Aider vs OpenAI Codex CLIUpdated 2026-03-16

SUMMARY

Aider (CLI) uses markdown configuration while OpenAI Codex CLI (CLI) uses markdown. They differ on 2 of 11 compared features. Both tools provide AI-assisted development but take different approaches to project configuration.

FEATURE COMPARISON

FeatureAiderOpenAI Codex CLI
Config formatmarkdownmarkdown
Hierarchy support
Global config
Project config
Subdirectory scoping
File inclusion / imports
Ignore file.aiderignore.codexignore
IDE integrationCLICLI
Schema / structureFree-form (Markdown)Free-form (Markdown)
Git committed
EncodingUTF-8UTF-8

FILE MAPPING

PurposeAiderOpenAI Codex CLI
Settings / configuration.aider.conf.yml~/.codex/config.toml
Ignore patterns.aiderignore.codexignore
Project instructionsCONVENTIONS.mdAGENTS.md

SIDE-BY-SIDE CODE SAMPLES

Aider
CONVENTIONS.md
markdown
# Project Conventions

## Language
- TypeScript strict mode, no `any` types
- Prefer const over let, never use var
- Use template literals over string concatenation

## Architecture
- Feature-based folder structure under src/features/
- Each feature exports through an index.ts barrel file
- Shared utilities go in src/lib/

## Error Handling
- Use Result<T, E> pattern for expected failures
- Throw only for programmer errors
- Always log errors with structured context

## Testing
- Colocate tests next to source files (*.test.ts)
- Use factories for test data, not fixtures
- Integration tests in __tests__/ directories

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
```

KEY DIFFERENCES

1. Hierarchy support

Aider: Not supported. OpenAI Codex CLI: Supported.

2. Ignore file

Aider: .aiderignore. OpenAI Codex CLI: .codexignore.

WHICH SHOULD I USE?

Recommendation

Choose based on your IDE/CLI preference. If you use Aider, configure .aider.conf.yml. If you use OpenAI Codex CLI, configure AGENTS.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

agentconfig.ing2026-03-16COMPARE(1)