Worked on the braintrust-sdk repository to enhance telemetry accuracy and instrumentation reliability for backend systems. Addressed a bug in OpenRouter callModel metrics that caused double-counting of aggregate token and cost metrics on parent spans when child turn spans were emitted. The solution preserved parent span output and metadata, ensuring that metrics now reflect per-turn usage only and improving cost attribution and trace clarity. The update aligned instrumentation behavior across both OpenRouter and OpenRouter Agent plugins. Validation included formatting checks, unit tests, and typings verification. Utilized TypeScript and API development skills to deliver a robust and maintainable backend fix.