boolean(true)) for enabled boolean flags, Some(FlagValue::Boolean(false)) for disabled flags, and None when the flag wasn't returned by the evaluation.

Note: client.is_feature_enabled(), client.get_feature_flag(), client.get_feature_flag_payload(), and client.get_feature_flags() still work during the migration period, but they're deprecated. Prefer evaluate_flags() for new code.

Step 2: Include feature flag information when capturing events

Note: This step is only required for events captured using our server-side SDKs or API.

There are two methods you can use to include feature flag information in your events:

Method 1: Pass the evaluated flags snapshot to the event

Pass the same flags object that you used for branching. This attaches the exact flag values from that evaluation and doesn't make another /flags request.

Rust
use posthog_rs::{EvaluateFlagsOptions, Event};

let flags = client.evaluate_flags(
    "distinct_id_of_your_user",
    EvaluateFlagsOptions::default(),
).await.unwrap();

if flags.is_enabled("flag-key") {
    // Do something differently for this user
}

let mut event = Event::new("event_name", "distinct_id_of_your_user");
event.with_flags(&flags);
client.capture(event).await.unwrap();

By default, this attaches every flag in the snapshot using $feature/<flag-key> properties and $active_feature_flags.

To reduce event property bloat, pass a filtered snapshot:

Rust
// Attach only flags accessed with is_enabled() or get_flag() before this call
let mut event = Event::new("event_name", "distinct_id_of_your_user");
event.with_flags(&flags.only_accessed());
client.capture(event).await.unwrap();

// Attach only specific flags
let mut event = Event::new("event_name", "distinct_id_of_your_user");
event.with_flags(&flags.only(&["checkout-flow", "new-dashboard"]));
client.capture(event).await.unwrap();

only_accessed() is order-dependent. If you call it before accessing any flags with is_enabled() or get_flag(), no feature flag properties are attached.

Method 2: Include the `$feature/feature_flag_name` property manually

In the event properties, include $feature/feature_flag_name: variant_key:

Rust
use posthog_rs::Event;

let mut event = Event::new("event_name", "distinct_id_of_your_user");
event.insert_prop("$feature/feature-flag-key", "variant-key").unwrap();
client.capture(event).await.unwrap();

Evaluating only specific flags

By default, evaluate_flags() evaluates every flag for the user. If you only need a few flags, pass flag_keys to request only those flags:

Rust
use posthog_rs::EvaluateFlagsOptions;

let flags = client.evaluate_flags(
    "distinct_id_of_your_user",
    EvaluateFlagsOptions {
        flag_keys: Some(vec!["checkout-flow".to_string(), "new-dashboard".to_string()]),
        ..Default::default()
    },
).await.unwrap();

Sending `$feature_flag_called` events

flags.get_flag_payload() doesn't send $feature_flag_called events and doesn't count as an access for only_accessed().

Blocking client

If you're using the blocking client (with default-features = false), the API is the same but without .await:

Rust
use posthog_rs::EvaluateFlagsOptions;

let flags = client.evaluate_flags(
    "distinct_id_of_your_user",
    EvaluateFlagsOptions::default(),
).unwrap();

if flags.is_enabled("flag-key") {
    // Do something differently for this user
}

There are two steps to implement feature flags in Elixir:

Step 1: Evaluate flags once

Call PostHog.FeatureFlags.evaluate_flags/1 once for the user, then read values from the returned snapshot.

Boolean feature flags

Elixir
{:ok, snapshot} = PostHog.FeatureFlags.evaluate_flags("distinct_id_of_your_user")

if PostHog.FeatureFlags.Evaluations.enabled?(snapshot, "flag-key") do
  # Do something differently for this user
  # Optional: fetch the payload
  payload = PostHog.FeatureFlags.Evaluations.get_flag_payload(snapshot, "flag-key")
end

Multivariate feature flags

Elixir
{:ok, snapshot} = PostHog.FeatureFlags.evaluate_flags("distinct_id_of_your_user")

enabled_variant = PostHog.FeatureFlags.Evaluations.get_flag(snapshot, "flag-key")

if enabled_variant == "variant-key" do
  # Do something differently for this user
  # Optional: fetch the payload
  payload = PostHog.FeatureFlags.Evaluations.get_flag_payload(snapshot, "flag-key")
end

PostHog.FeatureFlags.Evaluations.get_flag/2 returns the variant string for multivariate flags, true for enabled boolean flags, false for disabled flags, and nil when the flag wasn't returned by the evaluation.

