Using Codex (GPT 5.6 Luna High) for Dataset and Evals Work

Melvin Vivas · X post · 2026-08-27 · Open on X

Topics: AI Dev Tools & Productivity, Evaluation (Evals) & Testing · Level: beginner

Summary

Melvin Vivas finds that Codex with GPT 5.6 Luna on high reasoning effort is good enough to help with dataset and evals work, which is mostly simple tasks. The 5-hour usage limit goes a long way: after 2 hours of use he was at 94%, with 2h 45m left before the reset.

Key points

• For simple dataset and evals tasks, a mid-tier setup (Codex with GPT 5.6 Luna, high effort) is enough.
• Codex resets usage every 5 hours; 2 hours of work used 94% of the window.
• Use cheaper models for routine evals chores.

Resources mentioned

• 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
• 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