agent rewrote 340 lines to fix a two-line bug, third time this week it-failed

Constrain the blast radius mechanically instead of asking nicely.

What works for me, in order of effect:

• give it a failing test and say the only success condition is that test passing with no other test changing. Now there's a definition of done that isn't "improve the code"
• state the line budget out loud. "Fix this with the smallest possible diff. If the fix is more than 10 lines, stop and explain why before writing anything." The explain-first clause is the important half — it turns an unwanted refactor into a sentence you can say no to
• keep a rule in the repo config: no unrequested refactors, no renames, no new abstractions unless asked

The stop-and-explain pattern changed my day-to-day more than anything else. Most of the time it comes back with "it's three lines" and just does it.

Split the two things into two turns and two commits. Turn one: fix. Commit. Turn two: "now, if you think this file needs refactoring, propose it." Half the time the proposal is good and I take it, as its own commit with its own review.

The problem isn't that it refactors, it's that the fix and the refactor arrive in one diff and you can't approve one without the other. Git solves that, not prompting.

Smaller context helps more than people expect. Give it the whole file and it treats the whole file as in scope. Give it the function plus its test and the diff tends to stay inside the function.

I've started selecting the specific range rather than opening the file for small fixes. It sounds fussy and it measurably shrinks diffs.

Cheap trick in the meantime: git add -p and stage only the hunks you asked for, discard the rest. Takes 30 seconds, gets you the fix without the refactor, and doesn't depend on the model cooperating at all.

Worth saying plainly: 340-line diffs from a two-line ask are a review problem, and the review problem is now the actual bottleneck. If you're doing this fifteen times a day your throughput is bounded by how fast you can read, not by how fast it can write. Anything that makes diffs smaller is worth more than anything that makes generation faster.

this is why i don't let it write code. i ask it to explain the bug and i type the fix. way faster than reading 340 lines

Check whether a formatter or lint autofix is in the loop before you blame the model for all 340. Some of those lines might be Biome or Prettier reformatting on save with settings that differ from what's committed. I chased "it keeps rewriting my file" for a week and it was my editor.