Discover tools through tools/list and invoke one through tools/call with its name and arguments. A tool can declare outputSchema and return a matching structuredContent object. Include the serialized JSON in a text content block as well for older clients. Tool execution failures should use a tool result with isError true so the agent can correct its input or choose another action; protocol errors describe malformed requests or unsupported operations. Read-only and destructive annotations are hints, not authorization controls. Scope: MCP specification 2025-11-25. Reviewed September 19, 2026.