GPT-5.3-Codex Now Available in OpenAI's Responses API

Melvin Vivas · X post · 2026-03-01 · Open on X

Topics: LLM Fundamentals, AI Agents, Tool Use & MCP · Level: intermediate

Summary

OpenAI made GPT-5.3-Codex available to all developers through the Responses API. The creator notes this means you can now build agents on top of the Codex model.

Key points

• GPT-5.3-Codex is available to all developers in the OpenAI Responses API.
• You can now build your own agents and coding tools on the Codex model.
• The model docs live on developers.openai.com.

Resources mentioned

• GPT-5.3-Codex model documentation · docs · developers.openai.com · free
  OpenAI developer docs for the GPT-5.3-Codex model.
• OpenAI Responses API · docs · platform.openai.com · check price
  OpenAI's API for building model-powered apps and agents with tool use.
  Also in: OpenAI DevDay 2026 Recap: Dots, Agents API, Codex Cloud & Marketplace (Melvin Vivas on X · notes)

Try this

• Try building an agent with GPT-5.3-Codex through the Responses API.
• Build a coding agent powered by GPT-5.3-Codex via the Responses API.