Codex

Codex Skill Guide

Running a Task

1. If the user did not specify a model or reasoning effort, use the installed Codex default or ask once with the available user-question mechanism. Do not hardcode a model list; model names change over time.
2. Select the sandbox mode required for the task; default to --sandbox read-only unless edits or network access are necessary.
3. Run codex --version first. Stop and report the failure if Codex is unavailable.
4. Assemble the command with the appropriate options:

• -m, --model
• --config model_reasoning_effort=""
• --sandbox ; only use other modes when codex exec --help lists them
• -C, --cd
• --add-dir
• --skip-git-repo-check
• --dangerously-bypass-approvals-and-sandbox

1. Do not use --skip-git-repo-check by default. Use it only when the user explicitly asks to run outside a Git repository or has approved that boundary bypass for this command.
2. When continuing a previous session, use codex exec resume --last via stdin. Do not add model, reasoning, or sandbox flags on resume unless the user explicitly requests an override.
3. IMPORTANT: By default, append 2>/dev/null to codex exec commands to suppress thinking tokens (stderr). Only show stderr if the user explicitly requests it or if debugging is needed.
4. Run the command, capture stdout/stderr (filtered as appropriate), and summarize the outcome for the user.
5. 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."

Safe Prompt Passing

Do not build Codex commands with echo "user prompt" | ...; user text can contain quotes, substitutions, or newlines. Prefer a quoted heredoc so the shell never reinterprets prompt contents:

codex exec resume --last 2>/dev/null /dev/null` |
| Apply local edits | `workspace-write` | `--sandbox workspace-write 2>/dev/null` |
| Apply edits that need network access | `workspace-write` plus config | `--sandbox workspace-write -c 'sandbox_workspace_write.network_access=true' 2>/dev/null` |
| Permit extra write scope | Prefer `--add-dir` | Ask before adding extra writable directories |
| Permit broad file access | `danger-full-access` only after approval | Ask before adding `--sandbox danger-full-access` |
| Resume recent session | Inherited from original | `codex exec resume --last 2>/dev/null ` plus other flags `2>/dev/null` |

## Following Up
- After every `codex` command, use the available user-question mechanism to confirm next steps, collect clarifications, or decide whether to resume with `codex exec resume --last`.
- When resuming, pass the new prompt through stdin using a quoted heredoc. 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 (`--sandbox danger-full-access`, `--dangerously-bypass-approvals-and-sandbox`, `--dangerously-bypass-hook-trust`, `--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`.

## Gotchas

- `--skip-git-repo-check` bypasses an important cwd/worktree guard. Treat it like a boundary exception, not a default.
- `danger-full-access` and the `--dangerously-*` bypass flags are high-impact modes. Prefer `read-only`, then `workspace-write`, then modes explicitly listed by the installed CLI, then specific `--add-dir` grants before considering full access.
- If a prompt came from the user or another model, pass it as stdin or as a single already-quoted CLI argument. Never interpolate it into a shell string.

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [majiayu000](https://github.com/majiayu000)
- **Source:** [majiayu000/spellbook](https://github.com/majiayu000/spellbook)
- **License:** MIT
- **Homepage:** https://github.com/majiayu000/spellbook#quick-start

Install and usage instructions live in the source repository linked above.