A scripted edit to source code fails to match after a formatter has run

Symptoms

An agent's exact-text replacement (an edit tool, sed, a Python replace) reports that the text to replace is not found, in a file it wrote or read a few steps earlier. The code looks the same, except for where lines break.

Cause

A formatter (cargo fmt, rustfmt on save, black, prettier) reflowed the file between the read and the edit: a long call split over several lines, a match arm joined, a trailing comma added. The text the agent remembers no longer exists byte for byte. In the Hardwar project it happened several times after cargo fmt and cargo clippy --fix, while the crates were being written (observed).

What to do

Read the file again after any formatter run, and match on the formatted text. Keep an edit's match short and inside one statement, so a reflow elsewhere does not break it. Run the formatter once at the end of a set of edits rather than between them, then read the result. When a replacement fails, read the lines around it; do not retry the same text.