chore(ai): add AGENTS.md (#8555)

.github/copilot-instructions.md

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+# copilot-instructions.md
+
+This file provides guidance to Copilot Agent when working with code in this repository.
+
+## Codebase Overview
+
+Vitest is a next-generation testing framework powered by Vite. This is a monorepo using pnpm workspaces.
+
+## Essential references
+
+- Agent-specific guide: See [AGENTS.md](../AGENTS.md)

AGENTS.md

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+# Vitest AI Agent Guide
+
+This document provides comprehensive information for AI agents working on the Vitest codebase.
+
+## Project Overview
+
+Vitest is a next-generation testing framework powered by Vite. This is a monorepo using pnpm workspaces with the following key characteristics:
+
+- **Language**: TypeScript/JavaScript (ESM-first)
+- **Package Manager**: pnpm (required)
+- **Node Version**: ^18.0.0 || >=20.0.0
+- **Build System**: Vite + Rollup
+- **Monorepo Structure**: 15+ packages in `packages/` directory
+
+## Setup and Development
+
+### Initial Setup
+1. Run `pnpm install` to install dependencies
+2. Run `pnpm build` to build all packages
+3. Install Playwright browsers when working with browser features: `npx playwright install --with-deps`
+
+### Key Scripts
+- `pnpm build` - Build all packages
+- `pnpm dev` - Watch mode for development
+- `pnpm lint` - Run ESLint
+- `pnpm lint:fix` - Fix linting issues automatically
+- `pnpm typecheck` - Run TypeScript type checking
+
+## Testing
+
+### Running Tests
+- **All tests**: `CI=true pnpm test:ci`
+- **Examples**: `CI=true pnpm test:examples`
+- **Specific test suite**: `CI=true cd test/<test-folder> && pnpm test <test-file>`
+- **Core directory test**: `CI=true pnpm test <test-file>` (for `test/core`)
+- **Browser tests**: `CI=true pnpm test:browser:playwright` or `CI=true pnpm test:browser:webdriverio`
+
+### Testing Utilities
+- **`runInlineTests`** from `test/test-utils/index.ts` - You must use this for complex file system setups (>1 file)
+- **`runVitest`** from `test/test-utils/index.ts` - You can use this to run Vitest programmatically
+- **No mocking policy** - You must never mock anything in tests
+
+## Project Structure
+
+### Core Packages (`packages/`)
+- `vitest` - Main testing framework
+- `vite-node` - Vite SSR runtime
+- `browser` - Browser testing support
+- `ui` - Web UI for test results
+- `runner` - Test runner core
+- `expect` - Assertion library
+- `spy` - Mocking and spying utilities
+- `snapshot` - Snapshot testing
+- `coverage-v8` / `coverage-istanbul` - Code coverage
+- `utils` - Shared utilities
+- `mocker` - Module mocking
+
+### Test Organization (`test/`)
+- `test/core` - Core functionality tests
+- `test/browser` - Browser-specific tests
+- Various test suites organized by feature
+
+### Important Directories
+- `docs/` - Documentation (Vite-powered)
+- `examples/` - Example projects and integrations
+- `scripts/` - Build and development scripts
+- `.github/` - GitHub Actions workflows
+- `patches/` - Package patches via pnpm
+
+## Code Style and Conventions
+
+### Formatting and Linting
+- **Always run** `pnpm lint:fix` after making changes
+- Fix non-auto-fixable errors manually
+
+### TypeScript
+- Strict TypeScript configuration
+- Use `pnpm typecheck` to verify types
+- Configuration files: `tsconfig.base.json`, `tsconfig.build.json`, `tsconfig.check.json`
+
+### Code Quality
+- ESM-first approach
+- Follow existing patterns in the codebase
+- Use utilities from `@vitest/utils/*` when available. Never import from `@vitest/utils` main entry point directly.
+
+## Common Workflows
+
+### Adding New Features
+1. Identify the appropriate package in `packages/`
+2. Follow existing code patterns
+3. Add tests using testing utilities
+4. Run `pnpm build && pnpm typecheck && pnpm lint:fix`
+5. Add tests with relevant test suites
+
+### Debugging
+- Use VS Code: `⇧⌘B` (Shift+Cmd+B) or `Ctrl+Shift+B` for dev tasks
+- Check `scripts/` directory for specialized development tools
+
+### Documentation
+- Main docs in `docs/` directory
+- Built with `pnpm docs:build`
+- Local dev server: `pnpm docs`
+
+## Dependencies and Tools
+
+### Key Dependencies
+- **Vite** - Build tool and dev server
+- **Rollup** - Bundler
+- **ESLint** - Linting
+- **TypeScript** - Type checking
+- **Playwright** - Browser testing
+- **Chai/Expect** - Assertions
+- **Tinypool** - Worker threading
+- **Tinybench** - Benchmarking
+
+### Development Tools
+- **tsx** - TypeScript execution
+- **ni/nr** - Package manager abstraction
+- **bumpp** - Version bumping
+- **changelogithub** - Changelog generation
+
+## Browser Testing
+- Two modes: Playwright and WebDriverIO
+- Separate test commands for each
+- Component testing supported (Vue, React, Svelte, Lit, Marko)
+
+## Performance Considerations
+- This is a performance-critical testing framework
+- Pay attention to import costs and bundle size
+- Use lazy loading where appropriate
+- Consider worker thread implications
+
+## Troubleshooting
+
+### Common Issues
+- Ensure pnpm is used (not npm/yarn)
+- Build before running tests
+- Check Node.js version compatibility
+- Playwright browsers must be installed for browser tests
+
+### Getting Help
+- Check existing issues and documentation
+- Review CONTRIBUTING.md for detailed guidelines
+- Follow patterns in existing code

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Codebase Overview
+
+Vitest is a next-generation testing framework powered by Vite. This is a monorepo using pnpm workspaces.
+
+## Essential references
+
+- Agent-specific guide: See [AGENTS.md](AGENTS.md)