Capability Extensions

Per SEP-2133, both clients and servers can declare protocol extensions under the extensions member of their capabilities. Keys are extension identifiers using the reverse-DNS prefix convention (e.g. "io.modelcontextprotocol/tasks", "com.example/feature"); values are extension-defined configuration objects, with {} meaning “supported with no settings”.

On the server, declare extensions through the capabilities keyword, either as a plain hash or via the MCP::Server::Capabilities builder:

capabilities = MCP::Server::Capabilities.new
capabilities.support_tools
capabilities.support_extensions("com.example/feature" => { enabled: true })

server = MCP::Server.new(name: "my_server", capabilities: capabilities)

The declared extensions appear in the initialize result’s capabilities.extensions. Extensions the client declared during initialize are readable via server.client_capabilities[:extensions] (or session.client_capabilities[:extensions] for per-session transports).

On the modern lifecycle, the same declarations appear in the server/discover result, and the client’s extensions ride each request’s _meta envelope instead of an initialize handshake. Inside a handler, server_context.client_capabilities[:extensions] reads the current request’s declarations with envelope-first resolution, on either lifecycle.

On the client, pass extensions through connect:

client.connect(capabilities: { extensions: { "com.example/feature" => {} } })