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

[haacked]

💡 Motivation and Context

$feature_flag_called events carry the full enriched properties (Config.DefaultEventProperties, request context, system context) 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 minimal_flag_called_events in the local-evaluation definitions payload) and the flag's has_experiment is exactly false. Any missing signal — gate absent, has_experiment unknown, experiment-linked flag, legacy response shapes — sends the full legacy shape unchanged.

This is server-gated because rolling it 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 specifics for this SDK:

• The local-evaluation gate rides the poller's atomic flagsState swaps: lock-free reads, preserved across 304 Not Modified refreshes, reset on quota-limit. When local evaluation is inconclusive and the SDK falls back to a remote /flags request, the gate is reset and re-read from that response, since it is now the source of the flag value.
• Enforcement happens at serialization time in both wire formats (legacy /batch and capture-v1) from one shared allowlist, so DefaultEventProperties, request-context properties, system context, and the snapshot path's $feature/<key> / $feature_flag_payload extras are excluded structurally rather than deleted piecemeal.
• Kept properties: $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, $geoip_disable, $is_server, $session_id, $window_id, $device_id, plus $lib/$lib_version added at serialization. $geoip_disable and $is_server stay because they are processing-control/classification sentinels; the session/window/device IDs are linkage identifiers.
• If a BeforeSend hook replaces the message, the minimal marker is lost and the event degrades fail-safe to the full shape.
• No SDK configuration is added; the gate is server-controlled per project.

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

💚 How did you test it?

• go test -race -count=1 -timeout=5m ./... — ok
• go vet ./... — clean
• gofmt -l . — clean
• bin/api-diff — public API snapshot up to date

Test coverage added:

• Gate matrix for both sources (v2 /flags response and local-evaluation definitions payload): gated + no experiment, gated + experiment, gated + unknown has_experiment, ungated.
• Exact-key-set assertions on minimal events (remote single-flag, local-eval single-flag, and EvaluateFlags snapshot paths), so any property leaking into — or dropping out of — the minimal shape fails the test.
• Remote-fallback gate source in both directions: local gate on + ungated remote response stays full; local gate off + gated remote response goes minimal.
• 304 ETag-only state swap preserves the cached gate.
• Capture-v1 wire shape, including the $session_id top-level lift on a minimal event and $groups retention.

A changeset (.changeset/minimal-flag-called-events.md, minor) documents the behavior.

📝 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-go Compliance Report

Date: 2026-07-18 04:27:17 UTC
Duration: 169248ms

✅ All Tests Passed!

