`[2026-07-15]` **`re.sub`/`re.subn` INTERPRETS backslash-escapes in the REPLACEMENT string** — a `json.dumps`'d value (escaped `\n`) fed as the replacement leaked REAL newlines into a source file (broke `first_message.py` with an unterminated-string SyntaxError). Fix: use a FUNCTION replacement (`pat.subn(lambda m: new_block, src)`) or `str.replace` — the callable form bypasses escape processing. json.dumps itself escapes correctly; re.sub was the culprit.
