Marc Rasi focused on stabilizing the Model Context Protocol TypeScript SDK by addressing a critical issue in the SSE transport implementation. He delivered a targeted bug fix that normalized custom header propagation across fetch-based transport instances, ensuring headers were consistently passed and honored throughout all channels. Working primarily with TypeScript and leveraging his front end development and testing skills, Marc improved the reliability of SSE-based data streams and reduced header-related edge cases for downstream consumers. His work in the modelcontextprotocol/typescript-sdk repository demonstrated a deep understanding of transport mechanisms and contributed to more robust and predictable header handling in the SDK.