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, orgpt-5.3-codex) AND which reasoning effort to use (xhigh,high,medium, orlow) in a single prompt with two questions. - Select the sandbox mode required for the task; default to
--sandbox read-onlyunless 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 --lastvia 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/nullto allcodex execcommands 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
codexcommand, immediately useAskUserQuestionto confirm next steps, collect clarifications, or decide whether to resume withcodex 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 --versionor acodex execcommand 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.