Note: PostHog.FeatureFlags.check/2, PostHog.FeatureFlags.check!/2, PostHog.FeatureFlags.get_feature_flag_result/2, and PostHog.FeatureFlags.get_feature_flag_result!/2 still work during the migration period, but they're deprecated. Prefer evaluate_flags/1 for new code.

Step 2: Include feature flag information when capturing events

Note: This step is only required for events captured using our server-side SDKs or API.

There are two methods you can use to include feature flag information in your events:

Method 1: Put the evaluated flags snapshot in context

Put the same snapshot object that you used for branching into context. Subsequent captures from the same process attach the exact flag values from that evaluation and don't make another /flags request.

Elixir
{:ok, snapshot} = PostHog.FeatureFlags.evaluate_flags("distinct_id_of_your_user")

if PostHog.FeatureFlags.Evaluations.enabled?(snapshot, "flag-key") do
  # Do something differently for this user
end

PostHog.FeatureFlags.set_in_context(snapshot)
PostHog.capture("event_name", %{distinct_id: "distinct_id_of_your_user"})

By default, this attaches every flag in the snapshot using $feature/<flag-key> properties and $active_feature_flags.

To reduce event property bloat, put a filtered snapshot in context:

Elixir
{:ok, snapshot} = PostHog.FeatureFlags.evaluate_flags("distinct_id_of_your_user")

# Attach only flags accessed with enabled?/2, get_flag/2, or get_flag_payload/2 before this call
PostHog.FeatureFlags.Evaluations.enabled?(snapshot, "flag-key")
PostHog.FeatureFlags.set_in_context(
  PostHog.FeatureFlags.Evaluations.only_accessed(snapshot)
)

# Or attach only specific flags
PostHog.FeatureFlags.set_in_context(
  PostHog.FeatureFlags.Evaluations.only(snapshot, ["checkout-flow", "new-dashboard"])
)

only_accessed/1 is order-dependent. If you call it before accessing any flags with enabled?/2, get_flag/2, or get_flag_payload/2, no feature flag properties are attached.

Method 2: Include the `$feature/feature_flag_name` property manually

In the event properties, include $feature/feature_flag_name: variant_key:

Elixir
PostHog.capture("event_name", %{
  "$feature/feature-flag-key" => "variant-key",
  distinct_id: "distinct_id_of_your_user"
})

Evaluating only specific flags

By default, evaluate_flags/1 evaluates every flag for the user. If you only need a few flags, pass flag_keys to request only those flags:

Elixir
{:ok, snapshot} =
  PostHog.FeatureFlags.evaluate_flags(%{
    distinct_id: "distinct_id_of_your_user",
    flag_keys: ["checkout-flow", "new-dashboard"]
  })

Sending `$feature_flag_called` events

Capturing $feature_flag_called events enables PostHog to know when a flag was accessed by a user and provide analytics and insights on the flag. With evaluate_flags/1, the SDK sends this event when you call PostHog.FeatureFlags.Evaluations.enabled?/2 or PostHog.FeatureFlags.Evaluations.get_flag/2 for a flag.

PostHog.FeatureFlags.Evaluations.get_flag_payload/2 doesn't send $feature_flag_called events.

There are two steps to implement feature flags in .NET:

Step 1: Evaluate flags once

Call EvaluateFlagsAsync() once for the user, then read values from the returned snapshot.

Boolean feature flags

C#
var flags = await posthog.EvaluateFlagsAsync("distinct_id_of_your_user");

if (flags.IsEnabled("flag-key"))
{
    // Do something differently for this user
    // Optional: fetch the payload
    var matchedPayload = flags.GetFlagPayload("flag-key");
}

Multivariate feature flags

C#
var flags = await posthog.EvaluateFlagsAsync("distinct_id_of_your_user");

var enabledVariant = flags.GetFlag("flag-key")?.VariantKey;

if (enabledVariant == "variant-key") // replace "variant-key" with the key of your variant
{
    // Do something differently for this user
    // Optional: fetch the payload
    var matchedPayload = flags.GetFlagPayload("flag-key");
}

flags.GetFlag() returns a nullable FeatureFlag object. Check VariantKey for multivariate flags and IsEnabled for boolean flags. It returns null when the flag wasn't returned by the evaluation.

