feat(agent-core): strengthen the language-matching rule in the default system prompt

[RealKai42]

Related Issue

No linked issue — the problem is explained below.

Problem

The default system prompt's language-matching rule (reply and think in the user's language) was a single paragraph buried at the end of the "Prompt and Tool Use" section. Mid-prompt instructions carry the least weight, and in practice the model tends to drift back to English in exactly one scenario the rule never named: returning to address the user after a long stretch of English tool output. The rule also lacked a boundary for repository artifacts, so strengthening it naively would risk Chinese commit messages and code comments in English repos.

What changed

All changes are in packages/agent-core/src/profile/default/system.md:

• Added a dedicated # Language section near the top of the prompt (right after the identity/role block). It pins the language to the user's most recent messages, follows mid-session switches, applies to replies / reasoning / progress notes / questions, and explicitly names the "after long English tool output" drift scenario.
• Added a carve-out in the same section: artifacts that go into the repository (code comments, commit messages, PR descriptions, documentation) follow the project's existing conventions, not the conversation language.
• Removed the old language paragraph from "Prompt and Tool Use" and dropped the duplicated "in the same language as the user" clause from the pre-tool-call narration sentence, so the rule lives in one place.
• Added a matching bullet to # Ultimate Reminders, using the head/tail positions of the prompt where instructions carry the most weight.

Verified with the full agent-core test suite (215 files, 3429 passed; 3 pre-existing expected failures). prompt-placeholders.test.ts guards template syntax only and is unaffected by this prose change.

Checklist

• I have read the CONTRIBUTING document.
• I have linked a related issue, or explained the problem above.
• I have added tests that prove my feature works.
• Ran gen-changesets skill, or this PR needs no changeset.
• Ran gen-docs skill, or this PR needs no doc update.

[changeset-bot]

🦋 Changeset detected

Latest commit: ee89b42

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[pkg-pr-new]

pnpm dlx https://pkg.pr.new/@moonshot-ai/kimi-code@38a9b14

npx https://pkg.pr.new/@moonshot-ai/kimi-code@38a9b14

commit: 38a9b14