Developed and delivered a billing labels feature for Vertex AI’s generateContent requests in the langchain4j/langchain4j repository, enabling users to attach custom labels for improved cost attribution and reporting. The solution introduced a labels builder to both chat and streaming models, ensuring that labels are injected into every request when specified, while maintaining backward compatibility for existing integrations. The implementation achieved end-to-end parity with Google’s genai request structure and was validated through extensive unit testing in Java, with 88 targeted tests and over 1,300 core tests passing. The work emphasized robust API development and thorough validation in real-world usage scenarios.