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

• has_experiment: boolean() on the Result struct (default false); build_result/3 parses metadata.has_experiment from the /flags?v=2 response with strict boolean coercion (this SDK has no local evaluation).
• log_feature_flag_usage/4 always adds the property, covering both the legacy single-flag path and the Evaluations snapshot path (missing flags omit the property).
• The SDK compliance adapter's hand-built $feature_flag_called also includes the property via an extract_has_experiment/2 helper over the response it already parses.
• Minor sampo changeset (CHANGELOG is generated at release).

💚 How did you test it?

mix test: 336 tests, 0 failures, with new coverage for true / reported-false / absent on both send paths plus the struct default. mix format --check-formatted, mix credo --strict, and mix posthog.public_api --check all clean. Note: ran on Elixir 1.18.3/OTP 27 (the repo pins 1.20/OTP 29, not installed locally; mix.exs requires only ~> 1.17); CI covers the pinned toolchain.

📝 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

[github-actions]

posthog-elixir Compliance Report

Date: 2026-07-15 23:28:08 UTC
Duration: 117489ms

✅ All Tests Passed!

46/46 tests passed

---

Capture Tests

✅ 29/29 tests passed

View Details

Test	Status	Duration
Format Validation.Event Has Required Fields	✅	608ms
Format Validation.Event Has Uuid	✅	609ms
Format Validation.Event Has Lib Properties	✅	608ms
Format Validation.Distinct Id Is String	✅	608ms
Format Validation.Token Is Present	✅	608ms
Format Validation.Custom Properties Preserved	✅	608ms
Format Validation.Event Has Timestamp	✅	608ms
Retry Behavior.Retries On 503	✅	5614ms
Retry Behavior.Does Not Retry On 400	✅	2612ms
Retry Behavior.Does Not Retry On 401	✅	2612ms
Retry Behavior.Respects Retry After Header	✅	5614ms
Retry Behavior.Implements Backoff	✅	15612ms
Retry Behavior.Retries On 500	✅	5613ms
Retry Behavior.Retries On 502	✅	5614ms
Retry Behavior.Retries On 504	✅	5615ms
Retry Behavior.Max Retries Respected	✅	15624ms
Deduplication.Generates Unique Uuids	✅	618ms
Deduplication.Preserves Uuid On Retry	✅	5614ms
Deduplication.Preserves Uuid And Timestamp On Retry	✅	10619ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5615ms
Deduplication.No Duplicate Events In Batch	✅	615ms
Deduplication.Different Events Have Different Uuids	✅	611ms
Compression.Sends Gzip When Enabled	✅	608ms
Batch Format.Uses Proper Batch Structure	✅	608ms
Batch Format.Flush With No Events Sends Nothing	✅	606ms
Batch Format.Multiple Events Batched Together	✅	613ms
Error Handling.Does Not Retry On 403	✅	2609ms
Error Handling.Does Not Retry On 413	✅	2612ms
Error Handling.Retries On 408	✅	5615ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	609ms
Request Payload.Flags Request Uses V2 Query Param	✅	609ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	607ms
Request Payload.Flags Request Omits Authorization Header	✅	608ms
Request Payload.Token In Flags Body Matches Init	✅	608ms
Request Payload.Groups Round Trip	✅	608ms
Request Payload.Groups Default To Empty Object	✅	608ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	608ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	608ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	608ms
Request Lifecycle.No Flags Request On Init Alone	✅	4ms
Request Lifecycle.No Flags Request On Normal Capture	✅	607ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	1212ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	608ms
Retry Behavior.Retries Flags On 502	✅	911ms
Retry Behavior.Retries Flags On 504	✅	910ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	1209ms

[greptile-apps]

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

[turnipdabeets]

LGTM! get_in + == true keeps it crash-proof and always-boolean, both the legacy and Evaluations paths converge on the same Result field, and the true/false/absent cases are all asserted.