Open-source coding agents: Grok Build, Codex, and Claude Code

Melvin Vivas · X post · 2026-07-16 · Open on X

Topics: AI Dev Tools & Productivity, AI Agents, Tool Use & MCP · Level: intermediate

Summary

The creator points out that two major coding-agent harnesses, xAI's Grok Build and OpenAI's Codex, are now open source, while Anthropic's Claude Code is not. You can read the code of open-source agent harnesses to see how coding agents are built.

Key points

• Grok Build (xAI's coding agent) is open source.
• OpenAI Codex (the CLI coding agent) is open source.
• Claude Code is not open source yet.
• You can study open-source harnesses to learn how coding agents handle tool use, planning and editing.

Resources mentioned

• Grok Build · tool · x.ai · free
  xAI's agentic coding CLI; data sharing is on by default and can be turned off with /privacy.
  Also in: Setting up Grok Bot for coding with Grok Build and Cursor CLI (Melvin Vivas on X · notes), Reusable GPU devbox: PyTorch/CUDA plus six coding agents (Melvin Vivas on X · notes), Handing off an unfinished Codex session to Grok Build (Melvin Vivas on X · notes), Grok Build finishes a task Codex left hanging (Melvin Vivas on X · notes) and 10 more
• OpenAI Codex · tool · openai.com · paid
  OpenAI's coding agent. In the diagram it writes code, fixes review findings and drives the build loop. The creator also used it to make this video.
  Also in: An agent bot that installs and drives Codex on its own (Melvin Vivas on X · notes), Asking a Coder bot to install Codex (Melvin Vivas on X · notes), Sign in with ChatGPT: Setting Usage Limits for Each App (Melvin Vivas on X · notes), Codex Cloud Environments Must Be Saved & Published Before Use (Melvin Vivas on X · notes) and 240 more
• Claude Code · tool · code.claude.com · paid · recommended by both Bashiri Smith & Melvin Vivas
  Build agents and pipelines from the terminal; the guide's main agentic coding tool.
  Also in: Create Claude Code Plugins with /plugin-authoring (Melvin Vivas on X · notes), Claude Code mods: customize behavior and UI with plugins (Melvin Vivas on X · notes), AI Engineer Roadmap Overview: From ML Foundations to RAG, Agents & Ops (Bashiri Smith on Facebook · notes), SkillsBento: Free Plugin Marketplace for Codex and Claude Code (Melvin Vivas on X · notes) and 101 more