During February 2026, Yusuke Matsushita enhanced the modelcontextprotocol/go-sdk repository by implementing extensions support for ServerCapabilities and ClientCapabilities, addressing SEP-2133 compliance. He designed and integrated an Extensions field, along with helper methods to normalize nil settings to empty maps, preventing negotiation failures due to null values. His work included shallow-copy and clone logic for extension maps, ensuring correct capability handling across backend services. Using Go and focusing on API design and backend development, Yusuke developed comprehensive unit and conformance tests, achieving full green results across 30 scenarios and 40 assertions, demonstrating thoroughness and reliability in the delivered feature.