Codex

Codex Skill Guide

Running a Task

1. For a new session (resumes inherit the prior model/effort — see step 5), ask the user (via AskUserQuestion) which model AND which reasoning effort to use, in a single prompt with two questions. When the user expresses no preference, default to gpt-6-astra at high.
   • Model — default gpt-6-astra:
     • GPT-6: gpt-6-astra (frontier / most capable — default)
     • GPT-5.6: gpt-5.6-sol (reliable agentic workhorse), gpt-5.6-terra (balanced, everyday), gpt-5.6-luna (fast & affordable)
     • Legacy (kept for compatibility): gpt-5.5, gpt-5.4, gpt-5.4-mini, gpt-5.3-codex-spark, gpt-5.3-codex
   • Reasoning effort — default high: low, medium, high, xhigh, max, ultra.
     • max/ultra require a GPT-6 or GPT-5.6 model; ultra is only on astra/sol/terra (luna caps at max); legacy models cap at xhigh.
     • ultra = maximum reasoning with automatic task delegation (slowest and most expensive — reserve for the hardest jobs).
     • If the chosen effort exceeds the chosen model's maximum, fall back to that model's highest supported effort and tell the user.
2. Select the sandbox mode required for the task; default to --sandbox read-only unless edits or network access are necessary.
3. Assemble the command with the appropriate options:
   • -m, --model <MODEL>
   • --config model_reasoning_effort="<low|medium|high|xhigh|max|ultra>" (max/ultra only on GPT-6 / GPT-5.6 models; ultra only on astra/sol/terra — see step 1)
   • --sandbox <read-only|workspace-write|danger-full-access>
   • --full-auto
   • -C, --cd <DIR>
   • --skip-git-repo-check
   • "your prompt here" (as final positional argument)
4. Always use --skip-git-repo-check.
5. When continuing a previous session, use codex exec --skip-git-repo-check resume --last via stdin. When resuming don't use any configuration flags unless explicitly requested by the user e.g. if he species the model or the reasoning effort when requesting to resume a session. Resume syntax: echo "your prompt here" | codex exec --skip-git-repo-check resume --last 2>/dev/null. All flags have to be inserted between exec and resume.
6. IMPORTANT: By default, append 2>/dev/null to all codex exec commands to suppress thinking tokens (stderr). Only show stderr if the user explicitly requests to see thinking tokens or if debugging is needed.
7. IMPORTANT (stdin): codex exec always reads stdin and concatenates it with the positional prompt -- even when the prompt is fully supplied as a positional argument. If stdin is not closed, codex blocks forever. When invoking from a harness (background tasks, hooks, scripts where stdin is not a TTY but also not closed), explicitly redirect stdin: append </dev/null to the command, e.g. codex exec ... "prompt" </dev/null 2>/dev/null. Symptom of getting this wrong: zero bytes of stdout, zero CPU accumulated, process appears hung indefinitely.
8. Run the command, capture stdout/stderr (filtered as appropriate), and summarize the outcome for the user.
9. After Codex completes, inform the user: "You can resume this Codex session at any time by saying 'codex resume' or asking me to continue with additional analysis or changes."

Quick Reference

Execution timeouts

Codex produces no intermediate output — it writes the result only at completion. If the process is killed before finishing, the output file is silently empty (no error).

Preferred approach: run synchronously — eliminates timeout risk entirely and the conversation waits for the result anyway.

If running in background, set the execution timeout based on reasoning effort: