feat: send `$device_id` on feature flag requests #2708

andehen · 2025-12-08T11:55:56Z

Problem

We want to support using $device_id as a bucketing identifier in feature flags. See this RFC for full context.

In short, we want a stable identifier that does not change when users log in. To support that, we need to include the $device_id as part of the payload to the feature flag request service.

Changes

Include $device_id in the payload when fetching feature flags.

Release info Sub-libraries affected

Libraries affected

Checklist

Tests for new code
Accounted for the impact of any changes across different platforms
Accounted for backwards compatibility of any changes (no breaking changes!)
Took care not to unnecessarily increase the bundle size

If releasing new changes

Ran pnpm changeset to generate a changeset file
Added the "release" label to the PR to indicate we're publishing new versions for the affected packages

vercel · 2025-12-08T11:56:03Z

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Preview	Updated (UTC)
posthog-js	Ready	Preview	Dec 8, 2025 2:09pm
posthog-nextjs-config	Ready	Preview	Dec 8, 2025 2:09pm

greptile-apps

_{2 files reviewed, no comments}

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github-actions · 2025-12-08T12:00:04Z

Download bundle size visualization

github-actions · 2025-12-08T12:03:32Z

Size Change: +780 B (+0.02%)

Total Size: 5.11 MB

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`packages/browser/dist/array.full.js`	369 kB	+79 B (+0.02%)
`packages/browser/dist/array.full.no-external.js`	384 kB	+79 B (+0.02%)
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`packages/browser/dist/array.no-external.js`	176 kB	+77 B (+0.04%)
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_{compressed-size-action}

andehen added 2 commits December 8, 2025 12:39

send device_id on feature flag requests

7de4a30

add changeset

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andehen added the release label Dec 8, 2025

greptile-apps bot reviewed Dec 8, 2025

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include in mock

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vercel bot deployed to Preview – posthog-nextjs-config December 8, 2025 12:30 View deployment

vercel bot deployed to Preview – posthog-js December 8, 2025 12:31 View deployment

use isNull and isUndefined instead of direct checks

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feat: send `$device_id` on feature flag requests #2708

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feat: send `$device_id` on feature flag requests #2708

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