Note: posthog.IsFeatureEnabledAsync(), posthog.GetFeatureFlagAsync(), and Capture(..., sendFeatureFlags: true, ...) still work during the migration period, but they're deprecated. Prefer EvaluateFlagsAsync() for new code.

Step 2: Include feature flag information when capturing events

Note: This step is only required for events captured using our server-side SDKs or API.

There are two methods you can use to include feature flag information in your events:

Method 1: Pass the evaluated flags snapshot to `Capture()`

Pass the same flags object that you used for branching. This attaches the exact flag values from that evaluation and doesn't make another /flags request.

C#
var flags = await posthog.EvaluateFlagsAsync("distinct_id_of_your_user");

if (flags.IsEnabled("flag-key"))
{
    // Do something differently for this user
}

posthog.Capture(
    "distinct_id_of_your_user",
    "event_name",
    properties: null,
    groups: null,
    flags: flags
);

By default, this attaches every flag in the snapshot using $feature/<flag-key> properties and $active_feature_flags.

To reduce event property bloat, pass a filtered snapshot:

C#
// Attach only flags accessed with IsEnabled() or GetFlag() before this call
posthog.Capture(
    "distinct_id_of_your_user",
    "event_name",
    properties: null,
    groups: null,
    flags: flags.OnlyAccessed()
);

// Attach only specific flags
posthog.Capture(
    "distinct_id_of_your_user",
    "event_name",
    properties: null,
    groups: null,
    flags: flags.Only("checkout-flow", "new-dashboard")
);

Method 2: Include the `$feature/feature_flag_name` property manually

In the event properties, include $feature/feature_flag_name: variant_key:

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

Evaluating only specific flags

By default, EvaluateFlagsAsync() evaluates every flag for the user. If you only need a few flags, pass FlagKeysToEvaluate to request only those flags:

C#
var flags = await posthog.EvaluateFlagsAsync(
    "distinct_id_of_your_user",
    options: new AllFeatureFlagsOptions
    {
        FlagKeysToEvaluate = new[] { "checkout-flow", "new-dashboard" },
    }
);

Sending `$feature_flag_called` events

Capturing $feature_flag_called events enables PostHog to know when a flag was accessed by a user and provide analytics and insights on the flag. With EvaluateFlagsAsync(), the SDK sends this event when you call flags.IsEnabled() or flags.GetFlag() for a flag.

flags.GetFlagPayload() doesn't send $feature_flag_called events and doesn't count as an access for OnlyAccessed().

Advanced: Overriding server properties

Sometimes, you may want to evaluate feature flags using person properties, groups, or group properties that haven't been ingested yet, or were set incorrectly earlier.

For example:

C#
var flags = await posthog.EvaluateFlagsAsync(
    "distinct_id_of_the_user",
    options: new AllFeatureFlagsOptions
    {
        PersonProperties = new()
        {
            ["property_name"] = "value",
        },
        Groups = new()
        {
            new Group("your_group_type", "your_group_id")
            {
                ["group_property_name"] = "value",
            },
            new Group("another_group_type", "another_group_id")
            {
                ["group_property_name"] = "another value",
            },
        },
    }
);

if (flags.IsEnabled("flag-key"))
{
    // Do something differently for this user
}

Overriding GeoIP properties

The following GeoIP properties can be overridden:

$geoip_country_code
$geoip_country_name
$geoip_city_name
$geoip_city_confidence
$geoip_continent_code
$geoip_continent_name
$geoip_latitude
$geoip_longitude
$geoip_postal_code
$geoip_subdivision_1_code
$geoip_subdivision_1_name
$geoip_subdivision_2_code
$geoip_subdivision_2_name
$geoip_subdivision_3_code
$geoip_subdivision_3_name
$geoip_time_zone

Simply include any of these properties in the person_properties parameter alongside your other person properties when calling feature flags.

There are 3 steps to implement feature flags using the PostHog API:

Step 1: Evaluate the feature flag value using `flags`

flags is the endpoint used to determine if a given flag is enabled for a certain user or not.

Request

# 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();

Note: The groups key is only required for group-based feature flags. If you use it, replace group_type and group_id with the values for your group such as company: "Twitter".

Using evaluation context tags and runtime filtering without SDKs

When making direct API calls to the /flags endpoint, you can control which flags are evaluated using evaluation context tags and runtime filtering.

Evaluation contexts

To filter flags by evaluation context, include the evaluation_contexts field in your request body:

Note: The legacy parameter evaluation_environments is also supported for backward compatibility.

