Back to guides

Codex CLI Integration

Use any model with OpenAI's Codex CLI through LLM Gateway. One config file, full cost tracking.

Codex CLI is OpenAI's open-source terminal coding agent. By default it connects to OpenAI's API, but with LLM Gateway you can route it through a single gateway—use GPT-5.3 Codex, Gemini, Claude, or any of 180+ models while keeping full cost visibility.

One config file. No code changes. Full cost tracking in your dashboard.

Quick Start

Create or edit your Codex CLI config file at ~/.codex/config.toml:

1openai_base_url = "https://api.llmgateway.io/v1"
2model = "auto"
3model_reasoning_effort = "high"
4

5[tui]
6show_tooltips = false
7

8[model_providers.openai]
9name = "OpenAI"
10base_url = "https://api.llmgateway.io/v1"

Then set your API key:

1export OPENAI_API_KEY=llmgtwy_your_api_key_here

Now run Codex CLI as usual:

1codex

Why This Works

LLM Gateway's /v1 endpoint is fully OpenAI-compatible. Codex CLI sends requests to our gateway instead of OpenAI directly, and we route them to the right provider behind the scenes. This means:

  • Use any model — GPT-5.3 Codex, Gemini, Claude, or 180+ others
  • Keep your workflow — Codex CLI doesn't know the difference
  • Track costs — Every request appears in your LLM Gateway dashboard
  • Automatic caching — Repeated requests hit cache, saving money

Configuration Explained

Base URL

The openai_base_url and base_url fields point Codex CLI to LLM Gateway instead of OpenAI:

1openai_base_url = "https://api.llmgateway.io/v1"

Model Selection

Use auto to let LLM Gateway pick the best model, or set a specific one from the models page:

1model = "auto"
2# or pick a specific model
3model = "gpt-5.3-codex"

Reasoning Effort

Control how much reasoning the model uses. Options are low, medium, and high:

1model_reasoning_effort = "high"

Choosing Models

Use auto to let LLM Gateway pick the best model automatically, or choose a specific one from the models page:

1# let LLM Gateway pick the best model
2model = "auto"
3

4# or pick a specific model
5model = "gpt-5.3-codex"

What You Get

  • Any model in Codex CLI — GPT-5.3 Codex for heavy lifting, lighter models for routine tasks
  • Cost visibility — See exactly what each coding session costs
  • One bill — Stop managing separate accounts for OpenAI, Anthropic, Google
  • Response caching — Repeated requests hit cache automatically
  • Discounts — Check discounted models for savings up to 90%

Troubleshooting

Authentication errors

Make sure your OPENAI_API_KEY environment variable is set to your LLM Gateway API key (starts with llmgtwy_).

Model not found

Verify the model ID matches exactly what's listed on the models page. Model IDs are case-sensitive.

Connection issues

Check that base_url is set to https://api.llmgateway.io/v1 (note the /v1 at the end).

Get Started

  1. Sign up free — no credit card required
  2. Copy your API key from the dashboard
  3. Create the config file above
  4. Run codex and start coding

Questions? Check our docs or join Discord.