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

[haacked]

💡 Motivation and Context

$feature_flag_called events carry the full enriched properties dict (rails request context, super/context properties, custom event properties) 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 (minimalFlagCalledEvents in the v2 /flags response, or top-level minimal_flag_called_events in the local-evaluation flag definitions payload) and the evaluated flag's has_experiment is exactly false. Any missing signal (gate absent, has_experiment unknown, legacy response shape, request failure) sends the full legacy shape unchanged.

Server-gated because rolling this out unconditionally could break existing insights; announcement/comms come before enablement. The goal is to establish $feature_flag_called as a special system event.

Implementation notes:

• Single enforcement point: minimal events are built by slicing the allowlist in Client#capture after context merge and FieldParser assembly, so context properties and parser-added metadata can never leak in. Kept: $feature_flag, $feature_flag_response, $feature_flag_has_experiment, $feature_flag_id, $feature_flag_version, $feature_flag_reason, $feature_flag_request_id, $feature_flag_evaluated_at, $feature_flag_error, locally_evaluated, $groups, $process_person_profile, $session_id, $is_server, $lib, $lib_version (string and symbol forms).
• Local-evaluation gate lives on the poller: refreshed on each definitions poll, preserved across 304 Not Modified responses, reset when flags are unset on quota limit, and persisted through flag_definition_cache_provider implementations so it survives restarts in multi-worker setups.
• Remote evaluation reads the gate per /flags response. For mixed snapshots (evaluate_flags), the team-level response gate supersedes the poller gate; a response without the gate fails the whole snapshot safe to full events.
• Both $feature_flag_called emit paths are covered: the legacy single-flag path (get_feature_flag_result and friends) and the snapshot path (evaluate_flags / FeatureFlagEvaluations).

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

💚 How did you test it?

• bundle exec rspec — 630 examples, 0 failures
• bundle exec rubocop — 82 files inspected, no offenses
• bundle exec rake public_api:check — passing (no public API surface changes)

Coverage added:

• Gate matrix from both signal sources (local-eval definitions payload and v2 /flags response), across both emit paths: gated + no experiment → minimal; gated + experiment → full; gate missing / has_experiment missing → full.
• Exact-key-set assertions on minimal events, including $session_id and $is_server retention and stripping of context properties, $feature/<key>, and payload props.
• Regression test proving a caller-supplied _minimal_flag_called_event key inside capture properties cannot trigger minimization.
• Cache-provider tests simulating restart: gate stored on fetch, applied when definitions load from cached data.

Released as a minor changeset for posthog-ruby.

📝 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

---

Created with PostHog Code

[github-actions]

posthog-ruby Compliance Report

Date: 2026-07-18 05:22:04 UTC
Duration: 98183ms

✅ All Tests Passed!

46/46 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	110ms
Format Validation.Event Has Uuid	✅	107ms
Format Validation.Event Has Lib Properties	✅	109ms
Format Validation.Distinct Id Is String	✅	107ms
Format Validation.Token Is Present	✅	107ms
Format Validation.Custom Properties Preserved	✅	107ms
Format Validation.Event Has Timestamp	✅	108ms
Retry Behavior.Retries On 503	✅	5314ms
Retry Behavior.Does Not Retry On 400	✅	2109ms
Retry Behavior.Does Not Retry On 401	✅	2109ms
Retry Behavior.Respects Retry After Header	✅	8113ms
Retry Behavior.Implements Backoff	✅	15523ms
Retry Behavior.Retries On 500	✅	5212ms
Retry Behavior.Retries On 502	✅	5212ms
Retry Behavior.Retries On 504	✅	5214ms
Retry Behavior.Max Retries Respected	✅	15510ms
Deduplication.Generates Unique Uuids	✅	112ms
Deduplication.Preserves Uuid On Retry	✅	5212ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10317ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5215ms
Deduplication.No Duplicate Events In Batch	✅	114ms
Deduplication.Different Events Have Different Uuids	✅	109ms
Compression.Sends Gzip When Enabled	✅	109ms
Batch Format.Uses Proper Batch Structure	✅	106ms
Batch Format.Flush With No Events Sends Nothing	✅	5ms
Batch Format.Multiple Events Batched Together	✅	110ms
Error Handling.Does Not Retry On 403	✅	2110ms
Error Handling.Does Not Retry On 413	✅	2109ms
Error Handling.Retries On 408	✅	5213ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	107ms
Request Payload.Flags Request Uses V2 Query Param	✅	107ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	106ms
Request Payload.Flags Request Omits Authorization Header	✅	107ms
Request Payload.Token In Flags Body Matches Init	✅	106ms
Request Payload.Groups Round Trip	✅	107ms
Request Payload.Groups Default To Empty Object	✅	108ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	109ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	109ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	107ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	105ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	110ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	106ms
Retry Behavior.Retries Flags On 502	✅	209ms
Retry Behavior.Retries Flags On 504	✅	253ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	109ms