111/111 tests passed

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Capture_V1 Tests

✅ 94/94 tests passed

View Details

Test	Status	Duration
Endpoint And Method.Targets V1 Endpoint	✅	24ms
Endpoint And Method.Does Not Use Legacy Endpoints	✅	6ms
Required Headers.Has Authorization Bearer Header	✅	7ms
Required Headers.Has Content Type Json	✅	6ms
Required Headers.Has Posthog Sdk Info Format	✅	15ms
Required Headers.Has Posthog Attempt Header	✅	18ms
Required Headers.Has Posthog Request Id	✅	6ms
Required Headers.Has Posthog Request Timestamp	✅	6ms
Required Headers.Has User Agent	✅	7ms
Body Format.Body Has Created At And Batch	✅	6ms
Body Format.No Api Key In Body	✅	6ms
Body Format.No Sent At In Body	✅	7ms
Event Format.Event Has Required Root Fields	✅	6ms
Event Format.Event Uuid Is Valid	✅	5ms
Event Format.Event Timestamp Is Rfc3339	✅	7ms
Event Format.Distinct Id Is String	✅	6ms
Event Format.Distinct Id At Root Not Properties	✅	6ms
Event Format.Custom Properties Preserved	✅	6ms
Event Format.Set Properties Preserved	✅	6ms
Event Format.Set Once Properties Preserved	✅	6ms
Event Format.Groups Properties Preserved	✅	6ms
Event Format.Sdk Generates Uuid If Not Provided	✅	6ms
Event Format.Event Has Required Root Fields Batch	✅	9ms
Event Format.Event Uuid Is Valid Batch	✅	9ms
Event Format.Event Timestamp Is Rfc3339 Batch	✅	7ms
Event Format.Distinct Id Is String Batch	✅	20ms
Event Format.Distinct Id At Root Not Properties Batch	✅	9ms
Event Format.Custom Properties Preserved Batch	✅	8ms
Event Format.Set Properties Preserved Batch	✅	9ms
Event Format.Set Once Properties Preserved Batch	✅	8ms
Event Format.Groups Properties Preserved Batch	✅	9ms
Event Format.Sdk Generates Uuid If Not Provided Batch	✅	8ms
Batch Behavior.Multiple Events In Single Batch	✅	31ms
Batch Behavior.Batch Envelope Smoke	✅	10ms
Batch Behavior.Flush With No Events Sends Nothing	✅	4ms
Batch Behavior.Flush At Triggers Batch	✅	1007ms
Batch Behavior.Created At Reflects Batch Creation Time	✅	7ms
Deduplication.Generates Unique Uuids	✅	15ms
Deduplication.Different Events Same Content Different Uuids	✅	9ms
Deduplication.Preserves Uuid On Retry	✅	5135ms
Deduplication.Preserves Timestamp On Retry	✅	5136ms
Deduplication.Preserves Uuid And Timestamp On Batch Retry	✅	5138ms
Deduplication.No Duplicate Events In Batch	✅	31ms
Header Behavior On Retry.Attempt Header Starts At One	✅	7ms
Header Behavior On Retry.Attempt Header Increments On Retry	✅	10135ms
Header Behavior On Retry.Request Id Preserved On Retry	✅	5136ms
Header Behavior On Retry.Different Requests Have Different Request Ids	✅	2012ms
Header Behavior On Retry.Request Timestamp Changes On Retry	✅	5137ms
Response Format Validation.Success Response Has Uuid Keyed Results	✅	7ms
Response Format Validation.Success Response Has Ok For Each Event	✅	28ms
Response Format Validation.Success No Retry After When All Ok	✅	28ms
Response Format Validation.Success Retry After Present When Retry Events	✅	129ms
Response Format Validation.Success No Retry After When Drop Only	✅	27ms
Response Format Validation.Response Echoes Request Id	✅	7ms
Retry Behavior.Retries On 408	✅	5132ms
Retry Behavior.Retries On 500	✅	5136ms
Retry Behavior.Retries On 503	✅	5136ms
Retry Behavior.Retries On 504	✅	5136ms
Retry Behavior.Retryable Errors Have Retry After	✅	2132ms
Retry Behavior.Respects Retry After On Retryable Error	✅	8139ms
Retry Behavior.Does Not Retry On 400	✅	2133ms
Retry Behavior.Does Not Retry On 401	✅	2132ms
Retry Behavior.Does Not Retry On 402	✅	2133ms
Retry Behavior.Does Not Retry On 413	✅	2132ms
Retry Behavior.Does Not Retry On 415	✅	2133ms
Retry Behavior.Non Retryable Errors Have No Retry After	✅	2133ms
Retry Behavior.Implements Backoff	✅	15145ms
Retry Behavior.Max Retries Respected	✅	15146ms
Partial Batch Handling.Handles 200 Full Success	✅	2009ms
Partial Batch Handling.Handles 200 With All Ok	✅	3032ms
Partial Batch Handling.Does Not Retry Dropped Events	✅	3031ms
Partial Batch Handling.Does Not Retry Limited Events	✅	3031ms
Partial Batch Handling.Prunes Ok Events On Partial Retry	✅	5136ms
Partial Batch Handling.Prunes Dropped Events On Partial Retry	✅	5137ms
Partial Batch Handling.Retries Only Retry Events From Partial	✅	5134ms
Partial Batch Handling.Partial Retry Preserves Uuids	✅	5136ms
Partial Batch Handling.Partial Retry Attempt Header Increments	✅	5136ms
Partial Batch Handling.Partial Retry Request Id Preserved	✅	5136ms
Partial Batch Handling.Respects Retry After On Partial	✅	5136ms
Partial Batch Handling.Unknown Result Treated As Terminal	✅	3029ms
Partial Batch Handling.Mixed Ok Drop Limited No Retry	✅	3035ms
Compression.Sends Gzip Content Encoding	✅	16ms
Compression.No Content Encoding When Disabled	✅	6ms
Compression.Compressed Body Is Decompressible	✅	15ms
Error Handling.Does Not Retry On Unknown 4Xx	✅	2132ms
Event Options.Cookieless Mode Override	✅	7ms
Event Options.Disable Skew Correction Override	✅	6ms
Event Options.Process Person Profile Override	✅	6ms
Event Options.Product Tour Id Override	✅	6ms
Event Options.Unset Options Omitted	✅	6ms
Event Options.Options Override In Batch	✅	8ms
Geoip And Historical Migration.Geoip Disable Injected Into Properties	✅	6ms
Geoip And Historical Migration.Historical Migration Set In Body	✅	6ms
Geoip And Historical Migration.Historical Migration Absent By Default	✅	6ms

Feature_Flags Tests

✅ 17/17 tests passed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	✅	16ms
Request Payload.Flags Request Uses V2 Query Param	✅	16ms
Request Payload.Flags Request Hits Flags Path Not Decide	✅	16ms
Request Payload.Flags Request Omits Authorization Header	✅	15ms
Request Payload.Token In Flags Body Matches Init	✅	16ms
Request Payload.Groups Round Trip	✅	16ms
Request Payload.Groups Default To Empty Object	✅	16ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	✅	15ms
Request Payload.Disable Geoip Omitted Defaults To False	✅	16ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	✅	16ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	6ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	✅	29ms
Request Lifecycle.Mock Response Value Is Returned To Caller	✅	15ms
Retry Behavior.Retries Flags On 502	✅	17ms
Retry Behavior.Retries Flags On 504	✅	18ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	✅	16ms