feat: add advanced_feature_flags_dedup_per_session config option#3241
Merged
dustinbyrne merged 2 commits intomainfrom Mar 16, 2026
Merged
feat: add advanced_feature_flags_dedup_per_session config option#3241dustinbyrne merged 2 commits intomainfrom
dustinbyrne merged 2 commits intomainfrom
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When enabled, event deduplication is scoped to the current session ID. Each new session re-emits the event for every flag that is checked, fixing the case where experiments miss flag events that were cached before the experiment went live.
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Size Change: +3.54 kB (+0.05%) Total Size: 6.68 MB
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marandaneto
approved these changes
Mar 16, 2026
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Problem
Experiments measure usage of feature flags via the
$feature_flag_calledevent. The SDK deduplicates this event globally—once emitted and cached for a given flag+value, it is never re-emitted unlessidentify(with a new distinct ID) orresetis called. This means that if the event fires before an experiment goes live, the experiment will never see it for that user, skewing results.This deviates from how the mobile SDKs track
$feature_flag_calledevents, which is in-memory.Changes
Adds a new config option
advanced_feature_flags_dedup_per_session(defaultfalse) that scopes$feature_flag_calleddeduplication to the current session ID. When enabled, the persisted$flag_call_reportedcache is cleared whenever the session rotates, so each new session re-emits the event for every flag that is checked.Files changed:
packages/types/src/posthog-config.ts— Newadvanced_feature_flags_dedup_per_session: booleanconfig property with JSDoc.packages/browser/src/constants.ts— NewFLAG_CALL_REPORTED_SESSION_IDpersistence key, added to reserved properties.packages/browser/src/posthog-core.ts— Default value (false).packages/browser/src/posthog-featureflags.ts— Session-scoped dedup logic: compares current session ID to the stored one, clears the reported-flags map on mismatch.packages/browser/src/__tests__/featureflags.test.ts— 5 new tests covering session rotation, default-off behavior, within-session value tracking, multi-flag reset, and empty session ID safety.In a future revision, we'll create a new defaults in which this is on by default.
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