### Preflight Checklist - [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) 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? ## Description A Chinese token leaked into an otherwise-Vietnamese response. The model emitted 维护 ("maintain") instead of the Vietnamese word "duy trì".

Language-mixing: Chinese token leaks into Vietnamese output

Open 💬 0 comments Opened Jul 10, 2026 by vananhngo

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?

Description

A Chinese token leaked into an otherwise-Vietnamese response. The model
emitted 维护 ("maintain") instead of the Vietnamese word "duy trì".

Model

Opus 4.8 (1M context) — claude-opus-4-8[1m]

Environment

Claude Code — VSCode extension

Expected

Entire response in Vietnamese.

Actual

A single Chinese token appeared mid-sentence:
"...naming-convention.md là doc QA/Claude维护 thủ công..."
Expected: "...doc QA/Claude duy trì thủ công..."

Impact

Random language-mixing on common translatable tokens; makes Vietnamese
responses hard to read. Non-deterministic — hard to reproduce on demand.

What Should Happen?

Entire response in Vietnamese.

Error Messages/Logs

Steps to Reproduce

Use Claude Code (VSCode extension) with model Opus 4.8 (1M context).
Ask a question in Vietnamese and have the model answer in Vietnamese

(here: a question about a project doc, mixing Vietnamese prose with
English/technical terms like file names — "naming-convention.md",
"fetchData.js").

Observe the generated Vietnamese response.

Actual result

The model emitted a Chinese token 维护 ("maintain") mid-sentence instead of
the Vietnamese "duy trì":
"...naming-convention.md là doc QA/Claude维护 thủ công..."

Note on reproducibility

Non-deterministic — does not reproduce on every run. It appears to happen
on common translatable words when the sentence mixes Vietnamese prose with
English/technical tokens. No custom config or system prompt needed to
trigger; it surfaced during normal Vietnamese-language use.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Use Claude Code (VSCode extension) with model Opus 4.8 (1M context).

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

View original on GitHub ↗