fix(tools): fuzzy edit can overwrite a block whose interior drifted

[PierrunoYT]

Version / branch / commit

Audited on main at 903e3d13 (2026-08-23).

Problem

edit_file promises that old_string must match byte-for-byte, but blockAnchorReplacer falls back to matching the first and last lines plus an average interior similarity threshold of 0.65. It then replaces the entire on-disk span even when the interior differs from old_string.

Reproduction

Given an on-disk block containing:

func a() {
	x := 1
	y := 2
	z := 3
	return
}

request an edit whose old_string is identical except for y := 99. The fuzzy matcher accepts the block and reports success, replacing the real y := 2 line even though the requested old content never existed.

Expected behavior

A drifted interior must make the edit fail or require an exact re-read/retry. The tool must not discard content that differs from old_string.

Actual behavior

The whole span is replaced and the tool reports Successfully edited (replaced 1 occurrence).

Relevant code

• internal/tools/edit_replacers.go:30,154-249,466-481
• internal/tools/edit_file.go:109-131

Suggested fix

Reject fuzzy block matches when any interior line materially differs from the requested old block. Add a regression test using the y := 2 / y := 99 drift case and prove it fails before the fix.

[PierrunoYT]

I'm working on this.