Lessons for Agentic Coding: What should we do when code is cheap?

I had an idea to improve performance in one of the slowest but also one of the most critical parts of the codebase I own, so I asked Claude to re-write it. I gave it exact instructions. It got most things right but key things wrong. I caught the bugs and then asked it for some optimizations, and it came up with a number that were quite good. As I read the code, I saw more and more opportunities for improvement. To make a long story short, code that used to require upwards of 30 seconds in a particularly heinously ugly stress test now finishes in about 8ms.

My original code was terrible. That's indisputable. Maybe the bar for improvement was low. Still, the algorithms and optimizations that I was able to devise while using Claude Opus 4.6 surprised me. I don't often feel pleased with the cleverness of my work, but in this case the work really is stellar -- or at least enough of an improvement that it feels stellar.

Could I have written it without Claude? Yes, definitely. But I was able to produce the code in a few days while having a fever of 100-102, which I definitely couldn't have done on my own.

Moreover, it was plainly apparent to me, while I worked, that I was better able to think about high-level architecture and design because I wasn't stuck on the details of actually writing the code. The code itself, line by line, isn't difficult if you have familiarity with bitwise operations, but there's enough of it, with enough branches, that it's difficult as a whole and the work of writing it would have consumed much of my attention and energy.

Claude missed a huge amount. I improved performance by more than 95% after it told me there were no other opportunities for major optimizations.

Using the tool freed me, I found, to think more clearly, more deeply, and more effectively. Does the result create tech debt? I don't think so. I've pored over it and can't find anything lacking in style, design, or architecture. It's very well documented. Claude wrote tests, as I requested, for everything, including all the bugs that Claude missed and I caught. Test coverage is probably 100%, but, much more importantly, tests exhaustively cover cases, including edge cases, that would have, again, been difficult to enumerate and write by myself.

I doubt Claude could have done all this as well if the codebase and tests weren't already as mature as they are. I really wonder about the feasibility and advisability of greenfield software development with these tools. And a junior developer absolutely couldn't have accomplished what I did. The tool would have produced far worse work in the hands of someone who doesn't know what they were doing.

So I agree with you and disagree: I'm turning a corner on these tools, but I absolutely could not just let rip and trust it to do anything correctly. Moreover, I could not be less impressed by the MCPs written by people in my company. The bare tool by itself is better, though maybe that says more about my company, and my regards for the people I work with, than the tools.

Now.. the AI first engineer might still have to deal with hallucinated things. But.. they can also use the newfound cheapness of code to improve their workflow. Instead of just testing on localhost and manually deploying to prod, you can have a full dev, staging, prod pipeline for free. Tech debt can be one command from being refactored. The open source package that doesn’t quite do what you need it to do? Fork it and write a patch. The ai will be able to maintain the patch. Oh.. you need that bespoke feature for management? Np, done in a 1hr ai session.

Each of these things might be arguably insignificant on their own but net over a projects lifetime they really build up.

Does "basically free" to you mean for you just that someone else is paying the cost? That's a mentality that has only made the world worse when applied to a wider range of things. Be hesitant in that line of thinking, I suggest, and consider the future.