A couple of weeks ago I published a post with a tidy rule in it. When you add capability to an AI coding agent, reach for the lightest option first: a procedure file before a CLI, a CLI before a heavier integration, and only build the heavy machinery once you've proven you'll reuse it. My whole case rested on context cost. The heavy options load a lot of definitions up front and carry them every turn, so starting light keeps the window clean.
I still think the front half is right. But it isn't the rule I'd write now, because a reader took it apart in the comments and handed it back as something better. This post is about that exchange, because the rewrite was sharper than my original, and pretending I arrived at it alone would be both a lie and the less interesting story.
The hole, found in one comment
The first comment didn't argue with the rule. It walked straight to the blind spot. The moment a tool touches anything external or stateful, lightest-first reverses on you: a lightweight call that fails silently halfway through is harder to debug than a heavier tool that surfaces the failure cleanly. Pay the complexity up front.
My first instinct was to defend, and I did, a little. I said we were measuring different things, that I'd optimized for context cost while they were optimizing for failure observability, both real, different axes. I held the line by pointing out you can wrap a lightweight call to fail loudly, so the cheap path stays open.
