Codex Exec Skill

Use OpenAI Codex CLI to execute development tasks that involve code modifications. This skill modifies code.

When to Use

• User asks to "add", "create", "implement", or "generate" code
• User wants to refactor existing code
• User needs to fix a bug
• User wants to create tests
• User asks to "update" or "modify" code

Prerequisites

Verify Codex CLI is available:

codex --version  # Should display installed version

Basic Usage

Step 1: Understand the Task

Parse what needs to be done:

• What to create/modify?
• Which files are affected?
• What's the expected outcome?
• Any constraints or requirements?

Step 2: Gather Context

Before executing, understand current state:

git status  # Check for uncommitted changes
git diff    # See existing modifications

Read relevant files to understand patterns.

Step 3: Execute Codex

Run Codex with clear instructions:

codex exec "[TASK]

Follow these guidelines:
- Follow existing code patterns and conventions
- Add appropriate error handling
- Include necessary imports
- Maintain code quality and readability
- Use TypeScript types where applicable
- Add comments for complex logic

Project context:
- Language: [e.g., TypeScript, Python, Go, Rust]
- Framework: [e.g., React, Django, Express]
- Build tool: [e.g., Vite, webpack, cargo, go build]
- Coding style: [Reference style guide or linter config]"

Step 4: Verify Changes

After execution:

git status              # See what changed
git diff                # Review modifications
<lint-command>          # Check for errors
<test-command>          # Run tests

Step 5: Report Results

Provide clear summary:

• What files were modified/created
• What changes were made
• Verification results (lint, types, tests)
• Any issues or warnings

Example Tasks

Generate New Code

codex exec "Create a UserProfile component in src/components/ with:
- Props: name (string), email (string), avatar (string optional)
- Display user info in a card layout
- Use TypeScript types
- Follow existing component patterns
- Add CSS modules for styling"

Refactor Code

codex exec "Refactor the validation logic in src/components/LoginForm.tsx:
- Extract validation into separate src/utils/validation.ts file
- Create validateEmail and validatePassword functions
- Maintain all existing functionality
- Add TypeScript types"

Fix Bugs

codex exec "Fix the memory leak in src/hooks/useWebSocket.ts:
- The WebSocket connection is not being cleaned up
- Add proper cleanup in useEffect return function
- Ensure connection closes on unmount"

Add Features

codex exec "Add email validation to ContactForm.tsx:
- Use regex: /^[^\s@]+@[^\s@]+\.[^\s@]+$/
- Show error message below input field
- Validate on blur and submit
- Prevent submission if invalid
- Style error message in red"

Create Tests

codex exec "Create unit tests for src/utils/validation.ts:
- Test validateEmail with valid/invalid inputs
- Test validatePassword with edge cases
- Test error messages
- Use Jest and React Testing Library
- Aim for 100% coverage"

Execution Modes

Safe Mode (Default)

codex exec "[TASK]"
# Prompts for approval before each action

Preview Mode

codex exec "[TASK]" --dry-run
# Shows what would be done without executing

Automated Mode ⚠️

codex exec "[TASK]" --yes
# Auto-approves all actions - use with caution!

Only use --yes for:

• Low-risk tasks (formatting, comments)
• Isolated environments
• Well-tested operations

Never use --yes for:

• Production code
• Security-sensitive changes
• Database operations
• File deletions

Verification Workflow

After Codex executes, ALWAYS:

1. Review changes:

   git diff
   

2. Run linter:

   <lint-command>
   

3. Run tests:

   <test-command>
   

4. Manual testing:

   Start your development server and test the functionality manually.

5. Report status:

   • ✅ Changes applied successfully
   • ✅ Linter passed
   • ✅ Tests passed
   • ✅ Manual testing confirmed

Best Practices

✅ DO:

• Be specific in task descriptions
• Include file paths
• Specify expected behavior
• Review all changes with git diff
• Run linter and tests
• Test manually before declaring success

❌ DON'T:

• Use vague instructions like "make it better"
• Skip verification steps
• Use --yes for critical code
• Commit without reviewing
• Ignore errors or warnings

Error Handling

If execution fails:

1. Check the error message
2. Verify Codex authentication
3. Try with more specific instructions
4. Break task into smaller steps

If changes are incorrect:

1. Revert: git restore . or git restore <file>
2. Re-execute with better instructions
3. Or fix manually

If tests fail:

1. Review what broke
2. Ask Codex to fix the test failures
3. Or fix manually

Writing Effective Tasks

Good Task Description

"Add email validation to ContactForm.tsx using regex pattern.
Show error message below the input field on blur.
Prevent form submission if email is invalid.
Style the error message in red (#dc2626).
Follow existing form validation patterns."

Poor Task Description

"Add validation"  # Too vague
"Fix the form"    # No specifics

Safety Checklist

Before using codex-exec:

• Understand what will be modified
• Have clean git state (can rollback)
• Know which files will be affected
• Have tests to verify correctness

After using codex-exec:

• Reviewed changes with git diff
• Ran linter (language-specific)
• Ran type checker (if applicable)
• Ran tests (language-specific)
• Tested manually
• Ready to commit

Related Skills

• codex-ask: For understanding code before modifying
• codex-review: For reviewing changes after modification

Tips for Success

1. Start small: Test with simple tasks first
2. Be specific: Include file paths, requirements, constraints
3. Follow patterns: Reference existing code to follow
4. Verify always: Never skip the verification workflow
5. Iterate: Use codex-ask → codex-exec → codex-review cycle

Limitations

• Cannot run code or execute tests itself
• May not understand complex business logic
• Limited by context window size
• Requires manual verification
• Cannot guarantee correctness

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Remember: This skill MODIFIES code. Always review changes before committing!