Right, so it’s a trade off between contexts. There are two reasons to use subagents, parallelism and tailoring of context. For the second, there is the “personality” of the subagents as well as how much context is injected from the main agent. Ignoring the personality, you ideally want the injected context to be small and focused on a single task so the subagent doesn’t get distracted. You want the main agent to be orchestrating all the subagents, but not reading all the same files they are reading, otherwise you’ll be paying for the same tokens in multiple contexts. IMO, this is where prompt engineering comes in, to be able to guide the main agent as to where subagents are desired and where not.

That is true of Anthropic's implementation but not inherent in sub-agents in general.