Codex Skill Guide

Workflow

1. Confirm task mode:
   • New run: use codex exec.
   • Continue prior run: use codex exec ... resume --last with stdin prompt.
2. Set defaults unless user overrides:
   • Model: gpt-5.4.
   • Reasoning effort: ask user to choose xhigh, high, medium, or low.
   • Sandbox: read-only unless edits/network are required.
3. Build command with required flags:
   • Always include --skip-git-repo-check.
   • Add 2>/dev/null by default to suppress thinking tokens on stderr.
   • Show stderr only if user asks or debugging is needed.
4. Run command, summarize outcome, and ask what to do next.
5. After completion, remind user they can continue with codex resume.

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

Command Patterns

New run

codex exec --skip-git-repo-check \
  --model gpt-5.4 \
  --config model_reasoning_effort="high" \
  --sandbox read-only \
  "your prompt here" 2>/dev/null

Resume latest session

Use stdin and keep flags between exec and resume.

echo "your prompt here" | codex exec --skip-git-repo-check resume --last 2>/dev/null

When resuming, do not add configuration flags unless the user explicitly asks for changes (for example, different model or reasoning effort).

Model Options

| Model | Best for | Context window | Key features | | -------------- | ---------------------------------------------- | ----------------- | --------------------------------------------------------- | | gpt-5.4 ⭐ | Default for most coding tasks in Codex | N/A in this skill | OpenAI's recommended default for general-purpose coding | | gpt-5.4-pro | Harder problems that benefit from more compute | N/A in this skill | More compute for deeper reasoning on difficult tasks | | gpt-5-mini | Faster/cost-effective option for lighter tasks | N/A in this skill | Smaller GPT-5 model for lower-cost coding and chat tasks | | gpt-5.3-codex | Legacy specialized alternative | N/A in this skill | Prior Codex-tuned model; generally superseded by GPT-5.4 |

gpt-5.4 is the default for software engineering tasks.

Reasoning Effort

• 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)

Following Up

• After every run, ask for next steps or clarifications.
• When proposing another run, restate model, reasoning effort, and sandbox mode.
• For continuation, use stdin with resume --last.

Error Handling

• If codex --version or codex exec exits non-zero, report failure and ask before retrying.
• Ask permission before high-impact flags unless already granted: --full-auto, --sandbox danger-full-access, --skip-git-repo-check.
• If output includes warnings or partial results, summarize and ask how to proceed.

CLI Version

Use a current Codex CLI version that supports gpt-5.4. Check with:

codex --version

Use /model inside Codex to switch models, or set defaults in ~/.codex/config.toml.