feat(flags): minimize $feature_flag_called events for non-experiment flags

[haacked]

💡 Motivation and Context

$feature_flag_called events carry the full enriched properties (context envelope, super properties, $active_feature_flags, the $feature/<key> enumeration) on every flag call, even for flags not linked to an experiment. This PR trims those events to a strict allowlist iff the server-controlled gate is on (top-level minimalFlagCalledEvents in the v2 /flags response) AND the flag's has_experiment is exactly false. Any missing signal (field absent, legacy response shape, cached state without it, has_experiment unknown) sends the full legacy shape unchanged, and experiment-linked flags always send the full envelope. Server-gated because rolling this out unconditionally could break existing insights; announcement/comms come before enablement.

Implementation notes:

• All changes live in the pure-JVM posthog core module.
• The gate is persisted in preferences alongside the cached flags so it survives app restarts; the key is in ALL_INTERNAL_KEYS, so it cannot leak into events via registered super properties.
• The gate is parsed overwrite-not-merge: a response without the field flips it off, so full events resume as soon as the server stops sending it. It is also cleared on reset()/identity change.
• The allowlist is applied at the single capture choke point (captureStateless) after property assembly, building the minimal property set from the allowlist rather than deleting from the full set.
• On a real Android cold restart, cached flag details deserialize without has_experiment, so events stay full until the first fresh /flags response of the session (fail-safe, bounded staleness).

Part of a cross-SDK rollout; reference implementation and fuller context: PostHog/posthog-python#748

💚 How did you test it?

Ran locally:

• ./gradlew :posthog:test — 805 tests including the new ones: gated + no experiment → exact allowlist key set (super props, context envelope, $feature/<key>, $active_feature_flags stripped); gated + experiment → full; ungated / field absent → full; gate + minimal shape persist across a simulated SDK restart; gate parse/persist/flip-off/clear at the PostHogRemoteConfig level.
• ./gradlew :posthog-server:test
• ./gradlew spotlessCheck
• ./gradlew :posthog:apiCheck :posthog-server:apiCheck (after apiDump)

Could not run :posthog-android:testDebugUnitTest or Android lint in the authoring environment (no Android SDK available) — no Android-module sources changed, but CI must confirm.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

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Created with PostHog Code

[github-actions]

posthog-android Compliance Report

Date: 2026-07-18 04:29:31 UTC
Duration: 118311ms

✅ All Tests Passed!

46/46 tests passed

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Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	372ms
Format Validation.Event Has Uuid	✅	31ms
Format Validation.Event Has Lib Properties	✅	30ms
Format Validation.Distinct Id Is String	✅	27ms
Format Validation.Token Is Present	✅	28ms
Format Validation.Custom Properties Preserved	✅	25ms
Format Validation.Event Has Timestamp	✅	27ms
Retry Behavior.Retries On 503	✅	7025ms
Retry Behavior.Does Not Retry On 400	✅	4024ms
Retry Behavior.Does Not Retry On 401	✅	4023ms
Retry Behavior.Respects Retry After Header	✅	7029ms
Retry Behavior.Implements Backoff	✅	17024ms
Retry Behavior.Retries On 500	✅	7017ms
Retry Behavior.Retries On 502	✅	7020ms
Retry Behavior.Retries On 504	✅	7017ms
Retry Behavior.Max Retries Respected	✅	17037ms
Deduplication.Generates Unique Uuids	✅	39ms
Deduplication.Preserves Uuid On Retry	✅	7016ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	12032ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7020ms
Deduplication.No Duplicate Events In Batch	✅	33ms
Deduplication.Different Events Have Different Uuids	✅	23ms
Compression.Sends Gzip When Enabled	✅	19ms
Batch Format.Uses Proper Batch Structure	✅	18ms
Batch Format.Flush With No Events Sends Nothing	✅	12ms
Batch Format.Multiple Events Batched Together	✅	31ms
Error Handling.Does Not Retry On 403	✅	4023ms
Error Handling.Does Not Retry On 413	✅	4020ms
Error Handling.Retries On 408	✅	5026ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	33ms
Request Payload.Flags Request Uses V2 Query Param	✅	19ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	23ms
Request Payload.Flags Request Omits Authorization Header	✅	24ms
Request Payload.Token In Flags Body Matches Init	✅	20ms
Request Payload.Groups Round Trip	✅	24ms
Request Payload.Groups Default To Empty Object	✅	20ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	19ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	18ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	21ms
Request Lifecycle.No Flags Request On Init Alone	✅	8ms
Request Lifecycle.No Flags Request On Normal Capture	✅	19ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	37ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	24ms
Retry Behavior.Retries Flags On 502	✅	323ms
Retry Behavior.Retries Flags On 504	✅	324ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	22ms