AI Coding Agents Skill

Expert knowledge for Codex CLI and Claude Code CLI — the two leading AI coding agents.

Note: This skill documents both tools for reference. VMark development primarily uses Claude Code CLI. The Codex CLI sections are retained for completeness and cross-tool workflows.

When to Use

• Orchestrating complex coding tasks via CLI

• Configuring MCP servers for either tool

• Setting up automation pipelines (CI/CD)

• Troubleshooting authentication or sandbox issues

• Comparing capabilities between agents

• Custom agent/subagent configuration

Quick Reference

Starting Sessions

Task Codex CLI Claude Code CLI

Interactive session codex

claude

With prompt codex "fix the bug"

claude "fix the bug"

Non-interactive codex exec "task"

claude -p "task"

Resume last codex resume --last

claude -c

Resume by ID codex resume <id>

claude -r <id>

Safety Modes

Mode Codex CLI Claude Code CLI

Read-only -s read-only

--permission-mode plan

Workspace write -s workspace-write

(default)

Full access -s danger-full-access

--dangerously-skip-permissions

Auto mode --full-auto

--permission-mode default

YOLO mode --yolo

--dangerously-skip-permissions

Model Selection

Task Codex CLI Claude Code CLI

Select model -m gpt-5-codex

--model opus

Use local OSS --oss

N/A

Fallback model N/A --fallback-model sonnet

Codex CLI (OpenAI)

Installation

npm i -g @openai/codex

or

brew install --cask codex

Authentication

codex login # OAuth via ChatGPT codex login --with-api-key # Read API key from stdin codex login status # Check auth status codex logout # Remove credentials

Core Commands

codex

• Interactive Mode

codex # Start TUI codex "fix all TypeScript errors" # With initial prompt codex -i screenshot.png "explain" # With image codex --full-auto "refactor" # Low-friction mode codex --search "find docs" # Enable web search

codex exec

• Non-Interactive

codex exec "write tests" # Run and exit codex e "task" # Short alias echo "task" | codex exec - # From stdin codex exec --json "task" # JSONL output codex exec -o result.txt "task" # Save to file codex exec --output-schema schema.json "task" # Validate output

codex resume

• Continue Sessions

codex resume # Interactive picker codex resume --last # Most recent codex resume --all # Show all (any directory) codex resume <session-id> # Specific session codex resume <id> "continue with this" # With prompt

codex review

• Code Review

codex review # Review current branch vs main codex review --uncommitted # Review uncommitted changes codex review --base develop # Against specific branch codex review --commit abc123 # Review specific commit codex review "focus on security" # Custom instructions

codex apply

• Apply Cloud Task

codex apply <task-id> # Apply diff from cloud task

codex cloud

• Cloud Tasks (Experimental)

codex cloud # Browse cloud tasks codex cloud exec "task" --env <env-id> # Submit task codex cloud status <task-id> # Check status codex cloud diff <task-id> # Show diff codex cloud apply <task-id> # Apply changes

codex mcp

• MCP Server Management

codex mcp list # List servers codex mcp list --json # JSON output codex mcp get <name> # Server details codex mcp add <name> -- npx my-server # Add stdio server codex mcp add <name> --url https://... # Add HTTP server codex mcp add <name> --env API_KEY=xxx -- cmd # With env vars codex mcp remove <name> # Remove server codex mcp login <name> --scopes read,write # OAuth for HTTP codex mcp logout <name> # Remove OAuth

codex sandbox

• Run Sandboxed Commands

macOS

codex sandbox macos -- npm test codex sandbox seatbelt --full-auto -- ./script.sh

Linux

codex sandbox linux -- npm test codex sandbox landlock -- ./script.sh

codex completion

• Shell Completions

codex completion bash >> ~/.bashrc codex completion zsh >> ~/.zshrc codex completion fish > ~/.config/fish/completions/codex.fish

Slash Commands (Interactive)

Command Purpose

/model

Switch model (gpt-5-codex, gpt-5, etc.)

