Do you do any preprocessing of diffs to replace significant whitespace with some token that is easier to spot? In my experience, some LLMs cannot tell unchanged context from the actual changes. That's especially annoying with -U99999 diffs as a shortcut to provide full file context.

I've only ever had that problem when supplying a formatted diff alone. Once I moved to "provide the diff, and then also provide the entire contents of the file after the change", I've never had the problem. (I've also only seriously used GPT-5.0 high or more powerful models for this.)