fix: Dedupe feature flag called events by response

[marandaneto]

💡 Motivation and Context

Repeated feature flag reads currently emit duplicate $feature_flag_called events for the same distinct ID, flag, and returned response value. This adds per-supervisor deduplication so duplicate reads for the same response are suppressed while still emitting again when the response changes.

💚 How did you test it?

• mix format --check-formatted
• mix test test/posthog/feature_flags/evaluations_test.exs
• mix test

📝 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 sampo add to generate a changeset file

🤖 Agent context

Autonomy: Human-driven (agent-assisted)

A coding agent prepared this PR from the dedicated fix/ff-called-value-dedupe worktree at the user's request. The implementation keeps deduplication scoped to the PostHog supervisor by registering an Agent-backed cache under the existing registry, suppresses repeated $feature_flag_called captures for the same distinct ID/flag/response tuple, preserves no-op behavior when no supervisor is running, and adds tests for same-value dedupe and changed-value recapture.

[github-actions]

posthog-elixir Compliance Report

Date: 2026-06-24 20:42:22 UTC
Duration: 107381ms

⚠️ Some Tests Failed

42/45 tests passed, 3 failed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	610ms
Format Validation.Event Has Uuid	✅	610ms
Format Validation.Event Has Lib Properties	✅	610ms
Format Validation.Distinct Id Is String	✅	609ms
Format Validation.Token Is Present	✅	611ms
Format Validation.Custom Properties Preserved	✅	610ms
Format Validation.Event Has Timestamp	✅	610ms
Retry Behavior.Retries On 503	✅	5615ms
Retry Behavior.Does Not Retry On 400	✅	2613ms
Retry Behavior.Does Not Retry On 401	✅	2613ms
Retry Behavior.Respects Retry After Header	✅	5615ms
Retry Behavior.Implements Backoff	✅	15612ms
Retry Behavior.Retries On 500	✅	5616ms
Retry Behavior.Retries On 502	✅	5615ms
Retry Behavior.Retries On 504	✅	5615ms
Retry Behavior.Max Retries Respected	✅	15617ms
Deduplication.Generates Unique Uuids	✅	620ms
Deduplication.Preserves Uuid On Retry	✅	5615ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10620ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5617ms
Deduplication.No Duplicate Events In Batch	✅	614ms
Deduplication.Different Events Have Different Uuids	✅	612ms
Compression.Sends Gzip When Enabled	✅	610ms
Batch Format.Uses Proper Batch Structure	✅	609ms
Batch Format.Flush With No Events Sends Nothing	✅	607ms
Batch Format.Multiple Events Batched Together	✅	615ms
Error Handling.Does Not Retry On 403	✅	2610ms
Error Handling.Does Not Retry On 413	✅	2613ms
Error Handling.Retries On 408	✅	5616ms

Feature_Flags Tests

⚠️ 13/16 tests passed, 3 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	8ms
Request Payload.Flags Request Uses V2 Query Param	✅	7ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	6ms
Request Payload.Flags Request Omits Authorization Header	✅	6ms
Request Payload.Token In Flags Body Matches Init	✅	6ms
Request Payload.Groups Round Trip	✅	7ms
Request Payload.Groups Default To Empty Object	✅	7ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	✅	7ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	6ms
Request Payload.Disable Geoip Omitted Defaults To False	❌	6ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	7ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	609ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	10ms
Request Lifecycle.Mock Response Value Is Returned To Caller	❌	7ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	❌	609ms

Failures

request_payload.disable_geoip_omitted_defaults_to_false

Field 'geoip_disable' not found in /flags request body at path 'geoip_disable'. Available keys: ['groups', 'api_key', 'distinct_id', 'flag_keys_to_evaluate', 'group_properties', 'person_properties']

request_lifecycle.mock_response_value_is_returned_to_caller

Last action result missing field 'value'. Keys: ['error', 'success']

side_effect_events.get_feature_flag_captures_feature_flag_called_event

Expected 1 events with name '$feature_flag_called', got 0

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix: dedupe feature flag called events b..." | Re-trigger Greptile}

[martosaur]

Using agent as a cache is a bit dangerous for production as that means all feature flag calls will be synchronously routed through a single process, creating a bottleneck.

[marandaneto]

Using agent as a cache is a bit dangerous for production as that means all feature flag calls will be synchronously routed through a single process, creating a bottleneck.

Mm good point, any suggestion?

[martosaur]

Usually an ETS table is used in some form with an attached process that manages it's lifecycle/eviction/etc. There's also a way to piggyback on the Registry for convenience. Not sure which way I would personally go, would likely need to spend some time exploring to get a feel for what works best. I can take a look at this if you're ok with waiting.

[dustinbyrne]

this doesn't account for groups in the cache key

[dustinbyrne]

log_feature_flag_usage doesn't account for it either, but it should be able to be provided to FeatureFlags.check, get_feature_flag_result, evaluate_flags via the body

[marandaneto]

yeah groups aren't accounted for in a few SDKs, i think i'd do this as a follow up since i can check/update the spec and do all at once later

[marandaneto]

@martosaur sounds good, not in a rush
i changed to an ETS table, if you think this is better, we can merge this as is and make it better later if you come up with a better plan

[marandaneto]

@martosaur sounds good, not in a rush i changed to an ETS table, if you think this is better, we can merge this as is and make it better later if you come up with a better plan

merged this but happy to hear if theres a better way, we can fix forward.