feat(flags): add $feature_flag_has_experiment to $feature_flag_called events

[haacked]

💡 Motivation and Context

Part of a cross-SDK effort to shrink $feature_flag_called events. This first phase adds a $feature_flag_has_experiment boolean property to every $feature_flag_called event, reflecting the server's has_experiment signal: metadata.has_experiment in the /flags?v=2 response and has_experiment on /api/feature_flag/local_evaluation flag definitions. When the server does not report the field (older deployments, bootstrapped flags), the property is omitted, so it is tri-state: true, false, or absent (unknown).

This lets ingestion distinguish experiment-linked flag events (which need the full property set for exposure analysis) from the rest, and lets us measure the split before a later phase strips the expensive properties ($feature/<key>, $feature_flag_payload, per-event system metadata) from non-experiment flag events. This PR intentionally minimizes nothing: no properties are removed, no config options are added, and dedupe behavior is unchanged.

Changes

• FeatureFlagMetadata.hasExperiment (@SerializedName("has_experiment"), default false) parsed from /flags?v=2 metadata and round-tripping through both flag caches; the same field on FlagDefinition for /local_evaluation, threaded into buildFeatureFlagFromResult.
• All send paths set the property: PostHog.sendFeatureFlagCalled, PostHogStateless.sendFeatureFlagCalled, PostHogFeatureFlagEvaluations.recordAccess, and the compliance adapter's hand-built capture, defaulting to false when details are unavailable (null details, unknown flags, bootstrap/cache-degraded flags).
• posthog.api regenerated; minor changeset for posthog and posthog-server.

Known pre-existing gap (not introduced here): the server SDK's single-flag local-evaluation fast path (resolveFeatureFlag) doesn't populate flag details, so events from that path report false even for experiment flags, same as the existing $feature_flag_id/$feature_flag_version behavior there. The evaluateFlags snapshot path reports it correctly. This must be fixed before any event-minimization phase.

💚 How did you test it?

./gradlew :posthog:test :posthog-server:test :posthog:apiCheck :posthog-server:apiCheck spotlessCheck: BUILD SUCCESSFUL. New tests cover true / explicit-false / absent across the client, stateless, snapshot, Gson-parse, and local-evaluation paths. The :posthog-android module is untouched (and requires an Android SDK not present locally); the change lives in the pure-JVM posthog and posthog-server modules. The compliance adapter compiles (:sdk_compliance_adapter:compileKotlin).

📝 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

[github-actions]

posthog-android Compliance Report

Date: 2026-07-15 23:31:22 UTC
Duration: 118534ms

✅ 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	✅	406ms
Format Validation.Event Has Uuid	✅	38ms
Format Validation.Event Has Lib Properties	✅	31ms
Format Validation.Distinct Id Is String	✅	29ms
Format Validation.Token Is Present	✅	28ms
Format Validation.Custom Properties Preserved	✅	25ms
Format Validation.Event Has Timestamp	✅	30ms
Retry Behavior.Retries On 503	✅	7027ms
Retry Behavior.Does Not Retry On 400	✅	4026ms
Retry Behavior.Does Not Retry On 401	✅	4026ms
Retry Behavior.Respects Retry After Header	✅	7026ms
Retry Behavior.Implements Backoff	✅	17032ms
Retry Behavior.Retries On 500	✅	7019ms
Retry Behavior.Retries On 502	✅	7021ms
Retry Behavior.Retries On 504	✅	7018ms
Retry Behavior.Max Retries Respected	✅	17023ms
Deduplication.Generates Unique Uuids	✅	45ms
Deduplication.Preserves Uuid On Retry	✅	7016ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	12034ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	7019ms
Deduplication.No Duplicate Events In Batch	✅	37ms
Deduplication.Different Events Have Different Uuids	✅	29ms
Compression.Sends Gzip When Enabled	✅	23ms
Batch Format.Uses Proper Batch Structure	✅	23ms
Batch Format.Flush With No Events Sends Nothing	✅	15ms
Batch Format.Multiple Events Batched Together	✅	35ms
Error Handling.Does Not Retry On 403	✅	4024ms
Error Handling.Does Not Retry On 413	✅	4022ms
Error Handling.Retries On 408	✅	5034ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	36ms
Request Payload.Flags Request Uses V2 Query Param	✅	25ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	24ms
Request Payload.Flags Request Omits Authorization Header	✅	29ms
Request Payload.Token In Flags Body Matches Init	✅	23ms
Request Payload.Groups Round Trip	✅	26ms
Request Payload.Groups Default To Empty Object	✅	29ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	25ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	26ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	25ms
Request Lifecycle.No Flags Request On Init Alone	✅	9ms
Request Lifecycle.No Flags Request On Normal Capture	✅	25ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	38ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	27ms
Retry Behavior.Retries Flags On 502	✅	327ms
Retry Behavior.Retries Flags On 504	✅	334ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	26ms

[greptile-apps]

_{Reviews (1): Last reviewed commit: "Add $feature_flag_has_experiment to $fea..." | Re-trigger Greptile}

[turnipdabeets]

LGTM! All four send paths read the metadata null-safely, the Gson absent-field default is pinned by tests, and true/false/absent are covered on every layer. The greptile binary-compat P1s look dismissible to me since both classes are @PostHogInternal and posthog/posthog-server release in lockstep — same precedent as ensureExperienceContinuity.