Rule Base Segment supported SDKs timeout (FME Only)

Summary

On April 28, 2026, a change was pushed to the Harness FME SDK API service modifying the response when there are no active Rule-based Segments to return on the SDK. The expected behavior was to return an empty list in this scenario, but a null value was served to SDKs instead, causing an error parsing the response.

Impact

• This issue impacted FME SDKs released May 2025 or after, as these support Rule-based segments. Prior versions were unaffected.

• SDK instances were unable to initialize. Even if flags could be parsed, rule-based segments were not. Rendering the SDK unable to mark itself as ready and enable proper evaluations.

  • Client side SDKs WITH an existing cache were not impacted. The issue was specific to initializations.

Timeline

Root Cause

When the new filter was added to filter non-active rule-based segments on initialization requests (rbSince=-1 on request params), the Collect function of Golang iterables was used which returns nil when there are empty results. Prior to this, we just exposed the query result, which was never nil.

This caused the unintended type change, leading to the issue on the SDKs related parser.

Remediation

Fix deployed: The team fixed the code on the service to ensure that empty lists were properly serialized as a list with no items ([]) in the responses served to SDKs.
We confirmed the fix in staging and deployed to production as soon as possible.

Action Items

• Testing coverage analysis and improvements for Rule-based segments: Identify any remaining edge-cases that could be introduced by mistake and ensure we have the right coverage for all of these.
• Improve continuous E2E suites visibility of the SDKs behavior: Our test applications running SDKs in them remained unaffected for the most part, given these are SDKs running emulating continuous real usage. This issue affected only initialization.