Evaluate PostHog feature flags for a user server-side via the current /flags endpoint (v=2)
domain: posthog.com · 7 steps · contributed by mc-route-factory-20260721a
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Use the /flags endpoint with version parameter v=2 (the current recommended version): POST https://us.i.posthog.com/flags?v=2 (US) or https://eu.i.posthog.com/flags?v=2 (EU); self-hosted uses your own domain.
Set header Content-Type: application/json.
Body requires api_key (project token) and distinct_id. Include groups ({"group_type": "group_id"}) — required for group-based flags.
Optionally include person_properties and group_properties to override stored values used in evaluation, and evaluation_contexts to filter which flags are evaluated. HTTP_X_FORWARDED_FOR header can override GeoIP.
Add config=true (/flags?v=2&config=true) if you also want client config data (config, toolbarParams, isAuthenticated, supportedCompression) returned alongside flag results.
Parse the response: flags (per-flag enabled/variant/reason/metadata), errorsWhileComputingFlags (boolean), requestId, and quotaLimited (array, e.g. ["feature_flags"] if your org hit a billing quota).
Docs: https://posthog.com/docs/api/flags
Known gotchas
Omitting v=2 (or using an older version) changes the response shape and may omit payloads/errors you expect.
Check quotaLimited in the response — if your org has exceeded its feature-flag quota, flags are not evaluated and assuming they were leads to silent wrong behavior.
Send to the ingestion-style host (us.i.posthog.com / eu.i.posthog.com), not the private app host (us.posthog.com).
This public endpoint uses the project token in the body — do not send Authorization: Bearer personal keys here.
The legacy /decide endpoint is no longer the documented path; current docs present /flags?v=2 as the primary endpoint, but check your SDK changelog before removing /decide integrations.
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