str_replace guards the replacement with `if old_str not in content`, which
cannot reject an empty old_str -- `"" in content` is always true. So an
empty old_str reached `str.replace("", new_str)`, which inserts new_str at
every character boundary, and the tool rewrote the file while still
returning "OK":
old_str='', new_str='# H\n' -> OK, file silently prepended
old_str='', new_str='X', replace_all -> OK, 'XdXeXfX XmXaXiXnX(X)X:X\nX...'
The empty-file branch above it already handles this case (`if not content:
if not old_str: return "OK"`), and the existing test states the intent
directly: "An empty old_str is a no-op edit and remains a benign OK". That
contract just never held once the file had content.
The tool is registered by default (config.example.yaml) and its schema
declares old_str as a plain string with no minLength, so a model can emit
"" legitimately; read-before-write only compares a hash and lets it past.
Check old_str first so the no-op holds whatever the file contains. The
empty-file case folds into the same not-found branch, which keeps its
message and behaviour.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
str_replace returned "OK" whenever the target file was empty, silently
reporting success even when the model asked to replace a non-empty string
that could not possibly be present. Only short-circuit to "OK" when old_str
is also empty; otherwise return the standard not-found error.
