AI DevelopmentSimon Willison

Use subagents and custom agents in Codex

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“Subagents pattern is now widely supported across coding agents - from OpenAI Codex to Claude Code to Cursor, with custom agents definable as TOML files”

Key takeaways

  • OpenAI Codex launched subagents in general availability with default agents for 'explorer', 'worker', and 'default' - similar to Claude Code's implementation
  • Custom agents can be defined as TOML files in ~/.codex/agents/ with custom instructions and specific model assignments (including gpt-5.3-codex-spark for speed)
  • Subagents pattern has achieved cross-platform standardization - now supported by OpenAI Codex, Claude Code, Gemini CLI, Mistral Vibe, OpenCode, VS Code, and Cursor, signaling architectural convergence in AI coding tools
  • The pattern enables specialized agent orchestration (e.g., 'browser_debugger' reproduces issues, 'code_mapper' traces paths, 'ui_fixer' implements fixes) for complex debugging workflows
  • Platform convergence suggests subagents are becoming the standard architecture for agentic coding workflows, similar to how REST APIs became standard for web services

Why this matters for operators: Engineering teams evaluating AI coding assistants; understanding emerging standards in agentic workflows

I cover AI×GTM intelligence like this every Wednesday.

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This analysis was produced using the STEEPWORKS system — the same agents, skills, and knowledge architecture available in the GrowthOS package.