Tenghui Li enhanced developer-facing documentation for the Vertex AI RAG Engine within the google/adk-docs repository, focusing on improving onboarding and discoverability for agent developers. Tenghui authored a comprehensive section in built-in-tools.md, detailing the vertex_ai_rag_retrieval feature and providing a Python usage example to guide private data retrieval workflows. The work leveraged Markdown and Python to ensure clarity and practical guidance for integrating Vertex AI RAG with the Agent Development Kit. No runtime defects or regressions were introduced, reflecting careful attention to documentation quality. This contribution deepened the technical reference available to developers, supporting more productive ADK integrations.