OpenAI Codex CLI vs GitHub CopilotComparisonUpdated 2026-03-16
SUMMARY
OpenAI Codex CLI (CLI) uses markdown configuration while GitHub Copilot (IDE) uses markdown. They differ on 3 of 11 compared features. Both tools provide AI-assisted development but take different approaches to project configuration.
FEATURE COMPARISON
| Feature | OpenAI Codex CLI | GitHub Copilot |
|---|---|---|
| Config format | markdown | markdown |
| Hierarchy support | ✓ | ✓ |
| Global config | ✓ | ✗ |
| Project config | ✓ | ✓ |
| Subdirectory scoping | ✓ | ✓ |
| File inclusion / imports | ✗ | ✗ |
| Ignore file | .codexignore | ✗ |
| IDE integration | CLI | ✓ |
| Schema / structure | Free-form (Markdown) | Free-form (Markdown) |
| Git committed | ✓ | ✓ |
| Encoding | UTF-8 | UTF-8 |
FILE MAPPING
| Purpose | OpenAI Codex CLI | GitHub Copilot |
|---|---|---|
| Project instructions | AGENTS.md | .github/copilot-instructions.md |
| Project instructions | AGENTS.override.md | .github/instructions/*.instructions.md |
SIDE-BY-SIDE CODE SAMPLES
OpenAI Codex CLI
AGENTS.md
markdown
# AGENTS.md
## Project Overview
This is a Next.js 14 e-commerce application using the App Router,
Server Components, and Drizzle ORM with PostgreSQL.
## Code Conventions
- Use TypeScript strict mode throughout
- Prefer Server Components; use "use client" only when necessary
- Name files with kebab-case: `product-card.tsx`, not `ProductCard.tsx`
- Co-locate tests: `product-card.test.tsx` next to `product-card.tsx`
## Architecture
- `app/` — Next.js App Router pages and layouts
- `components/` — Shared UI components
- `lib/` — Utilities, database client, auth helpers
- `actions/` — Server Actions for mutations
## Testing
- Use Vitest for unit tests, Playwright for E2E
- Mock database calls using `lib/test-utils.ts` helpers
- Every new component needs at least one test
## Build Commands
```bash
pnpm dev # Start dev server
pnpm build # Production build
pnpm test # Run Vitest
pnpm test:e2e # Run Playwright
pnpm lint # ESLint check
```
GitHub Copilot
.github/copilot-instructions.md
markdown
# Copilot Instructions
## Code Style
- Use TypeScript strict mode for all new files
- Prefer functional components with hooks over class components
- Use named exports, not default exports
## Libraries
- Use date-fns for date manipulation (not moment.js)
- Use zod for runtime validation
- Prefer fetch over axios for HTTP requests
## Testing
- Write tests using Vitest
- Use Testing Library for component tests
- Aim for integration tests over unit tests
## Conventions
- Use camelCase for variables, PascalCase for components
- Prefix interfaces with I (e.g., IUserProps)
- Keep files under 300 lines; extract if longer
KEY DIFFERENCES
OpenAI Codex CLI: Supported. GitHub Copilot: Not supported.
OpenAI Codex CLI: .codexignore. GitHub Copilot: Not supported.
OpenAI Codex CLI: CLI. GitHub Copilot: Supported.
WHICH SHOULD I USE?
Choose based on your IDE/CLI preference. If you use OpenAI Codex CLI, configure AGENTS.md. If you use GitHub Copilot, configure .github/copilot-instructions.md. Many projects include config files for multiple tools so each team member can use their preferred editor.
PORTABILITY TIP
sync-instructions.sh
bash
# Maintain a single source of truth:
cp AI-INSTRUCTIONS.md CLAUDE.md
cp AI-INSTRUCTIONS.md .cursorrules
cp AI-INSTRUCTIONS.md AGENTS.md
SEE ALSO
Tool pages:
File references:
agentconfig.ing2026-03-16COMPARE(1)