I was converting all the views in my Rails app from HAML to ERB. It was doing each one perfectly, so I told it to do the rest. It went through a few, then asked me if it could write a program, and run that. I thought, hey, cool, sure. I get it; it was trying to save tokens. Clever! However -- you know where this is going -- despite knowing all the rules, and demonstrating it could apply them, the program it wrote made a total dog's breakfast out of the rest of the files. Thankfully, I've learned to commit my working copy before big "AI" changes, and I just revert when it barfs. I forced Claude to do the rest "manually" at great token expense, but it did it correctly. I've asked it to write other scripts, which it has also mangled. So I haven't been impressed at Claude's "tool writing" capability yet, and I'm jealous of people who seem to have good luck.