That's good to know. The open weight models are still pretty awful getting stuck in this loop

Agreed. For the others, I asked Claude to write a structural edit tool: s-expressions only, insert/replace/... When passed 'tree' it prints a file like

<line>: <s-expr .-separated path>

then eg when passed 'replace 2.1 "(+ x y)"' it returns a new file (as a list of s-exprs).