Comment by moinism

I agree, technically, "sub agent" is also another tool. But I think it's important to differentiate tools with deterministic input/output from those with reasoning ability. A simple 'Tool' will take the input and try to execute, but the 'subagent' might reason that the action is unnecessary and that the required output already exists in the shared context. Or it can ask a clarifying question from the main agent before using its tools.