Codex Agent Skill

Codex Skill Guide

Running a Task

• Do NOT specify a model by default. The Codex CLI is configured with a ChatGPT account, and explicit model flags (-m gpt-5-codex, -m gpt-5, -m o4-mini) all fail with "not supported when using Codex with a ChatGPT account." Omitting -m lets Codex use its default model, which works. Only add -m if the user explicitly requests a specific model.
• Select the sandbox mode required for the task; default to --sandbox read-only unless edits or network access are necessary.
• Assemble the command with the appropriate options:
• --sandbox <read-only|workspace-write|danger-full-access>
• --full-auto
• -C, --cd <DIR>
• --skip-git-repo-check
• -m, --model <MODEL> (only if user explicitly requests)
• --config model_reasoning_effort="<high|medium|low>" (only if user explicitly requests)
• Always use --skip-git-repo-check.
• 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. 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.
• 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.
• Run the command, capture stdout/stderr (filtered as appropriate), and summarize the outcome for the user.
• 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

| Use case | Sandbox mode | Key flags |

| --- | --- | --- |

| Read-only review or analysis | read-only | --sandbox read-only 2>/dev/null |

| Apply local edits | workspace-write | --sandbox workspace-write --full-auto 2>/dev/null |

| Permit network or broad access | danger-full-access | --sandbox danger-full-access --full-auto 2>/dev/null |

| Resume recent session | Inherited from original | echo "prompt" \| codex exec --skip-git-repo-check resume --last 2>/dev/null (no flags allowed) |

| Run from another directory | Match task needs | -C <DIR> plus other flags 2>/dev/null |

Following Up

• After every codex command, immediately use AskUserQuestion to confirm next steps, collect clarifications, or decide whether to resume with codex exec resume --last.
• When resuming, pipe the new prompt via stdin: echo "new prompt" | codex exec resume --last 2>/dev/null. The resumed session automatically uses the same model, reasoning effort, and sandbox mode from the original session.
• Restate the chosen model, reasoning effort, and sandbox mode when proposing follow-up actions.

Error Handling

• Stop and report failures whenever codex --version or a codex exec command exits non-zero; request direction before retrying.
• Before you use high-impact flags (--full-auto, --sandbox danger-full-access, --skip-git-repo-check) ask the user for permission using AskUserQuestion unless it was already given.
• When output includes warnings or partial results, summarize them and ask how to adjust using AskUserQuestion.