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();

Only flags where at least one evaluation tag matches (or flags with no tags at all) will be returned. For example:

Flag with evaluation context tags ["production", "api", "backend"] + request with ["production", "web"] = ✅ Flag evaluates ("production" matches)
Flag with evaluation context tags ["staging", "api"] + request with ["production", "web"] = ❌ Flag doesn't evaluate (no tags match)
Flag with evaluation context tags ["web", "mobile"] + request with ["production", "web"] = ✅ Flag evaluates ("web" matches)
Flag with no evaluation context tags = ✅ Always evaluates (backward compatibility)

Runtime detection

Evaluation runtime (server vs. client) is automatically detected based on your request headers and user-agent. This determines which flags are available based on their runtime setting (server-only, client-only, or all).

How runtime is detected:

User-Agent patterns - The system analyzes the User-Agent header:
- Client-side patterns: Mozilla/, Chrome/, Safari/, Firefox/, Edge/ (browsers), or mobile SDKs like posthog-android/, posthog-ios/, posthog-react-native/, posthog-flutter/
- Server-side patterns: posthog-python/, posthog-ruby/, posthog-php/, posthog-java/, posthog-go/, posthog-node/, posthog-dotnet/, posthog-elixir/, python-requests/, curl/
Browser-specific headers - Presence of these headers indicates client-side:
- Origin header
- Referer header
- Sec-Fetch-Mode header
- Sec-Fetch-Site header
Default behavior - If runtime can't be determined, the system includes flags with no runtime requirement and those set to "all"

Examples of runtime detection:

JavaScript
// 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
# 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
)

Terminal
# 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

JavaScript
// 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"
    })
});

Combining evaluation context tags and runtime filtering

Both features work together as sequential filters:

JavaScript
// 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

This allows precise control over which flags are evaluated in different contexts, helping optimize costs and improve security by ensuring flags only evaluate where intended.

Response

The response varies depending on whether you include the config=true query parameter:

Basic response (`/flags?v=2`)

Use this endpoint when you only need to evaluate feature flags. It returns a response with just the flag evaluation results.

Note: If a feature flag is associated with an experiment that has a holdout group, users in the holdout receive a variant value in the format holdout-{holdout_id} (e.g., holdout-727). You can detect holdout users by checking if the variant starts with holdout-.

JSON
{
  "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"
}

Full response with configuration (`/flags?v=2&config=true`)

Use this endpoint when you need both feature flag evaluation and PostHog configuration information (useful for client-side SDKs that need to initialize PostHog):

JSON
{
  "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
      }
    }
  }
}

Note: errorsWhileComputingFlags will return true if we didn't manage to compute some flags (for example, if there's an ongoing incident involving flag evaluation).
This enables partial updates to currently active flags in your clients.

Quota limiting

If your organization exceeds its feature flag quota, the /flags endpoint will return a modified response with quotaLimited.

For basic response (/flags?v=2):

JSON
{
  "flags": {},
  "errorsWhileComputingFlags": false,
  "quotaLimited": ["feature_flags"],
  "requestId": "d4d89b14-9619-4627-adf2-01b761691c2e"
}

For full response with configuration (/flags?v=2&config=true):

JSON
{
  "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
}

When you receive a response with quotaLimited containing "feature_flags", it means:

Your feature flag evaluations have been temporarily paused because you've exceeded your feature flag quota
If you want to continue evaluating feature flags, you can increase your quota in your billing settings under Feature flags & Experiments or contact support

Step 2: Include feature flag information when capturing events

Note: This step is only required for events captured using our server-side SDKs or API.

To do this, include the $feature/feature_flag_name property in your event:

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)

Step 3: Send a `$feature_flag_called` event

To track usage of your feature flag and view related analytics in PostHog, submit the $feature_flag_called event whenever you check a feature flag value in your code.

You need to include two properties with this event:

$feature_flag_response: This is the name of the variant the user has been assigned to e.g., "control" or "test"
$feature_flag: This is the key of the feature flag in your experiment.

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)

Advanced: Overriding server properties

Sometimes, you may want to evaluate feature flags using person properties, groups, or group properties that haven't been ingested yet, or were set incorrectly earlier.

