Agent Skills: Codex Skill Guide

Codex Skill Guide

Running a Task

1. Default to gpt-5.1-codex model. Ask the user (via AskUserQuestion) which reasoning effort to use (high, medium, or low). User can override model if needed (see Model Options below).
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="<high|medium|low>"
   • --sandbox <read-only|workspace-write|danger-full-access>
   • --full-auto
   • -C, --cd <DIR>
   • --skip-git-repo-check
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. Run the command, capture stdout/stderr (filtered as appropriate), and summarize the outcome for the user.
8. 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 |

Model Options

| Model | Best for | Context window | Key features | | --- | --- | --- | --- | | gpt-5.2-codex ⭐⭐ | Code review flagship: xhigh reasoning, security analysis, architecture review | 400K input / 128K output | 79% SWE-bench Pro, best for reviews | | gpt-5.1-codex ⭐ | Flagship model: Software engineering, agentic coding workflows | 400K input / 128K output | 76.3% SWE-bench, adaptive reasoning, $1.25/$10.00 | | gpt-5.1-codex-mini | Cost-efficient coding (4x more usage allowance) | 400K input / 128K output | Near SOTA performance, $0.25/$2.00 | | gpt-5.1-thinking | Ultra-complex reasoning, deep problem analysis | 400K input / 128K output | Adaptive thinking depth, runs 2x slower on hardest tasks |

GPT-5.1-Codex Advantages: 76.3% SWE-bench (vs 72.8% GPT-5), 30% faster on average tasks, better tool handling, reduced hallucinations, improved code quality. Knowledge cutoff: September 30, 2024.

Reasoning Effort Levels:

• xhigh - Maximum quality (code review, security analysis, architecture review) - requires gpt-5.2-codex
• high - Complex tasks (refactoring, architecture, security analysis, performance optimization)
• medium - Standard tasks (refactoring, code organization, feature additions, bug fixes)
• low - Simple tasks (quick fixes, simple changes, code formatting, documentation)

Cached Input Discount: 90% off ($0.125/M tokens) for repeated context, cache lasts up to 24 hours.

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.

Code Review Mode

For automated code reviews with maximum quality, use gpt-5.2-codex with xhigh reasoning:

Review Command Pattern

codex exec --skip-git-repo-check \
  -m gpt-5.2-codex \
  -c model_reasoning_effort="xhigh" \
  --sandbox read-only \
  --full-auto \
  "[review prompt with diff]" 2>/dev/null

Review Output Format

Structure findings with priority levels:

• P0 - Critical: Security vulnerabilities, data loss, crashes
• P1 - High: Logic errors, significant bugs, performance issues
• P2 - Medium: Code quality, maintainability concerns
• P3 - Low: Style, minor improvements, suggestions

Each finding should include:

• Title (max 80 chars)
• File path and line range
• Confidence score (0-1)
• Detailed explanation

End with overall verdict: "patch is correct" or "patch is incorrect" with justification.

CLI Version

Requires Codex CLI v0.57.0 or later for GPT-5.1 model support. The CLI defaults to gpt-5.1-codex on macOS/Linux and gpt-5.1 on Windows. Check version: codex --version

Use /model slash command within a Codex session to switch models, or configure default in ~/.codex/config.toml.