Codex CLI

Codex CLI Integration Skill

This skill enables Claude Code to invoke OpenAI Codex CLI (gpt-5.3-codex with high reasoning) for one-shot code review, approach validation, and cross-AI verification.

When to Use This Skill

Ideal Use Cases

1. Second Opinion / Cross-Validation

   • Verify your implementation approach before writing code
   • Get a different AI perspective on your solution
   • Validate architectural decisions

2. Code Review

   • Review uncommitted changes before commit
   • Review changes against a base branch before PR
   • Security-focused code review

3. Task Verification

   • Confirm implementation meets requirements
   • Validate that refactoring preserved behavior
   • Check for missed edge cases

4. Approach Validation

   • Before starting complex implementation
   • When choosing between multiple approaches
   • For architectural decisions

When NOT to Use

• Simple, quick tasks (overhead not worth 1-10 min wait)
• Tasks requiring interactive conversation/refinement
• When immediate response is critical
• Trivial changes (typos, formatting)

Core Commands

Approach Validation

codex exec "Review this approach: [description]. Is it sound? What are the tradeoffs?" -m gpt-5.3-codex --full-auto -o /tmp/codex-result.md > /dev/null 2>&1 && echo "Codex completed"
# Then: Read tool /tmp/codex-result.md

Code Review - Uncommitted Changes

codex exec review --uncommitted -m gpt-5.3-codex --full-auto > /tmp/codex-review.md 2>&1
# Then: Read tool /tmp/codex-review.md

Code Review - Against Branch

codex exec review --base main -m gpt-5.3-codex --full-auto > /tmp/codex-review.md 2>&1
# Then: Read tool /tmp/codex-review.md

Custom Analysis

codex exec "[prompt]" -m gpt-5.3-codex --full-auto -o /tmp/codex-result.md > /dev/null 2>&1 && echo "Codex completed"
# Then: Read tool /tmp/codex-result.md

Important Notes

1. One-Shot Only: Codex runs non-interactively. No follow-up questions possible.

2. Takes Time: Expect 1-10 minutes depending on complexity. Use run_in_background for long tasks.

3. Model: Always use -m gpt-5.3-codex for explicit model selection.

4. Full Auto: --full-auto enables workspace-write sandbox with auto-approval.

5. Output File: Use -o /tmp/codex-result.md to capture just the final response.

6. CRITICAL - Token Optimization: Always redirect stdout to /dev/null and read from file. See "Output Handling" below.

Output Handling Best Practice

Why this matters: Without optimization, Bash returns ~4700+ tokens of verbose output (reasoning steps, tool calls, MCP logs). With optimization, you get ~30 tokens + clean file read.

For codex exec (custom prompts)

# Run with output file + suppress verbose stdout
codex exec "prompt" -m gpt-5.3-codex --full-auto -o /tmp/codex-result.md > /dev/null 2>&1 && echo "Codex completed"

Then read result with Read tool (not cat): /tmp/codex-result.md

For codex exec review (no -o flag support)

# Redirect all output to file
codex exec review --uncommitted -m gpt-5.3-codex --full-auto > /tmp/codex-review.md 2>&1

Then read result with Read tool: /tmp/codex-review.md

Workflow

1. Run codex command with output redirection
2. Read result file with Read tool (not Bash cat)
3. Summarize findings to user

Critical: Provide File Paths in Prompts

Codex has NO context from Claude Code conversation. Always include explicit file paths in your prompts:

For Task Review

codex exec "Review the task specification at tasks/task-2026-01-09-feature/tech-decomposition.md
Is the implementation plan complete? Any gaps or risks?" -m gpt-5.3-codex --full-auto -o /tmp/codex-result.md > /dev/null 2>&1
# Read: /tmp/codex-result.md

For Code Review with Context

codex exec "Review implementation in these files:
- backend/src/application/sessions/use-cases/create-session.use-case.ts
- backend/src/infrastructure/web/dto/sessions/create-session.dto.ts

Check against requirements in: tasks/task-2026-01-09-feature/tech-decomposition.md" -m gpt-5.3-codex --full-auto -o /tmp/codex-result.md > /dev/null 2>&1
# Read: /tmp/codex-result.md

For Approach Validation

codex exec "I'm implementing the feature described in tasks/task-2026-01-09-feature/tech-decomposition.md

My approach:
1. [Step 1]
2. [Step 2]

Is this aligned with the requirements?" -m gpt-5.3-codex --full-auto -o /tmp/codex-result.md > /dev/null 2>&1
# Read: /tmp/codex-result.md

Best Practice

Always include:

• Task file path - so Codex knows the requirements
• Implementation file paths - so Codex knows what to review
• Directory path - for broader context (e.g., backend/src/application/sessions/)

Background Execution Pattern

For tasks that take longer, run in background:

# Run in background using Bash tool with run_in_background=true
codex exec "[complex prompt]" -m gpt-5.3-codex --full-auto -o /tmp/codex-result.md > /dev/null 2>&1 && echo "Codex completed"

Then read result with Read tool: /tmp/codex-result.md

Example Workflows

Pre-Implementation Validation

codex exec "I'm about to implement [feature] using [approach].
Review this plan:
1. [Step 1]
2. [Step 2]
3. [Step 3]

Is this approach sound? What issues might I encounter?" -m gpt-5.3-codex --full-auto -o /tmp/codex-result.md > /dev/null 2>&1
# Read: /tmp/codex-result.md

Security Review

# Built-in review command (redirect to file)
codex exec review --uncommitted -m gpt-5.3-codex --full-auto > /tmp/codex-review.md 2>&1
# Read: /tmp/codex-review.md

# Or with custom focus:
codex exec "Review the uncommitted changes for security vulnerabilities including XSS, injection, auth issues" -m gpt-5.3-codex --full-auto -o /tmp/codex-result.md > /dev/null 2>&1
# Read: /tmp/codex-result.md

Cross-AI Verification

codex exec "I implemented [feature]. The key files are:
- path/to/file1.ts
- path/to/file2.ts

Verify the implementation is correct and complete." -m gpt-5.3-codex --full-auto -o /tmp/codex-result.md > /dev/null 2>&1

## What's New in Codex CLI 0.96.0

Highlights from the latest CLI release:
- `unified_exec` enabled on non-Windows platforms for smoother exec flows.
- Websocket rate-limit signaling via the new `codex.rate_limits` event.
- `thread/compact` available in the v2 app-server API with async behavior and status tracking.

Source: https://developers.openai.com/codex/changelog
# Read: /tmp/codex-result.md

See Also

• templates.md - Prompt templates for common operations
• reference.md - Complete command and flag reference