[MODEL] Opus 4.7 defaults to Argentine Spanish (voseo) from first turn, ignoring neutral-Spanish CLAUDE.md rules

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Type of Behavior Issue

Claude ignored my instructions or configuration

What You Asked Claude to Do

Any Spanish-language prompt in a session where CLAUDE.md (global and project-level) contains explicit rules requiring castellano neutro latinoamericano (neutral Latin American Spanish), using tú (tuteo), and forbidding voseo and Argentine regionalisms.

Example prompts: "explícame cómo funciona este módulo", "ayúdame a redactar un mensaje en español", "hazme un resumen de los cambios".

What Claude Actually Did

claude-opus-4-7 responds in Argentine Spanish from the very first turn, using:

• Voseo conjugation: vos tenés, vos sos, vos hacés, vos querés, vos podés
• Voseo imperatives: hacé, tené, andá, poné, mirá, fijate
• Argentine regionalisms: dale, che, boludo, pibe, laburo/laburar, quilombo, posta, copado, bárbaro, guita, acá (as default instead of aquí)
• Rioplatense register: informal tone inappropriate for professional/technical contexts

This happens on turn 1, before any context accumulation, with strict rules active in CLAUDE.md at multiple levels (global ~/.claude/CLAUDE.md, project ./CLAUDE.md) plus persistent feedback memory entries explicitly forbidding voseo.

Expected Behavior

With explicit CLAUDE.md rules enforcing castellano neutro, Claude should:

• Use tú for second person singular, never vos
• Conjugate in tuteo: tú tienes, tú haces, tú eres, tú puedes
• Use imperatives in tú form: haz, ten, ve, pon, mira
• Use neutral vocabulary: computadora, celular, auto, archivo, trabajar, genial
• Avoid regional slang from any country (Argentina, Spain, Mexico, etc.)
• Apply this behavior from the first response in the session, not after correction

The same ruleset works correctly on Sonnet 4.6, Opus 4.6, and Opus 4.1.

Files Affected

N/A — model output / behavior issue, not a file modification issue.

Permission Mode

I don't know / Not sure

Can You Reproduce This?

Yes, every time with the same prompt

Steps to Reproduce

1. Configure global ~/.claude/CLAUDE.md with an explicit neutral-Spanish rule, for example:

```markdown
## Spanish Style
When communicating in Spanish, use castellano neutro latinoamericano.

### Pronouns and conjugation

• Use tú for second person singular (tú eres, tú tienes, tú haces)
• NEVER use vos / voseo (vos sos, vos tenés, vos hacés)
• Imperatives in tú form: "haz", "ten", "ve", "pon" (NOT "hacé", "tené", "andá", "poné")

### Forbidden Argentine modismos
che, boludo, pibe, mina, laburo/laburar, quilombo, posta, re, copado, joya, dale, bárbaro, zarpado, fiaca, guita, morfar, acá (as default), allá (as default)
```

1. Start a fresh session with claude-opus-4-7 selected.
2. Send any Spanish-language prompt (e.g. "explícame cómo funciona este repo").
3. Observe that the first response contains voseo and/or Argentine regionalisms within the first few sentences.
4. Repeat with different prompts — behavior is consistent and reproducible.

Repro does not require long context, many tool calls, or compaction. It reproduces on turn 1.

Claude Model

Opus

(Specifically claude-opus-4-7 / Opus 4.7 with 1M context — the dropdown does not let me select a specific sub-version.)

Relevant Conversation

Happy to share sanitized transcripts on request. Pattern: any Spanish prompt yields voseo + regionalisms from the first response. Mid-conversation corrections produce 1–2 clean responses, then the model drifts back.

Impact

Medium - Extra work to undo changes

Output is technically usable but unsuitable for professional Spanish-language work aimed at a neutral Latin American audience (the majority of ~400M Spanish speakers). Every response must be post-edited or regenerated. For long/expensive Opus 4.7 responses (architecture analyses, refactor plans), regeneration wastes significant usage window.

Claude Code Version

2.1.119 (Claude Code)

Platform

Anthropic API

Additional Context

Relationship to #51686:

This issue is related to but distinct from #51686, which describes voseo leaking into neutral-Spanish output after extended context / long sessions (attention drift). That issue's repro is "works at first, drifts later."

This issue's repro is different: Opus 4.7 defaults to Argentine Spanish on turn 1 of a fresh session, even with strict rules active from the start. This suggests a post-training / RLHF weighting bias specific to Opus 4.7, rather than a context-degradation problem.

Evidence for a model-level baseline bias rather than context drift:

• Reproduces on turn 1 with no accumulated context.
• Same CLAUDE.md ruleset honored correctly on Sonnet 4.6, Opus 4.6, Opus 4.1.
• Increasing rule strength (capitals, "FORBIDDEN", "NEVER", multiple enforcement layers) does not change the baseline output.
• The mitigations proposed in #51686 (structured config, heartbeat re-injection, PreCompact checkpoints) address context drift but would not help if the baseline output distribution is already skewed toward Argentine Spanish before any drift occurs.

Hypothesis:

Opus 4.7's training data mix, RLHF annotator pool, or constitutional fine-tuning appears to weight Argentine Spanish as the default Spanish variant. Possible causes:

• Overrepresentation of Argentine Spanish in the post-training corpus
• RLHF annotator pool with Rioplatense baseline
• Misconfigured locale default treating "Spanish" as "es-AR"

Impact on user base:

Latin America has 20+ countries and ~400M Spanish speakers. Only ~45M are in the voseo region (Argentina + Uruguay + parts of Central America). Defaulting to Argentine Spanish alienates the majority of Spanish-speaking users and makes Opus 4.7 effectively unusable for professional work targeting a neutral LatAm audience.

Request:

• Confirm whether this is reproducible internally
• Confirm whether this is an intentional default for Opus 4.7 or a regression
• Provide guidance on any configuration that reliably forces neutral LatAm Spanish on Opus 4.7
• Fix in a future Opus 4.x release: "Spanish" should default to neutral, not regional Argentine