Automatically generated Chat title randomly mixes CJK characters into an English title

- VS Code Version: 1.136.0-insider
- Commit: 6b83849594a181ba0dcadea99844b6f1f42bbcc9
- GitHub Copilot Chat Version: 0.64.2026090105
- OS: Windows 11, connected to WSL
- Remote OS: Ubuntu 26.04
- UI language: English
- Remote locale: C.UTF-8
- Feature (e.g. agent/edit/ask mode):: agent
- Selected model (e.g. GPT 4.1, Claude 3.7 Sonnet): GPT-5.6 Sol (but gpt-4o-mini-2024-07-18 is being used to generate the buggy titles).

Steps to Reproduce:

NOTE: These are my repro steps, which probably won't repro with your because it does not include the workspace instruction files, but I also checked all 38 applicable instruction files and this session's five serialized system-prompt/tool files; none contained CJK characters before the malformed title was generated.

1. Open a new Chat session in VS Code Insiders.
2. Enter this English prompt:

`do you know why my CMake Build queue is backlogged with 9 builds? I something wrong going on or can something be improved?`

3. Wait for VS Code/Copilot to assign the automatic title.

Expected:

The generated title should remain in the language of the prompt, for example:

`CMake build queue backlog cause`

Actual:

The generated title was:

`CMake build queue backlog原因`

`原因` means "cause" or "reason" in Chinese and Japanese. It is semantically
appropriate, but unexpectedly switches languages within an otherwise English
title.

Additional information:

- The title was generated automatically and was not manually edited.
- The prompt and visible conversation were entirely in English.
- VS Code's UI language was English, and the WSL locale was `C.UTF-8`.
- I have observed this once and have not yet established consistent reproduction.

This appears to be a second occurrence of [#321853](https://github.com/microsoft/vscode/issues/321853), including the exact same
unexpected suffix, `原因`.

I attempted to export the Language Model Request from
`Developer: Show Chat Debug View`. Unfortunately, the title request had already
been evicted from the view. The request logger retains 100 entries, and 156
distinct requests were recorded after this title request.

The following correlation metadata remains in the GitHub Copilot Chat output:

- Backend request ID: `4da56004-c962-444a-b773-26f38bc1b806`
- Local Debug View request ID: `f36446b7`
- Timestamp: `2026-09-01T23:27:31.640Z`
- Model: `gpt-4o-mini-2024-07-18`
- Debug name: `title`
- Result: `success`
- Duration: `666 ms`

The complete request/response export is no longer locally recoverable, but the
backend request ID above may allow it to be located in service logs.