Codex Skill Guide

Codex Skill Guide

Running a Task

• Ask the user (via `AskUserQuestion`) which model to run (`gpt-5.5`, `gpt-5.4`, `gpt-5.4-mini`, `gpt-5.3-codex-spark`, or `gpt-5.3-codex`) AND which reasoning effort to use (`xhigh`, `high`, `medium`, or `low`) in a **single prompt with two questions**.
• 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="<xhigh|high|medium|low>"`
• `--sandbox <read-only|workspace-write|danger-full-access>`
• `--full-auto`
• `-C, --cd <DIR>`
• `--skip-git-repo-check`
• `"your prompt here"` (as final positional argument)
• 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."

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.

Critical Evaluation of Codex Output

Codex is powered by OpenAI models with their own knowledge cutoffs and limitations. Treat Codex as a **colleague, not an authority**.

Guidelines

• **Trust your own knowledge** when confident. If Codex claims something you know is incorrect, push back directly.
• **Research disagreements** using WebSearch or documentation before accepting Codex's claims. Share findings with Codex via resume if needed.
• **Remember knowledge cutoffs** - Codex may not know about recent releases, APIs, or changes that occurred after its training data.
• **Don't defer blindly** - Codex can be wrong. Evaluate its suggestions critically, especially regarding:
• Model names and capabilities
• Recent library versions or API changes
• Best practices that may have evolved

When Codex is Wrong

• State your disagreement clearly to the user
• Provide evidence (your own knowledge, web search, docs)
• Optionally resume the Codex session to discuss the disagreement. **Identify yourself as Claude** so Codex knows it's a peer AI discussion. Use your actual model name (e.g., the model you are currently running as) instead of a hardcoded name:

   echo "This is Claude (<your current model name>) following up. I disagree with [X] because [evidence]. What's your take on this?" | codex exec --skip-git-repo-check resume --last 2>/dev/null

• Frame disagreements as discussions, not corrections - either AI could be wrong
• Let the user decide how to proceed if there's genuine ambiguity

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`.