GPT-5.6 Luna at Highest Reasoning Handled a Release in Codex

Melvin Vivas · X post · 2026-08-31 · Open on X

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

Summary

A field report: Codex with GPT-5.6 Luna set to the highest reasoning effort did well on the creator's JSONL viewer release, and it didn't use up the 5-hour usage limit.

Key points

• GPT-5.6 Luna at the highest reasoning setting handled real release work (the JSONL viewer v1.3.0) well.
• The work stayed within Codex's 5-hour usage limit.
• Running a lighter model at high reasoning effort can be a cost-efficient alternative to a heavier model.

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
• donvito/jsonl-viewer · repo · github.com · free
  Open-source tool for viewing and editing .jsonl files (fine-tuning datasets) and agent traces from Codex, Claude Code, Pi and Hermes.
  Also in: jsonl-viewer: Live Traces for Codex Orchestrator and Subagents (Melvin Vivas on X · notes), See live Codex traces with the jsonl-viewer tool (Melvin Vivas on X · notes), Melvin Vivas's open-source AI tools, built with Codex (Melvin Vivas on X · notes), Analyzing Codex Traces with jsonl-viewer (Melvin Vivas on X · notes) and 12 more

Try this

• Try GPT-5.6 Luna at the highest reasoning effort in Codex before switching to a heavier model.