download_agent wrote response.text, which requests decodes with a guessed
charset when the S3 response carries none. For some UTF-8 files the guess
is wrong (e.g. mac_iceland), so non-ASCII literals were silently corrupted
and the agent could fail at import. Write response.content unchanged.

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>