Codex-Claude Engineering Loop Skill

Core Workflow Philosophy

This skill implements a balanced engineering loop:

• Claude Code: Architecture, planning, and execution
• Codex: Validation and code review
• Continuous Review: Each AI reviews the other's work
• Context Handoff: Always continue with whoever last cleaned up

Phase 1: Planning with Claude Code

1. Start by creating a detailed plan for the task
2. Break down the implementation into clear steps
3. Document assumptions and potential issues
4. Output the plan in a structured format

Phase 2: Plan Validation with Codex

1. Ask user (via AskUserQuestion):
   • Model: gpt-5 or gpt-5-codex
   • Reasoning effort: low, medium, or high
2. Send the plan to Codex for validation:

   echo "Review this implementation plan and identify any issues:
   [Claude's plan here]
   
   Check for:
   - Logic errors
   - Missing edge cases
   - Architecture flaws
   - Security concerns" | codex exec -m  --config model_reasoning_effort="" --sandbox read-only

3. Capture Codex's feedback

Phase 3: Feedback Loop

If Codex finds issues:

1. Summarize Codex's concerns to the user
2. Refine the plan based on feedback
3. Ask user (via AskUserQuestion): "Should I revise the plan and re-validate, or proceed with fixes?"
4. Repeat Phase 2 if needed

Phase 4: Execution

Once the plan is validated:

1. Claude implements the code using available tools (Edit, Write, Read, etc.)
2. Break down implementation into manageable steps
3. Execute each step carefully with proper error handling
4. Document what was implemented

Phase 5: Cross-Review After Changes

After every change:

1. Send Claude's implementation to Codex for review:
   • Bug detection
   • Performance issues
   • Best practices validation
   • Security vulnerabilities
2. Claude analyzes Codex's feedback and decides:
   • Apply fixes immediately if issues are critical
   • Discuss with user if architectural changes needed
   • Document decisions made

Phase 6: Iterative Improvement

1. After Codex review, Claude applies necessary fixes
2. For significant changes, send back to Codex for re-validation
3. Continue the loop until code quality standards are met
4. Use codex exec resume --last to continue validation sessions:

   echo "Review the updated implementation" | codex exec resume --last

Note: Resume inherits all settings (model, reasoning, sandbox) from original session

Recovery When Issues Are Found

When Codex identifies problems:

1. Claude analyzes the root cause
2. Implements fixes using available tools
3. Sends updated code back to Codex for verification
4. Repeats until validation passes

When implementation errors occur:

1. Claude reviews the error/issue
2. Adjusts implementation strategy
3. Re-validates with Codex before proceeding

Best Practices

• Always validate plans before execution
• Never skip cross-review after changes
• Maintain clear handoff between AIs
• Document who did what for context
• Use resume to preserve session state

Command Reference

Phase	Command Pattern	Purpose
Validate plan	echo "plan" | codex exec --sandbox read-only	Check logic before coding
Implement	Claude uses Edit/Write/Read tools	Claude implements the validated plan
Review code	echo "review changes" | codex exec --sandbox read-only	Codex validates Claude's implementation
Continue review	echo "next step" | codex exec resume --last	Continue validation session
Apply fixes	Claude uses Edit/Write tools	Claude fixes issues found by Codex
Re-validate	echo "verify fixes" | codex exec resume --last	Codex re-checks after fixes

Error Handling

1. Stop on non-zero exit codes from Codex
2. Summarize Codex feedback and ask for direction via AskUserQuestion
3. Before implementing changes, confirm approach with user if:
   • Significant architectural changes needed
   • Multiple files will be affected
   • Breaking changes are required
4. When Codex warnings appear, Claude evaluates severity and decides next steps

The Perfect Loop

Plan (Claude) → Validate Plan (Codex) → Feedback →
Implement (Claude) → Review Code (Codex) →
Fix Issues (Claude) → Re-validate (Codex) → Repeat until perfect

This creates a self-correcting, high-quality engineering system where:

• Claude handles all code implementation and modifications
• Codex provides validation, review, and quality assurance