Client capabilities this tool's handler requires to run. On the draft protocol, tools/call rejects with a -32003 MissingRequiredClientCapabilityError (whose data.requiredCapabilities lists the unmet ones) when the request's declared client capabilities do not cover these. Empty (the default) means no gating.
A registered tool: its descriptor plus the handler that executes it. The handler always returns a ToolResponse; the CallToolResult-returning registration overloads are adapted to one that always completes.