External AI CLIs: Codex & Gemini

Run OpenAI Codex CLI or Google Gemini CLI for second opinions and external reviews.

Prerequisites

Tool	Install	Config
Codex	npm install -g @openai/codex	codex login / ~/.codex/config.toml
Gemini	brew install gemini-cli	Google auth / ~/.gemini/settings.json

Check availability: codex --version / gemini --version

When to Use

• User wants a second opinion from a different AI model
• Code review from an external perspective
• Verification of an approach or architecture
• User explicitly requests codex or gemini

Recommended Models

No "latest" alias exists for either CLI — you must pin specific model names.

Codex (SWE-optimized)

Model	Use Case
gpt-5.3-codex	Best. Latest agentic coding model, SOTA on SWE-Bench Pro.
gpt-5.2-codex	Previous generation. Still capable.
gpt-5.1-codex-max	Extended reasoning variant.

Set default in ~/.codex/config.toml: model = "gpt-5.3-codex"

Gemini

Model	Use Case
gemini-3.1-pro-preview	Best. Latest model (Feb 2026). Requires previewFeatures: true.
gemini-3-pro-preview	Previous generation preview.
gemini-3-flash-preview	Faster, good for quick checks.
gemini-2.5-pro	Stable GA fallback if preview models have capacity issues.
gemini-2.5-flash	Fast GA fallback.

Always pin the model with -m in non-interactive/headless calls. The user's
~/.gemini/settings.json may use auto-gemini-3 routing, which can select Flash for
prompts it classifies as "simple" — not what you want for code reviews.

If you hit 429 MODEL_CAPACITY_EXHAUSTED on preview models, fall back to -m gemini-2.5-pro.
This is a server capacity issue (not quota) and mostly affects oauth-personal auth.
Enterprise API key (gemini-api-key auth with billing) has better capacity allocation.

Set default in ~/.gemini/settings.json:

{ "model": { "name": "gemini-3.1-pro-preview" } }

Codex CLI

Subcommands

Command	Purpose
codex review	Git-aware code review (non-interactive)
codex exec	Non-interactive prompt execution (always pass --skip-git-repo-check)
codex resume	Resume previous interactive session
codex apply	Apply latest agent diff via git apply

Code Review (most common use)

Important: codex review treats --base, --uncommitted, --commit, and [PROMPT] as mutually exclusive modes. You cannot combine a custom prompt with --base or --uncommitted. To review with custom instructions, pipe the diff to codex exec instead.

# Review uncommitted changes (default instructions)
codex review --uncommitted

# Review branch against main (default instructions)
codex review --base main

# Review specific commit (default instructions)
codex review --commit <SHA>

# Review with custom instructions — pipe diff to exec
git diff main...HEAD | codex exec --skip-git-repo-check -s read-only "Focus on error handling and security"
git diff HEAD | codex exec --skip-git-repo-check -s read-only "Check for race conditions"

# Review with title context
codex review --uncommitted --title "Add user auth middleware"

Non-Interactive Execution

# Freeform analysis
codex exec --skip-git-repo-check "Analyze the auth module for security issues"

# Specify model
codex exec --skip-git-repo-check -m o3 "Review this codebase architecture"

# Full-auto mode (sandboxed, auto-approves)
codex exec --skip-git-repo-check --full-auto "Refactor the test helpers"

# JSONL event output
codex exec --skip-git-repo-check --json "List all TODO comments" -o /tmp/result.txt

# Read-only sandbox
codex exec --skip-git-repo-check -s read-only "Audit dependencies for vulnerabilities"

Key Flags

Flag	Purpose
-m, --model <MODEL>	Model selection (e.g., o3)
-c key=value	Override config (TOML format)
-s, --sandbox <MODE>	read-only, workspace-write, danger-full-access
--full-auto	Sandboxed auto-execution
--skip-git-repo-check	Skip git repo validation (required for exec)
-C, --cd <DIR>	Set working directory
--search	Enable web search tool
--json	JSONL event output (exec only)
-o, --output-last-message <FILE>	Write last message to file (exec only)

Gemini CLI

Non-Interactive (Headless)

# Non-interactive prompt (exits when done)
gemini -p "Review this codebase for architectural issues"

# Auto-approve all actions
gemini -y -p "Fix the failing tests"

# Structured output
gemini -o json -p "List the public API surface of src/auth/"

Interactive

# Interactive with initial prompt
gemini -i "Help me debug the auth flow"

# Resume last session
gemini -r latest

# Include additional directories
gemini --include-directories ../shared-lib "Review cross-repo dependencies"

Key Flags

Flag	Purpose
-m, --model <MODEL>	Model selection
-p, --prompt <TEXT>	Non-interactive (headless) mode
-i, --prompt-interactive <TEXT>	Run prompt then stay interactive
-y, --yolo	Auto-approve all actions
--approval-mode <MODE>	default, auto_edit, yolo
-r, --resume <ID>	Resume session (latest or index)
--include-directories <DIRS>	Additional workspace directories
-o, --output-format <FMT>	text, json, stream-json

Usage Patterns

Get a Second Opinion on Changes

# Codex (git-aware, understands diffs natively)
codex review --uncommitted

# Gemini (prompt-based)
gemini -p "Review the uncommitted changes in this repo for bugs and security issues"

Review a Design Decision

codex exec --skip-git-repo-check "Evaluate the architecture in src/auth/. Is the token refresh approach sound?"

gemini -p "Analyze src/auth/ and critique the token refresh strategy"

Review a Branch Before PR

# Default review (no custom prompt needed)
codex review --base main

# Custom review instructions — pipe diff to exec
git diff main...HEAD | codex exec --skip-git-repo-check -s read-only "Review for correctness, test coverage, and maintainability"

Important

• Both CLIs operate on the working directory. Do NOT pipe file contents via $(cat file) — let them read files directly.
• If a command fails with a syntax error, run <tool> --help to check current flags before retrying.
• If an upgrade notice appears, inform the user. Do NOT auto-upgrade.
• Codex config values use TOML format: -c model_reasoning_effort="high"

Core Principle

Use codex review for git-aware code reviews. Use codex exec or gemini -p for freeform analysis. Let each CLI read files from the working directory rather than shell-expanding content into arguments.