What this means in practice
MCP servers connect their own declared tools through the SDK’s MCP client.
Choose the extension point that fits whether you are adding an action, a method, or an external service.
Practical guidance
There are three distinct ways to extend the toolset. A custom tool adds an operation for your own account or application. A skill packages instructions and a repeatable method so the agent can follow the same approach across tasks. A sub-agent takes a bounded piece of work that can be handled separately. An MCP server connects external tools through the SDK's MCP client.
Start by naming the missing capability, then choose the extension that matches it. If you need a new action, define a custom tool; if you need consistent procedure, write a skill; if the work divides cleanly, delegate a bounded task; if a service already exposes MCP tools, connect that server. Keep permissions narrow and expose only the operations the agent needs. This makes extensions easier to understand and safer to maintain.