DeepSeek V4's `reasoning_content` channel inherits the system
prompt's English bias even when users write in Chinese, so the
visible thinking trace stays in English alongside (sometimes
mixed-language) replies.
Adds a `## Language` section near the top of `base.md` directing
the model to mirror the user's language in *both*
`reasoning_content` and the final reply, with a carve-out so
identifiers, file paths, tool names, and log lines stay in their
original form (translating `read_file` to `读取文件` would break
tool calls). Default remains English when no clear signal is
present, so existing behaviour is preserved.
Includes a structural test in `crates/tui/src/prompts.rs` that
asserts the section ships in every mode (Agent / Yolo / Plan).
Wording is intentionally not asserted on, per the existing test
module's "don't fail on prose" comment.
Reported via the project Telegram community (#588).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Hunter Bown
a239968f5b