Use GPT 5.6 Luna Instead of Sol to Make Codex Limits Last

Melvin Vivas · X post · 2026-08-18 · Open on X

Topics: AI Dev Tools & Productivity, LLM Fundamentals · Level: beginner

Summary

The creator says GPT 5.6 Luna gives the best price-to-performance. He suggests using Luna instead of the larger Sol model in Codex so your usage limits last longer.

Key points

• The creator rates GPT 5.6 Luna best on price versus performance.
• Defaulting to Luna instead of Sol in Codex stretches your usage limits.
• General lesson: pick the cheapest model that is good enough for the task, not always the biggest one.

Resources mentioned

• GPT 5.6 Luna · tool · openai.com · paid
  The OpenAI model used inside Codex for the demo. The transcript gives the variant name as 'Soul', which is unclear.
  Also in: Set Codex subagent model and reasoning to save usage limits (Melvin Vivas on X · notes), Use GPT-5.6 Luna in Codex for Terminal Tasks (Melvin Vivas on X · notes), Match Reasoning Effort to Task Length in Codex (Astra/Sol) (Melvin Vivas on X · notes), Run Coworker desktop agents cheaply with GPT-5.6 Luna on OpenRouter (Melvin Vivas on X · notes) and 24 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

Try this

• Use GPT 5.6 Luna instead of Sol in Codex to stretch your usage limits.