UnicodeEncodeError: 'charmap' codec can't encode character

The error

UnicodeEncodeError: 'charmap' codec can't encode character '→' in position 12: character maps to <undefined>

Why it happens

When Python writes to a console window, it writes UTF-8. When its output goes to a pipe or a file, which is how coding agents read it, Python falls back to the Windows code page, usually cp1252. cp1252 has no arrows, no box-drawing characters and few letters outside Western Europe, so the first one Python tries to print raises.

Files have the same problem: open() without an encoding uses the code page too.

The fix

Run Python in UTF-8 mode, for one command, or for every shell you open from now on:

python -X utf8 script.py
setx PYTHONUTF8 1

Or fix it inside the script, before the first print, and name the encoding when you open files:

import sys
sys.stdout.reconfigure(encoding="utf-8")

with open(path, "w", encoding="utf-8") as f:
    f.write(text)

PEP 686 makes UTF-8 mode the default from Python 3.15.