# Language / i18n Configuration

Closes: [Issue #173](https://github.com/Chachamaru127/claude-code-harness/issues/173) — English version / how to set the global output language.

User-facing responses (skill outputs, slash commands, hook messages) follow the explicit session or project language. **When no language is configured, the default is English.** Use Japanese only when explicitly requested via one of the three mechanisms below.

This file is the SSOT. `CLAUDE.md` Language section is a short pointer to here. The README links here under "Language".

---

## How to switch language

There are three ways to set the language. **Resolution precedence, highest first:**

1. A per-message session instruction (you explicitly ask the agent to switch).
2. The project config `.claude-code-harness.config.yaml` (`i18n.language`).
3. The `CLAUDE_CODE_HARNESS_LANG` env var.

If none are set, the default is `en`. Note that the project config takes
priority over the env var: when `i18n.language` is present it wins, and
`CLAUDE_CODE_HARNESS_LANG` only applies as a fallback for projects that do not
set it.

### 1. Project-level config — `.claude-code-harness.config.yaml`

For a per-project setting that persists across sessions, set the language in the harness config file the runtime reads, `.claude-code-harness.config.yaml`:

```yaml
i18n:
  language: en   # en | ja (default: en)
```

This applies to every Claude Code session run inside the project, and the language directive is re-injected on every turn so responses stay in the configured language. Commit this file to make the choice repository-wide.

> Note: `harness.toml` is the v4/v5 build source consumed by `harness sync`; it does **not** currently carry the runtime language setting. Set `i18n.language` in `.claude-code-harness.config.yaml`.

### 2. Per-session env var — `CLAUDE_CODE_HARNESS_LANG`

For a project that does **not** pin `i18n.language`, export the env var before launching Claude Code to choose the language for that session:

```bash
export CLAUDE_CODE_HARNESS_LANG=ja
claude            # this session uses Japanese
```

Or inline:

```bash
CLAUDE_CODE_HARNESS_LANG=ja claude
```

> If the project config sets `i18n.language`, it wins over this env var. To
> override a config-pinned language for a single session, use a per-message
> session instruction (below).

### 3. Per-message session instruction

For ad-hoc switching mid-session, tell the agent in your own words:

```
> 以後は日本語で返答してください
```

```
> Please respond in English from now on.
```

This wins over both `.claude-code-harness.config.yaml` and `CLAUDE_CODE_HARNESS_LANG` for the rest of the session.

---

## What does NOT change

- Machine-readable values (JSON schema fields, contract verdicts, log keys) stay English regardless of the language setting.
- Source code identifiers, file names, commit hashes, command names stay as-is.
- Conventional Commits prefixes (`feat:` / `fix:` / `docs:` ...) stay English.

If you see Japanese in a place that should be machine-readable (e.g. a JSON contract field), please open an Issue — that's a bug, not an i18n setting question.

---

## Supported languages

| Code | Status |
|---|---|
| `en` | Supported (default) |
| `ja` | Supported |
| other (`zh`, `ko`, `de`, etc.) | Not officially supported — session instruction can request best-effort, but skill outputs may include English fallbacks |

---

## Related

- `CLAUDE.md` § Language (short pointer to this file)
- `README.md` § Language (link to this file)
- `.claude-code-harness.config.yaml` (`i18n.language`, read by the runtime and hooks)