[BUG]

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

In a Spanish-language Claude Code session, I explicitly asked Claude to stop using voseo ("vos", "tenés", "querés") and any regional Spanish (lunfardo/modismos), and to use neutral/formal Spanish only. Claude
acknowledged this, even added a project rule (CLAUDE.md) codifying it.
Despite that, it kept slipping back into voseo forms in later replies, across at least two further corrections in the same session.

What Should Happen?

Expected behavior:
Once explicitly corrected on a language/register preference, Claude
should apply it consistently for the rest of the session, not need to
be corrected multiple times for the same thing.

Actual behavior:
The same mistake (voseo) recurred at least 3 times in one session after
being corrected each time.

Error Messages/Logs

Steps to Reproduce

Steps to reproduce:

1. Start a Claude Code session in Spanish.
2. Ask Claude to use neutral Spanish only — no voseo, no regional slang.
3. Continue working normally.
4. Observe "vos"/"tenés"/"querés" resurface in later responses, despite
5. Claude confirming the correction each time it was pointed out.

Claude Model

Sonnet (default)

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

v2.1.258

Platform

Anthropic API

Operating System

Other Linux

Terminal/Shell

Other

Additional Information

_No response_