claude-code - 💡(How to fix) Fix [BUG] CLAUDE.md language/dialect instructions drift during long sessions (Spanish voseo leaks into neutral-Spanish output) [2 comments, 2 participants]

Preflight Checklist

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

What's Wrong?

CLAUDE.md language instructions regarding Spanish dialect/register are respected at the start of a session but silently ignored as the conversation grows longer or after tool-heavy interactions.

Specifically, I configure Claude Code to reply in neutral Latin American Spanish (tuteo) and to never use Rioplatense voseo (vos, tenés, sos, querés, podés, mirá, etc.). Early in a session this is honored. After enough context has accumulated, typically during long and demanding prompts, voseo forms start leaking in, sometimes mid-response and inconsistently within the same message.

This is not a cosmetic issue. Several voseo verb forms collide semantically with their neutral-Spanish equivalents. For example:

• corregí in neutral Spanish is first person past tense, meaning "I corrected it"
• corregí in Rioplatense voseo is second person imperative, meaning "[you] correct it"

So when Claude says corregí el archivo in a drifted response, it becomes genuinely ambiguous whether Claude is reporting a past action it performed or issuing an instruction to me. Multiplied across a long response, the output stops being reliably readable and has to be re-parsed sentence by sentence.

The failure mode is silent. There is no indication that the language rule has been deprioritized.

This appears to share a root cause with #34779 (accent marks and ñ being dropped in long sessions). Same "works at first, drifts later" pattern, different surface symptom. I left a comment there but opening a fresh report as the original issue is already closed.

What Should Happen?

Language and dialect instructions defined in CLAUDE.md (global and project-level) and in long-term memory should be respected consistently for the entire duration of the session, independent of conversation length, tool-call count, or post-compaction state.

If a user explicitly forbids a particular linguistic register (as in my case, voseo), that constraint should not degrade as context fills up.

Error Messages/Logs

No error messages. The failure is silent, the rule is simply no longer applied.

Steps to Reproduce

1. Place the following rule in ~/.claude/CLAUDE.md:

   ## Idioma (OBLIGATORIO)
   - SIEMPRE responder en español neutro latinoamericano (tuteo).
   - PROHIBIDO usar voseo argentino o uruguayo bajo cualquier circunstancia (vos, tenés, sos, querés, podés, mirá, etc.).
   - Si una respuesta contiene voseo, se considera incorrecta.
   - Usar formas: "tú", "tienes", "quieres", "puedes", "mira".

1. Reinforce it at project level in ./CLAUDE.md:

   **Como primera tarea en cada respuesta se debe revisar y aplicar las instrucciones escritas en `## Idioma (OBLIGATORIO)` en `~/.claude/CLAUDE.md`**

1. Add a persistent feedback memory entry:

   ---
   name: "Feedback: prohibición estricta de voseo"
   type: feedback
   ---
   Prohibido usar voseo en cualquier forma.

1. Start a session in Spanish and work normally on coding tasks, reviews, refactors, whatever. First several messages will respect the rule.
2. After extended interaction (many tool calls, long accumulated context, or a particularly long single response), observe that voseo forms (tenés, mirá, corregí as imperative, hacé, fijate, etc.) start appearing in the output.
3. The drift is inconsistent: some sentences remain in neutral Spanish, others flip to voseo within the same response.

The effect is most noticeable, and most costly, on long and expensive responses such as complex refactors or architecture analyses, which are exactly the responses the user is least willing to discard and re-generate.

Claude Model

Not sure / Multiple models

Is this a regression?

No, this never worked

Last Working Version

No response

Claude Code Version

2.1.116 (Claude Code)

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

When I'm running Opus at max capacity on a demanding prompt, an architecture review, a long refactor, a deep analysis, a single reply can easily consume around 40% of my usage window. When that reply comes back drifted into voseo, it isn't simply "written in a different style". The verb forms, pronouns and sentence structure don't map onto the dialect I actually read in, so the text stops functioning as readable Spanish for me. My brain has to stop on each sentence and re-derive who the subject is and whether a given verb is a past tense, a command, or a statement.

A rough English analogy would be handing a standard-English reader a long block of heavy Scots or Glaswegian: every word is technically English, but the conjugations, pronouns and idioms diverge enough that the passage stops parsing as English for that reader. The cost is comprehension, not aesthetics, and it lands on the most expensive responses of the session. The realistic options at that point are to regenerate the reply (burning another large chunk of the usage window on something that should have been right the first time) or to rewrite it by hand. Neither is great for an instruction layer that the documentation describes as authoritative.

Worth mentioning on the configuration side: the voseo prohibition isn't a single line tucked away hoping to be picked up. It's declared in three independent places, the global ~/.claude/CLAUDE.md, the project-level CLAUDE.md, and a persistent feedback memory entry, all active at the same time and all saying the same thing. The drift still happens, which is what makes me reasonably confident the problem isn't on the rule-authoring side. Whatever mechanism is deprioritizing these instructions later in the session is doing so regardless of how many layers restate them.