# Codex

> Invokes Codex CLI for code analysis, refactoring, or automated editing. Use when the user asks to run codex exec, codex resume, or references OpenAI Codex. Use when this capability is needed.

- Skill: `tomevault-io/codex-8` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds@latest add tomevault-io/codex-8`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tomevault-io/codex-8/raw
- Safety review: pending (external: skill-scanner PASS, skillspector WARNING)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: tomevault-io (https://skillmd.com/u/tomevault-io)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/tomevault-io/codex-8

---


# Codex Skill Guide

## When to Use Codex
- **Tricky Debugging**: Exceptional at finding elusive bugs that are hard to track down (mystery bugs, race conditions, edge cases)
- **Security Analysis**: Industry-leading vulnerability discovery - found zero-day CVEs in production frameworks, autonomous patch generation
- **Code Review**: Comprehensive security-focused code reviews, identifying vulnerabilities and anti-patterns
- **Complex Refactoring**: Large-scale code transformations with deep understanding of codebase context
- **Agentic Coding**: Multi-step autonomous software engineering tasks

<instructions>

## Running a Task

1. Use `gpt-5.3-codex` model. Ask the user (via `AskUserQuestion`) which reasoning effort to use (`xhigh`, `high`, `medium`, or `low`). Default to `medium` if the user does not specify.
2. Select the sandbox mode required for the task; default to `--sandbox read-only` unless edits or network access are necessary.
3. 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` (include after confirming with user on first use per session)
4. When continuing a previous session, use `codex exec --skip-git-repo-check resume --last` via stdin. When resuming, only add configuration flags if explicitly requested by the user (e.g., different model or reasoning effort). Resume syntax: `echo "your prompt here" | codex exec [flags] resume --last 2>/dev/null`. Flags must be placed between `exec` and `resume`.
5. **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.
6. Run the command, capture stdout/stderr (filtered as appropriate), and summarize the outcome for the user.
7. **After Codex edits files**, verify changes before proceeding:
   - Run `git diff` to review modifications
   - Run tests if applicable (`npm test`, `pytest`, etc.)
   - Only commit or continue after validation passes
8. **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."

### Task Checklist

```
- [ ] 1. Select model (default: gpt-5.3-codex) and reasoning effort
- [ ] 2. Select sandbox mode (default: read-only)
- [ ] 3. Assemble command with flags
- [ ] 4. Get permission for high-impact flags (if --full-auto or danger-full-access)
- [ ] 5. Run command with 2>/dev/null
- [ ] 6. Summarize outcome
- [ ] 7. Verify changes (git diff, tests) if edits made
- [ ] 8. Inform user about resume option
```

</instructions>

### 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 [flags] resume --last 2>/dev/null` (flags between exec/resume) |
| Run from another directory | Match task needs | `-C <DIR>` plus other flags `2>/dev/null` |

## Examples

<example>
**User**: "Review this file for security vulnerabilities"

**Claude assembles**:
```bash
codex exec --skip-git-repo-check -m gpt-5.3-codex --config model_reasoning_effort="high" --sandbox read-only 2>/dev/null
```

**After completion**: "Analysis complete. Found 2 potential SQL injection vulnerabilities in `db/queries.ts`. You can resume this Codex session at any time by saying 'codex resume' or asking me to continue with additional analysis."
</example>

<example>
**User**: "Fix the race condition bug in the worker pool"

**Claude assembles**:
```bash
codex exec --skip-git-repo-check -m gpt-5.3-codex --config model_reasoning_effort="high" --sandbox workspace-write --full-auto 2>/dev/null
```

**After edits**: Runs `git diff` to show changes, runs `npm test` to verify fix, then: "Fixed the race condition by adding mutex locks. Tests pass. You can resume this session with 'codex resume'."
</example>

<example>
**User**: "Continue analyzing that code" (after previous session)

**Claude assembles**:
```bash
echo "Continue the security analysis, focusing on authentication flows" | codex exec --skip-git-repo-check resume --last 2>/dev/null
```

**Note**: No model/sandbox flags needed—inherited from original session.
</example>

## Reasoning Effort Levels

Model: `gpt-5.3-codex` (400K input / 128K output). Check [Codex releases](https://github.com/openai/codex/releases) for current pricing and benchmarks.

| Reasoning | Best for |
| --- | --- |
| `xhigh` | Zero-day vulnerability discovery, deep architecture analysis, multi-hour agentic tasks |
| `high` | Security analysis, complex refactoring, performance optimisation, debugging race conditions |
| `medium` (default) | Feature additions, bug fixes, code review, standard refactoring |
| `low` | Quick fixes, formatting, documentation, simple changes |

Cached input tokens receive a significant discount. Repeated context within 24 hours benefits from this automatically.

## 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 a recent Codex CLI version for gpt-5.3-codex model support (check: `codex --version`). See [Codex releases](https://github.com/openai/codex/releases) for latest. The CLI defaults to `gpt-5.3-codex` on all platforms.

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

---
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<!-- tomevault:4.0:skill_md:2026-04-13 -->