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AGENTS.md

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+# AGENTS.md - Agent Coding Guidelines
+
+## Project Overview
+
+This is a pseudo-localization web application (a demo for testing internationalization). It runs entirely in the browser with vanilla JavaScript, HTML, and CSS. No build system or package manager is required.
+
+## Development Commands
+
+### Running the Application Locally
+
+```bash
+# Start a local HTTP server (Python 3)
+python3 -m http.server 8000
+
+# Then open http://localhost:8000 in your browser
+```
+
+### Code Quality Checks
+
+All checks run via Python scripts in the `tests/` directory:
+
+```bash
+# i18n translation quality check
+python3 tests/check_i18n.py
+
+# JSON syntax and duplicate key validation
+python3 tests/check_json.py
+
+# Check files end with newline
+python3 tests/check_trailing_newline.py
+
+# JavaScript linting via Biome (auto-downloads on first run)
+python3 tests/check_biome.py
+
+# Run all checks
+python3 tests/check_i18n.py && python3 tests/check_json.py && python3 tests/check_trailing_newline.py && python3 tests/check_biome.py
+```
+
+### Biome Linting (Direct)
+
+```bash
+# Install biome first, then run:
+biome lint src/
+```
+
+Note: There is no test runner for unit tests. The "tests" directory contains validation/check scripts only.
+
+## Code Style Guidelines
+
+### General Principles
+
+- This is a vanilla JavaScript project (no frameworks, no build step)
+- Use ES modules (`import`/`export` syntax)
+- Keep code simple and readable
+
+### File Organization
+
+- Source files go in `src/` directory
+- Entry point is `index.html`
+- Data files (i18n, character mappings) in `data/`
+- Check scripts in `tests/`
+
+### Imports
+
+```javascript
+// Use relative imports with .js extension
+import { state, loadHistory } from "./state.js";
+import { $ } from "./dom.js";
+
+// Group imports logically, one per line
+import { loadI18n, applyLanguage } from "./i18n.js";
+```
+
+### Naming Conventions
+
+- **Variables/functions**: camelCase (e.g., `loadI18n`, `charLib`)
+- **Constants**: SCREAMING_SNAKE_CASE (e.g., `LANGUAGE_NAMES_FALLBACK`)
+- **DOM elements**: Use descriptive names, avoid single letters
+- **File names**: kebab-case (e.g., `check_i18n.py`, `app.js`)
+
+### Functions
+
+- Use async/await for asynchronous operations
+- Keep functions focused and small
+- Use meaningful parameter names
+- Add no comments unless explaining complex logic
+
+```javascript
+// Good: async function with descriptive name
+export async function loadI18n() {
+  const basePath = window.BASE_PATH || './';
+  const response = await fetch(`${basePath}data/i18n.json`);
+  state.i18nData = await response.json();
+}
+
+// Good: early returns for clarity
+function getLangFallback(langCode) {
+  if (state.i18nData[langCode]) {
+    return langCode;
+  }
+  // ... more logic
+}
+```
+
+### Error Handling
+
+- Use try/catch for potentially failing operations
+- Provide fallback values when appropriate
+- Log errors appropriately (console.warn/error)
+
+```javascript
+try {
+  const data = await fetch(url).then(r => r.json());
+} catch (_e) {
+  return defaultValue; // Use _e convention for unused catch variables
+}
+```
+
+### State Management
+
+- Use a centralized state object (see `src/state.js`)
+- Keep state mutations predictable
+- Use `getState()` for safe state access when needed
+
+### DOM Manipulation
+
+- Use the `$()` helper from `src/dom.js` for element selection
+- Prefer `textContent` over `innerHTML` for text updates
+- Use `innerHTML` only when inserting dynamic HTML strings
+
+### UI Components (MWC - Material Web Components)
+
+- **Prefer MWC components**: The project loads `@material/web`, so prefer MWC components over custom HTML elements
+- Available components include:
+  - `md-text-button` - Text button
+  - `md-filled-button` - Filled button
+  - `md-outlined-button` - Outlined button
+  - `md-filled-text-field` - Text field input
+  - `md-radio` - Radio button
+  - `md-checkbox` - Checkbox
+  - `md-switch` - Switch
+  - `md-slider` - Slider
+  - `material-icons` - Icon font
+- Only consider custom implementation when MWC components cannot meet the requirements
+
+### Data Handling
+
+- JSON files in `data/` must be valid JSON
+- i18n keys must be consistent across all language files
+- Check translations with `tests/check_i18n.py` before committing
+
+### String Handling
+
+- Use template literals for string concatenation
+- Prefer string interpolation over concatenation
+
+### Linting Rules (Biome)
+
+The project uses Biome with these enabled rules:
+- `recommended`: true
+- `style/useTemplate`: warn (prefer template literals)
+- `style/useConst`: warn (prefer const)
+- `complexity/useLiteralKeys`: warn
+- `correctness/noUnusedImports`: warn
+- `correctness/noUnusedVariables`: warn
+- `correctness/useParseIntRadix`: warn
+- `suspicious/useIterableCallbackReturn`: warn
+
+### Git Workflow
+
+- All checks in `.github/workflows/check-code.yml` must pass before PR merge
+- Checks run automatically on pull requests
+- Use descriptive commit messages
+
+### Adding New Features
+
+1. Update `data/i18n.json` with new translation keys if needed
+2. Add new JavaScript logic in appropriate `src/` module
+3. Run all check scripts before committing
+4. Ensure no regressions in existing functionality
+
+### Common Patterns
+
+```javascript
+// Module pattern - export functions at end or inline
+export function myFunction() { }
+
+// State access pattern
+import { state, getState } from "./state.js";
+
+// DOM query pattern
+import { $ } from "./dom.js";
+const element = $("my-element-id");
+```
+
+### File Extensions
+
+- JavaScript: `.js`
+- HTML: `.html`
+- CSS: `.css`
+- JSON: `.json`
+- Python check scripts: `.py`
+
+### HTTP Server Requirements
+
+The app uses `window.BASE_PATH` for path configuration. When serving locally:
+```javascript
+// In browser console or HTML before loading scripts:
+window.BASE_PATH = './';
+```
+
+---
+
+For questions, refer to README.md or examine existing source files in `src/`.