/approvals

Change approval policy

/compact

Summarize conversation, free context

/diff

Show git diff

/review

Analyze working tree

/status

Show config and token usage

/mcp

List available MCP tools

/mention

Attach files

/fork

Branch conversation

/resume

Reopen previous session

/new

Fresh conversation

/init

Create AGENTS.md scaffold

/feedback

Submit logs/diagnostics

/quit , /exit

Exit CLI

Configuration (~/.codex/config.toml )

model = "gpt-5-codex" approval_policy = "on-request"

[sandbox] mode = "workspace-write"

[features] web_search = true

[profiles.ci] model = "gpt-4.1" approval_policy = "never"

Global Flags

-m, --model <MODEL> Model selection -s, --sandbox <MODE> read-only|workspace-write|danger-full-access -a, --ask-for-approval <P> untrusted|on-failure|on-request|never -c, --config <KEY=VALUE> Override config -C, --cd <DIR> Working directory -i, --image <FILE> Attach image(s) -p, --profile <NAME> Config profile --full-auto Low-friction mode --yolo Bypass all safety (DANGEROUS) --search Enable web search --add-dir <DIR> Grant additional write access --enable <FEATURE> Enable feature flag --disable <FEATURE> Disable feature flag --oss Use local OSS model

Claude Code CLI (Anthropic)

Installation

npm install -g @anthropic-ai/claude-code

Authentication

claude # First run prompts login claude setup-token # Set up long-lived token

Requires Claude Pro/Max subscription OR API key

Core Commands

claude

• Interactive Mode

claude # Start REPL claude "explain this project" # With prompt claude -c # Continue last conversation claude -r "session-name" # Resume by name/ID claude --model opus # Select model claude --chrome # Enable Chrome integration claude --ide # Auto-connect to IDE

claude -p

• Print Mode (Non-Interactive)

claude -p "explain this function" # Query and exit cat file | claude -p "explain" # Process piped input claude -p --output-format json "q" # JSON output claude -p --output-format stream-json "q" # Streaming JSON claude -p --max-turns 3 "task" # Limit agent turns claude -p --max-budget-usd 5 "task" # Spending limit claude -p --json-schema '{...}' "q" # Validate output schema

claude mcp

• MCP Server Management

claude mcp list # List servers claude mcp get <name> # Server details claude mcp add <name> <cmd> # Add stdio server claude mcp add -t http <name> <url> # Add HTTP server claude mcp add -e KEY=val <name> -- cmd # With env vars claude mcp add -H "Auth: Bearer x" <name> <url> # With headers claude mcp add -s project <name> <cmd> # Project scope claude mcp remove <name> # Remove server claude mcp serve # Run as MCP server claude mcp add-from-claude-desktop # Import from desktop app claude mcp reset-project-choices # Reset approvals

claude plugin

• Plugin Management

claude plugin list # List plugins claude plugin install <name> # Install plugin claude plugin install <name>@marketplace # From specific marketplace claude plugin uninstall <name> # Remove plugin claude plugin enable <name> # Enable disabled plugin claude plugin disable <name> # Disable plugin claude plugin update <name> # Update plugin claude plugin validate <path> # Validate manifest claude plugin marketplace # Manage marketplaces

claude update

• Self-Update

claude update # Check and install updates

claude doctor

• Diagnostics

claude doctor # Check health/issues

claude install

• Native Build

claude install # Install native build claude install stable # Specific version claude install latest # Latest version

Slash Commands (Interactive)

Command Purpose

/init

Generate CLAUDE.md

/clear

Reset context

/compact

Summarize conversation

/bug

Report issues

/doctor

Run diagnostics

/model

Switch model

/config

View/edit settings

/permissions

Manage permissions

/memory

View/edit memory

/project:<cmd>

Project-specific commands

/user:<cmd>

User-specific commands

Custom Commands

Create .claude/commands/fix-issue.md :

Fix GitHub issue #$ARGUMENTS

1. Read the issue details
2. Identify the problem
3. Implement the fix
4. Write tests
5. Create a commit

Usage: /project:fix-issue 1234

Configuration

User settings (~/.claude/settings.json ):

{ "model": "claude-sonnet-4-5-20250929", "verbose": false, "theme": "dark" }

Project settings (.claude/settings.json ):

{ "allowedTools": ["Bash(git:)", "Read", "Edit"], "disallowedTools": ["Bash(rm:)"] }

CLI Flags

Core

-p, --print Non-interactive mode -c, --continue Continue last conversation -r, --resume <ID> Resume specific session -v, --version Show version

Model & Config

--model <MODEL> sonnet|opus|haiku or full name --fallback-model <MODEL> Fallback when overloaded --settings <FILE> Load settings JSON --setting-sources <LIST> user,project,local --session-id <UUID> Use specific session ID

