[Bug] Model ignores Chilean Spanish tuteo instruction, drifts to Rioplatense voseo in long contexts
Bug Description
Title: Model drifts from instructed Chilean Spanish (tuteo) into Rioplatense voseo mid-session
Environment:
- Model: Opus 4.8 (claude-opus-4-8[1m])
- Surface: Claude Code CLI
- System prompt explicitly instructs: "In Spanish, use neutral Chilean Spanish with tuteo (tú). No voseo, no Rioplatense." — present in both ~/.claude/CLAUDE.md and the active output style.
Expected: All Spanish replies stay in neutral Chilean tuteo (tú / tienes / quieres) for the entire session, as instructed.
Actual: Replies start correctly in Chilean tuteo, then drift into Rioplatense voseo (vos / tenés / querés / dale / mirá) partway through the conversation. The drift correlates with longer contexts — the further the conversation
progresses, the more the explicit system-prompt instruction is ignored.
Impact: The user is Chilean. Being addressed in Argentine voseo despite an explicit, repeated instruction is jarring and reads as the model overriding a stated user preference. A negative instruction at the top of context is not holding
against the model's base tendency toward over-represented Rioplatense Spanish.
Repro:
- Set a system prompt / output style instructing neutral Chilean Spanish with tuteo, explicitly forbidding voseo/Rioplatense.
- Hold a multi-turn Spanish conversation of moderate length.
- Observe register shift from "tú/tienes" to "vos/tenés" without any user prompt change.
Suggested fix: Strengthen adherence to explicit regional-Spanish register instructions, particularly resistance to register drift over long contexts. The model should not fall back to a default Spanish variant when a specific one is
instructed.
Environment Info
- Platform: darwin
- Terminal: ghostty
- Version: 2.1.170
- Feedback ID: 827e40ed-930e-4a79-8cba-e2905726f9ca
Errors
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