For example:

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())

Overriding GeoIP properties

To override the GeoIP properties used to evaluate a feature flag, provide an IP address in the HTTP_X_FORWARDED_FOR when making your /flags request:

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())

The list of properties that this overrides:

$geoip_city_name
$geoip_country_name
$geoip_country_code
$geoip_continent_name
$geoip_continent_code
$geoip_postal_code
$geoip_time_zone

Adding feature flag code

Boolean feature flags

Multivariate feature flags

Ensuring flags are loaded before usage

Callback parameters

Reloading feature flags

Overriding server properties

Automatic overrides

Default overridden properties

Request timeout

Feature flag error handling

Method 1: Using hooks

Example 1: Using a boolean feature flag

Example 2: Using a multivariate feature flag

Example 3: Using a flag payload

Method 2: Using the PostHogFeature component

Payloads

Request timeout

Error handling

Step 1: Evaluate flags once

Boolean feature flags

Multivariate feature flags

Step 2: Include feature flag information when capturing events

Method 1: Pass the evaluated flags snapshot to capture()

Method 2: Include the $feature/feature_flag_name property manually

Evaluating only specific flags

Sending $feature_flag_called events

Advanced: Overriding server properties

Overriding GeoIP properties

Request timeout

Step 1: Evaluate flags once

Boolean feature flags

Multivariate feature flags

Step 2: Include feature flag information when capturing events

Method 1: Pass the evaluated flags snapshot to capture()

Method 2: Include the $feature/feature_flag_name property manually

Evaluating only specific flags

Sending $feature_flag_called events

Advanced: Overriding server properties

Overriding GeoIP properties

Request timeout

Step 1: Evaluate flags once

Boolean feature flags

Multivariate feature flags

Step 2: Include feature flag information when capturing events

Method 1: Pass the evaluated flags snapshot to capture()

Method 2: Include the $feature/feature_flag_name property manually

Evaluating only specific flags

Sending $feature_flag_called events

Advanced: Overriding server properties

Overriding GeoIP properties

Request timeout

Step 1: Evaluate flags once

Boolean feature flags

Multivariate feature flags

Step 2: Include feature flag information when capturing events

Method 1: Pass the evaluated flags snapshot to capture()

Method 2: Include the $feature/feature_flag_name property manually

Evaluating only specific flags

Sending $feature_flag_called events

Advanced: Overriding server properties

Overriding GeoIP properties

Request timeout

Step 1: Evaluate flags once

Boolean feature flags

Multivariate feature flags

Step 2: Include feature flag information when capturing events

Method 1: Pass the evaluated flags snapshot to Capture

Method 2: Include the $feature/feature_flag_name property manually

Evaluating only specific flags

Sending $feature_flag_called events

Advanced: Overriding server properties

Overriding GeoIP properties

Request timeout

Method 1: Using hooks

Example 1: Boolean feature flags

Example 2: Multivariate feature flags

Method 2: Loading the flag directly

Ensuring flags are loaded before usage

Reloading flags

Method 1: Pass the evaluated flags snapshot to `capture()`

Method 2: Include the `$feature/feature_flag_name` property manually

Sending `$feature_flag_called` events

Method 1: Pass the evaluated flags snapshot to `capture()`

Method 2: Include the `$feature/feature_flag_name` property manually

Sending `$feature_flag_called` events

Method 1: Pass the evaluated flags snapshot to `capture()`

Method 2: Include the `$feature/feature_flag_name` property manually

Sending `$feature_flag_called` events

Method 1: Pass the evaluated flags snapshot to `capture()`

Method 2: Include the `$feature/feature_flag_name` property manually

Sending `$feature_flag_called` events

Method 1: Pass the evaluated flags snapshot to `Capture`

Method 2: Include the `$feature/feature_flag_name` property manually

Sending `$feature_flag_called` events

Method 1: Pass the evaluated flags snapshot to `capture()`

Method 2: Include the `$feature/feature_flag_name` property manually

Sending `$feature_flag_called` events

Method 2: Include the `$feature/feature_flag_name` property manually

Sending `$feature_flag_called` events

Method 2: Include the `$feature/feature_flag_name` property manually

Sending `$feature_flag_called` events

Method 1: Pass the evaluated flags snapshot to `Capture()`

Method 2: Include the `$feature/feature_flag_name` property manually

Sending `$feature_flag_called` events

Step 1: Evaluate the feature flag value using `flags`

Basic response (`/flags?v=2`)

Full response with configuration (`/flags?v=2&config=true`)

Step 3: Send a `$feature_flag_called` event