System Prompt

--system-prompt <TEXT> Replace default prompt --append-system-prompt <T> Append to default --system-prompt-file <F> Replace with file (print only) --append-system-prompt-file Replace with file (print only)

Agent & Tools

--agent <NAME> Specify agent --agents <JSON> Define custom subagents --tools <LIST> Restrict built-in tools --allowedTools <LIST> Auto-approve tools --disallowedTools <LIST> Remove tools from context

Permissions

--permission-mode <MODE> acceptEdits|bypassPermissions|default|delegate|dontAsk|plan --dangerously-skip-permissions Skip all prompts (DANGEROUS) --allow-dangerously-skip-permissions Enable bypass option

Output

--output-format <FMT> text|json|stream-json --input-format <FMT> text|stream-json --include-partial-messages Include streaming chunks --verbose Verbose logging --debug [FILTER] Debug mode with filtering

Advanced

--max-turns <N> Limit agent turns (print only) --max-budget-usd <AMT> Spending limit (print only) --json-schema <SCHEMA> Validate JSON output --chrome / --no-chrome Chrome integration --ide IDE auto-connect --fork-session Create new session on resume --no-session-persistence Don't save session --add-dir <DIRS> Additional directories --plugin-dir <DIRS> Load plugins --disable-slash-commands Disable all skills --mcp-config <FILES> MCP server configs --strict-mcp-config Only use specified MCP --betas <HEADERS> Beta API headers

Custom Subagents

claude --agents '{ "reviewer": { "description": "Code reviewer. Use after changes.", "prompt": "You are a senior code reviewer...", "tools": ["Read", "Grep", "Glob"], "model": "sonnet" } }'

Common Patterns & Edge Cases

CI/CD Integration

Codex in GitHub Actions:

• name: Run Codex run: | echo "${{ secrets.OPENAI_API_KEY }}" | codex login --with-api-key codex exec --json -o result.txt "fix linting errors"

Claude in CI:

• name: Run Claude env: ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }} run: | claude -p --output-format json "review this PR" > review.json

Handling Rate Limits

Codex: Automatic backoff built-in.

Claude: Use --fallback-model :

claude -p --fallback-model haiku "quick task"

Working with Large Codebases

Codex: Use /compact to free context

In session: /compact

Claude: Use /compact or start fresh

claude --no-session-persistence -p "analyze src/"

Multi-Directory Access

Codex

codex --add-dir ../shared-lib --add-dir ../config

Claude

claude --add-dir ../shared-lib ../config

Structured Output

Codex:

codex exec --output-schema schema.json "generate API spec"

Claude:

claude -p --json-schema '{"type":"object","properties":{"name":{"type":"string"}}}' "extract data"

Image Input

Codex:

codex -i screenshot.png "explain this UI" codex -i img1.png -i img2.png "compare these"

Claude:

Via file reference in prompt

claude "analyze the image at ./screenshot.png"

Session Forking

Codex: /fork in session

Claude

claude -r "session-id" --fork-session "try alternative approach"

MCP Server Debugging

Codex:

codex mcp list --json | jq .

Claude:

claude --debug "mcp" --mcp-config ./mcp.json

Troubleshooting

Authentication Issues

Problem Codex Claude

Not logged in codex login status

claude doctor

Token expired codex logout && codex login

claude setup-token

API key issues Check OPENAI_API_KEY

Check ANTHROPIC_API_KEY

Sandbox Issues

Problem Solution

Permission denied Use --add-dir for specific directories

Can't run commands Check sandbox mode, use workspace-write

Network blocked Sandbox may block network; use danger-full-access carefully

MCP Server Issues

Problem Solution

Server not found Check mcp list , verify installation

Connection failed Check server logs, verify URL/command

Auth required Use mcp login (Codex) or add headers (Claude)

Performance Issues

Problem Solution

Slow responses Use lighter model (gpt-4.1-mini / haiku)

Context overflow Use /compact to summarize

High costs Set --max-budget-usd (Claude)

Best Practices

• Start with read-only for exploration, escalate as needed

• Use sessions - resume work instead of starting fresh

• Create AGENTS.md/CLAUDE.md for project-specific instructions

• Leverage MCP servers for external integrations

• Use structured output in CI/CD for parsing

• Set spending limits with --max-budget-usd

• Review diffs before applying (/diff , codex cloud diff )

• Commit checkpoints before major changes