Please don't take offense to this very dumb question:

Why can't you do the planning ? Figure out what needs to be done , break it down into small tasks and then ask the agent to execute those small tasks?

When we executed projects in the past, this is what I would do as a lead: figure out the overall software architecture and delegate the tasks to developers.

This way I always knew how the system worked and could extend it as needed. I am not in development role anymore but I am trying to understand why we are delegating planning and software architecture to coding agents?

The kinds of detailed (and excellent) plans Opus or Fable can generate on our large code base would take me maybe 1-2 days to work through and they do in 10-20 minutes.

Maybe I spent 2-4 hours reviewing it, checking things with colleagues etc.

Then I press "go" and maybe an hour later I have a tested system ready for manual review.

It's plans are at least as good as any I've seen. Their weakness is if there are unstated assumptions I have about how things need to be done, so most of my time is now getting those assumptions stated properly and then reviewing.

Why wouldn't I use this? It's the best tool I've used in my 30 years of professional programming.

I could do the planning but I don't, for the same reason that I could write the source code but I don't, for the same reason that I could write the machine code but I don't.

whatever you delegated in the past probably also required planning by the engineer that went down and got it done, most planning done by agents is at this same level, agent explores the codebase, understands where to touch, tradeoffs, code-level architecture, and ask the user for more context or balance with assumptions and other patterns already present in code