fix: no-op on PostHog API failures

[marandaneto]

💡 Motivation and Context

The .NET SDK should not crash the host application when PostHog is non-operational, including invalid project tokens or API/auth failures. Some public methods awaited API calls directly and could surface exceptions to the caller.

Changes:

• Return ApiResult(0) from direct ingestion APIs when PostHog API calls fail.
• Swallow/log API failures from FlushAsync, LoadFeatureFlagsAsync, and local feature flag loading paths while returning safe defaults.
• Update ETag quota-limit expectations to match no-throw public API behavior.
• Add regression coverage for direct ingestion, flush, and local flag loading API failures.

💚 How did you test it?

• dotnet test tests/UnitTests/UnitTests.csproj --no-restore --filter "FullyQualifiedName~TheApiFailureNoOpBehavior|FullyQualifiedName~TheDisabledClient|FullyQualifiedName~TheLoadFeatureFlagsAsyncMethod|FullyQualifiedName~ETagSupport"
• dotnet test tests/UnitTests/UnitTests.csproj --no-restore --filter "FullyQualifiedName~TheQuotaLimitBehavior|FullyQualifiedName~FeatureFlagsTests"

📝 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

[github-actions]

posthog-dotnet Compliance Report

Date: 2026-06-01 06:43:17 UTC
Duration: 589ms

⚠️ 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	❌	42ms
Request Payload.Flags Request Uses V2 Query Param	❌	20ms
Request Payload.Flags Request Hits Flags Path Not Decide	❌	5ms
Request Payload.Flags Request Omits Authorization Header	❌	5ms
Request Payload.Token In Flags Body Matches Init	❌	4ms
Request Payload.Groups Round Trip	❌	5ms
Request Payload.Groups Default To Empty Object	❌	5ms
Request Payload.Person Properties Distinct Id Auto Populated When Caller Omits It	❌	4ms
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	✅	179ms
Request Lifecycle.Two Flag Calls Produce Two Remote Requests	❌	7ms
Request Lifecycle.Mock Response Value Is Returned To Caller	❌	4ms
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'

[marandaneto]

this was flaky already so not really related to this PR but it was failing quite often

[greptile-apps]

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
src/PostHog/PostHogClient.cs:1192-1195
The `FlushAsync` catch filter omits the `ArgumentException` and `NullReferenceException` exclusions that every other catch block introduced in this PR carries. This means a programming error surfacing from `_asyncBatchHandler.FlushAsync()` (e.g., a `NullReferenceException` deep in the batch handler) would be silently logged instead of propagating, hiding real bugs. All sibling catch blocks explicitly let these exception types through (`when (e is not ArgumentException and not NullReferenceException and not OperationCanceledException)`); this one should match.

```suggestion
        catch (Exception e) when (e is not ArgumentException and not NullReferenceException and not OperationCanceledException)
        {
            _logger.LogErrorApiCallFailed(e, nameof(FlushAsync));
        }
```

_{Reviews (1): Last reviewed commit: "test: isolate exception source file lock" | Re-trigger Greptile}