fix: include group context in $feature_flag_called dedupe key

[gustavohstrassburger]

💡 Motivation and Context

In PostHogClient.TryCaptureDedupedFeatureFlagCalledEvent (src/PostHog/PostHogClient.cs), the per-distinctId MemoryCache key only included (distinctId, featureKey, cacheKeyValue). For group-scoped flags this meant that when the same user was evaluated under a different group, no new $feature_flag_called event was fired — causing per-group exposure undercount for experiments scoped to a group key.

Mirrors the posthog-node fix in PostHog/posthog-js#3658 (which closes PostHog/posthog-js#3651). Both SDKs share the same dedupe shape, so backend evaluation needs the same change.

💚 How did you test it?

Added three xUnit tests in tests/UnitTests/Features/FeatureFlagsTests.cs:

• CapturesSeparateFeatureFlagCalledEventsPerGroupContext — same user, same flag, two different group contexts → two events fired.
• DedupesFeatureFlagCalledAcrossRepeatedCallsUnderSameGroupContext — repeated access under the same group → one event.
• DedupesFeatureFlagCalledAcrossGroupInsertionOrder — same groups added in different insertion order → one event (canonicalized via OrderBy(GroupType, StringComparer.Ordinal)).

dotnet test --filter "FullyQualifiedName~Feature" is green: 322 tests pass on net8.0, 312 on netcoreapp3.1.

📝 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-dotnet Compliance Report

Date: 2026-05-28 15:33:26 UTC
Duration: 556ms

⚠️ Some Tests Failed

2/16 tests passed, 14 failed

---

Feature_Flags Tests

⚠️ 2/16 tests passed, 14 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	❌	44ms
Request Payload.Flags Request Uses V2 Query Param	❌	25ms
Request Payload.Flags Request Hits Flags Path Not Decide	❌	6ms
Request Payload.Flags Request Omits Authorization Header	❌	5ms
Request Payload.Token In Flags Body Matches Init	❌	5ms
Request Payload.Groups Round Trip	❌	5ms
Request Payload.Groups Default To Empty Object	❌	4ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	❌	5ms
Request Payload.Disable Geoip False Propagates As Geoip Disable False	❌	5ms
Request Payload.Disable Geoip Omitted Defaults To False	❌	5ms
Request Payload.Flag Keys To Evaluate Contains Only Requested Key	❌	5ms
Request Lifecycle.No Flags Request On Init Alone	✅	3ms
Request Lifecycle.No Flags Request On Normal Capture	✅	182ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	❌	7ms
Request Lifecycle.Mock Response Value Is Returned To Caller	❌	5ms
Side Effect Events.Get Feature Flag Captures Feature Flag Called Event	❌	5ms

Failures

request_payload.request_with_person_properties_device_id

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.flags_request_uses_v2_query_param

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.flags_request_hits_flags_path_not_decide

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.flags_request_omits_authorization_header

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.token_in_flags_body_matches_init

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.groups_round_trip

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.groups_default_to_empty_object

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.person_properties_distinct_id_auto_populated_when_caller_omits_it

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.disable_geoip_false_propagates_as_geoip_disable_false

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.disable_geoip_omitted_defaults_to_false

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_payload.flag_keys_to_evaluate_contains_only_requested_key

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_lifecycle.two_flag_calls_produce_two_remote_requests

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

request_lifecycle.mock_response_value_is_returned_to_caller

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

side_effect_events.get_feature_flag_captures_feature_flag_called_event

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'

[greptile-apps]

Prompt To Fix All With AI

Fix the following 2 code review issues. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 2
tests/UnitTests/Features/FeatureFlagsTests.cs:297-375
**Three `[Fact]` tests could be a parameterised `[Theory]`**

The three new tests cover the same logical property — dedup count as a function of group context — and share nearly identical scaffolding. Per the team's preference for parameterised tests, grouping them into a `[Theory]` with `[MemberData]` (or at least the first two as `[InlineData]`) would remove the duplicated container/handler/client setup and make the intent clearer at a glance.

### Issue 2 of 2
src/PostHog/PostHogClient.cs:776-780
**Potential cache-key collision with delimiter characters**

The canonical key format concatenates `GroupType` and `GroupKey` with `=` and joins entries with `;`. If either field contains those delimiters, two distinct group configurations can produce the same string, causing a false dedup where a `$feature_flag_called` event is suppressed for a genuinely different group context. Escaping the components before joining eliminates the ambiguity.

```suggestion
        var pairs = groups
            .Select(g => (g.GroupType, g.GroupKey))
            .OrderBy(p => p.GroupType, StringComparer.Ordinal)
            .Select(p => Uri.EscapeDataString(p.GroupType) + "=" + Uri.EscapeDataString(p.GroupKey));
        return string.Join(";", pairs);
```

_{Reviews (1): Last reviewed commit: "chore: add changeset" | Re-trigger Greptile}

[haacked]

Nice fix. A few suggestions and nits inline; nothing blocking.

[haacked]

suggestion: This is the second order-independent group-canonicalizer in the repo. FeatureFlagCacheKey.Generate at src/PostHog/Features/FeatureFlagCacheKey.cs:38-65 already does the same job for the flag-evaluation cache, using OrderBy(g => g.GroupType) (default culture-sensitive comparer) and , between entries. This one uses StringComparer.Ordinal and ;. Two helpers solving the same sub-problem will drift the next time either is touched, and a future reader will assume the difference is meaningful when it isn't.

Worth a follow-up (not this PR): align both on StringComparer.Ordinal (the right choice for cache keys — deterministic across cultures, faster), or factor a small Group.OrderedByType(IEnumerable<Group>) helper that both call sites compose from.

Same point on placement: this helper doesn't touch any PostHogClient state, so it would sit more naturally next to GroupCollection.

[gustavohstrassburger]

Will address this in a follow up PR.