Codex Skill for Codex

Codex Skill Guide

Running a Task

• Default to gpt-5.2 model. Ask the user (via AskUserQuestion) which reasoning effort to use (xhigh,high, medium, or low). User can override model if needed (see Model Options below).
• 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:
• -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
• 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 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.
• 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."

Safety Boundaries

• Do not run codex exec until the user has explicitly asked to use Codex for the task.
• Do not select --full-auto or a broader sandbox mode than the task requires.
• Do not overwrite user files, create commits, or perform destructive operations without explicit user approval.
• Do not use networked or danger-full-access runs unless the user requested work that actually needs them.

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-max | Max model: Ultra-complex reasoning, deep problem analysis | 400K input / 128K output | 76.3% SWE-bench, adaptive reasoning |

| gpt-5.2 ⭐ | Flagship model: Software engineering, agentic coding workflows | 400K input / 128K output | 76.3% SWE-bench, adaptive reasoning |

| gpt-5.2-mini | Cost-efficient coding (4x more usage allowance) | 400K input / 128K output | Near SOTA performance |

| gpt-5.1-thinking | Ultra-complex reasoning, deep problem analysis | 400K input / 128K output | Adaptive thinking depth, runs 2x slower on hardest tasks |

GPT-5.2 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 - Ultra-complex tasks (deep problem analysis, complex reasoning, deep understanding of the problem)
• 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: Cached context is substantially cheaper than first-pass context. Check current vendor pricing before quoting costs.

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.

CLI Version

Requires Codex CLI v0.57.0 or later for GPT-5.2 model support. The CLI defaults to gpt-5.2 on macOS/Linux and gpt-5.2 on Windows. Check version: codex --version

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