feat(flags): minimize $feature_flag_called events for non-experiment flags in the compliance adapter

[haacked]

💡 Motivation and Context

The Flutter SDK never captures $feature_flag_called in Dart: on iOS and Android the native posthog-ios / posthog-android SDKs capture it over the method channel, and on web it delegates to posthog-js. Flutter apps will therefore get minimal events via the native SDK releases, and this repo's production code needs no change.

This PR implements the minimal-event contract in sdk_compliance_adapter — the only capture path this repo owns, and the same place the $feature_flag_has_experiment support landed in #475 — so the cross-SDK compliance harness can validate the behavior:

• Emit a minimal $feature_flag_called event iff the v2 /flags response carries top-level minimalFlagCalledEvents == true and the evaluated flag's has_experiment == false.
• Any missing signal (field absent, legacy response shape, has_experiment unreported) → full event, exactly as today.
• The minimal shape is a strict allowlist built from scratch; for this adapter that keeps $feature_flag, $feature_flag_response, $feature_flag_has_experiment, $lib, and $lib_version, and drops the $feature/<key> enumeration. distinct_id/token remain as transport fields of the adapter's batch protocol.

The motivation is trimming non-experiment $feature_flag_called events down to a strict allowlist of useful properties. The gate is server-controlled, so nothing changes for any team until it is enabled. Experiment-linked flags always keep the full envelope since experiment exposure analysis reads it.

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

💚 How did you test it?

• flutter test in sdk_compliance_adapter (unit test files): 11/11 passed, including 5 new tests — gated + no experiment → exactly the allowlist key set; gated + experiment → full; ungated → full; gated + unreported has_experiment → full; the gate tracks the latest flags response.
• flutter test in posthog_flutter: 193/193 passed (unchanged).
• dart analyze .: no issues.
• dart format --set-exit-if-changed sdk_compliance_adapter: clean.

No CHANGELOG entry, matching adapter-only precedent (#475).

📝 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-flutter Compliance Report

Date: 2026-07-18 04:27:32 UTC
Duration: 96866ms

✅ All Tests Passed!

45/45 tests passed

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Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	141ms
Format Validation.Event Has Uuid	✅	118ms
Format Validation.Event Has Lib Properties	✅	117ms
Format Validation.Distinct Id Is String	✅	116ms
Format Validation.Token Is Present	✅	116ms
Format Validation.Custom Properties Preserved	✅	118ms
Format Validation.Event Has Timestamp	✅	118ms
Retry Behavior.Retries On 503	✅	5331ms
Retry Behavior.Does Not Retry On 400	✅	2117ms
Retry Behavior.Does Not Retry On 401	✅	2118ms
Retry Behavior.Respects Retry After Header	✅	8122ms
Retry Behavior.Implements Backoff	✅	15448ms
Retry Behavior.Retries On 500	✅	5226ms
Retry Behavior.Retries On 502	✅	5227ms
Retry Behavior.Retries On 504	✅	5226ms
Retry Behavior.Max Retries Respected	✅	15446ms
Deduplication.Generates Unique Uuids	✅	125ms
Deduplication.Preserves Uuid On Retry	✅	5225ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10336ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5232ms
Deduplication.No Duplicate Events In Batch	✅	126ms
Deduplication.Different Events Have Different Uuids	✅	117ms
Compression.Sends Gzip When Enabled	✅	117ms
Batch Format.Uses Proper Batch Structure	✅	115ms
Batch Format.Flush With No Events Sends Nothing	✅	109ms
Batch Format.Multiple Events Batched Together	✅	125ms
Error Handling.Does Not Retry On 403	✅	2115ms
Error Handling.Does Not Retry On 413	✅	2117ms
Error Handling.Retries On 408	✅	5225ms

Feature_Flags Tests

✅ 16/16 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	14ms
Request Payload.Flags Request Uses V2 Query Param	✅	10ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	10ms
Request Payload.Flags Request Omits Authorization Header	✅	10ms
Request Payload.Token In Flags Body Matches Init	✅	10ms
Request Payload.Groups Round Trip	✅	10ms
Request Payload.Groups Default To Empty Object	✅	9ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	10ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	10ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	9ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	10ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	113ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	18ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	10ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	116ms