Codex CLI
OpenAI's open-source coding agent that runs in the terminal, released in April 2025.
- Developer
- OpenAI
- Released
- 2025-04all 2025 milestones
- Instructions file
- AGENTS.md
Connected
- OpenAIAmerican artificial intelligence research organization
- AGENTS.mdAn open, tool-neutral Markdown convention for giving coding agents project instructions, introduced by OpenAI in August 2025 and now stewarded by the Linux Foundation's Agentic AI Foundation.
- OpenAI Codexcode-generating large language model developed by OpenAI
- Claude CodeAnthropic's agentic coding tool that reads a codebase, edits files, runs commands and verifies its work from the terminal, IDE or web.
Linked from
More agent tooling & protocols
- agent-html-kitAn open-source template kit and Claude Code skill (kit.js) that lets agents emit content-only HTML components while design lives in one shared file, published with a 72-run HTML vs Markdown benchmark.
- CLAUDE.mdA Markdown file of standing instructions that Claude Code loads at the start of every session, at user, project and folder level.
- Karpathy-inspired CLAUDE.md rulesA widely starred public CLAUDE.md with four behaviour rules: think before coding, simplicity first, surgical changes and goal-driven execution.
- Claude Code hooksShell commands that Claude Code runs automatically at lifecycle events such as before or after a tool call, so rules can be enforced by code rather than requested in prose.
- Claude Code subagentsSpecialised agents with their own instructions, tools and context window that the main Claude Code session can delegate tasks to.
- Model Context Protocol (MCP)An open protocol, introduced by Anthropic in November 2024, that standardises how AI applications connect to external tools and data sources.