If I ask a model to do a change involving editing a file, and it starts investigating internals of my build system, then sure it might not be counterproductive or break the task, and might have taken only extra 30 seconds; but for all I know, my quick rejection of a shell invocation (with a simple "irrelevant to the task" comment) might have just saved me half of today's Opus tokens, which already makes it worth it.

I think he has a point. I noticed the same with skills that invoke another agent harness.

I just have a skill to review the changes in the current worktree.

By default, it will put lots of instructions about locating the changes into the prompt, like explaining how to use git diff.

These instructions are obviously unnecessary. I can believe that the same issue of needlessly verbose prompts might exist with subagent spawning.

I would not go to customize that one however. With skills, it is a more natural fix.