Agent Skills: Codex CLI

Codex CLI

Run OpenAI Codex CLI locally for second-opinion audits, code review, and non-interactive task execution.

Prerequisites

Codex CLI must be installed and authenticated:

1. Install: npm install -g @openai/codex
2. Auth: codex login
3. Verify: codex --version

Core Execution Pattern

Use codex exec for delegated prompts (non-interactive):

codex exec "Your prompt here"

When the user says "codex prompt", treat it as:

codex exec "<user prompt>"

Model Guidance

Prefer gpt-5.4 unless the user asks for a different model.

Default reasoning effort is medium. Only escalate when explicitly needed:

• Use medium for most coding, reviews, refactors, second-opinion audits, and one-off tasks
• Use high only for: repeated/failing tasks that need deeper analysis, complex multi-step planning, or when the user explicitly asks for deeper reasoning

codex exec -c 'model="gpt-5.4"' -c 'model_reasoning_effort="medium"' "Your prompt"

Fallback note:

• If gpt-5.4 is unavailable, fall back to the newest available GPT-5 or Codex model
• Keep reasoning effort explicit for reproducible behavior

Critical: Argument Compatibility Rules

These rules prevent CLI errors. Follow them exactly.

codex exec

• -m flag works: codex exec -m gpt-5.4 "prompt"
• -c config overrides work: codex exec -c 'model="gpt-5.4"' "prompt"
• Stdin with - replaces the prompt — do NOT pass both - and a quoted prompt string
• Correct stdin: cat file.txt | codex exec -s read-only -c 'model="gpt-5.4"' -
• WRONG: cat file.txt | codex exec -s read-only - "Some extra prompt" (two positional args)

codex review

• -m flag does NOT work with codex review — use -c 'model="gpt-5.4"' instead
• --commit <SHA> and [PROMPT] are MUTUALLY EXCLUSIVE — cannot combine them
• --base <BRANCH> and [PROMPT] CAN be combined
• --uncommitted and [PROMPT] CAN be combined
• WRONG: codex review --commit abc123 "Review for security" (will error)
• Correct: codex review --commit abc123 -c 'model="gpt-5.4"'
• Correct: codex review --base main "Focus on security"

Workaround for reviewing commits with custom instructions

Since --commit cannot take a prompt, pipe the diff to codex exec instead:

git diff <SHA>~1..<SHA> > /tmp/diff.txt && cat /tmp/diff.txt | codex exec -s read-only -c 'model="gpt-5.4"' -c 'model_reasoning_effort="medium"' -

This is the recommended pattern for reviewing specific commits with custom review instructions.

Commands

Non-Interactive Execution

# Basic task (default model + medium effort)
codex exec -c 'model="gpt-5.4"' -c 'model_reasoning_effort="medium"' "Audit this logic for edge cases"

# Full-auto mode (sandboxed, lower friction)
codex exec --full-auto -c 'model="gpt-5.4"' -c 'model_reasoning_effort="medium"' "Implement the requested refactor"

# Read-only sandbox (analysis only)
codex exec -s read-only -c 'model="gpt-5.4"' -c 'model_reasoning_effort="medium"' "Find bugs in this code path"

# Workspace-write sandbox
codex exec -s workspace-write -c 'model="gpt-5.4"' -c 'model_reasoning_effort="medium"' "Apply the fix and update tests"

# Custom working directory
codex exec -C /path/to/project -c 'model="gpt-5.4"' "Evaluate this repository"

# Save final output to file
codex exec -o output.txt -c 'model="gpt-5.4"' "Summarize key risks"

# Pipe context from stdin (no additional prompt argument!)
cat context.txt | codex exec -s read-only -c 'model="gpt-5.4"' -c 'model_reasoning_effort="medium"' -

# Pipe diff for commit review with custom instructions
git diff HEAD~1..HEAD > /tmp/diff.txt && cat /tmp/diff.txt | codex exec -s read-only -c 'model="gpt-5.4"' -c 'model_reasoning_effort="medium"' -

Code Review

Use codex review for repository diffs. Model must be set via -c, not -m.

# Review uncommitted changes
codex review --uncommitted -c 'model="gpt-5.4"' -c 'model_reasoning_effort="medium"'

# Review against a base branch
codex review --base main -c 'model="gpt-5.4"' -c 'model_reasoning_effort="medium"'

# Review a specific commit (NO prompt allowed with --commit)
codex review --commit abc123 -c 'model="gpt-5.4"' -c 'model_reasoning_effort="medium"'

# Custom review instructions (only with --base or --uncommitted, NOT --commit)
codex review --uncommitted -c 'model="gpt-5.4"' "Focus on security issues"
codex review --base main -c 'model="gpt-5.4"' "Check for performance regressions"

Important Flag Placement

--search and -a/--ask-for-approval are top-level flags. Put them before exec or review.

Correct:

codex --search -a on-request exec "Your prompt"
codex --search -a on-request review --uncommitted

WRONG:

codex exec --search "Your prompt"
codex exec -a on-request "Your prompt"

Useful Flags

| Flag | Description | |------|-------------| | -c 'model="gpt-5.4"' | Set model (works everywhere, preferred over -m) | | -m | Model shorthand (works with exec only, NOT review) | | -s | Sandbox: read-only, workspace-write, danger-full-access | | -a | Approval policy (top-level flag, before subcommand) | | -C | Working directory | | -o | Write last message to file | | --full-auto | Sandboxed auto-execution (-a on-request -s workspace-write) | | --json | JSONL event output | | --search | Enable web search tool (top-level flag) | | --add-dir | Additional writable directories | | -c key=value | Override any config value |

Best Practices

• Default to medium reasoning effort — it covers most use cases well
• Only escalate to high for repeated failures, complex planning, or explicit user request
• Prefer codex exec for delegated prompts instead of interactive codex
• Start with -s read-only for audits and second opinions
• Use --full-auto only when you expect autonomous edits
• Always use -c 'model="gpt-5.4"' (not -m) for codex review commands
• For commit reviews with custom instructions, pipe git diff to codex exec -
• Keep prompts explicit about expected output format
• Add -o when another tool or agent must consume the result
• Run codex review --uncommitted before committing as a quick extra pass
• Run background tasks with run_in_background and check output via file path