Codex Desktop Third-Party API Quick Start: Configure the responses API

Codex Desktop reads configuration from the .codex directory in your user home. By creating auth.json and config.toml, you can make the desktop app call an OpenAI-compatible third-party API such as NixAPI.

This guide assumes Codex Desktop is already installed. You only need to create two config files and restart the app.

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Prerequisites

You need:

1. Codex Desktop installed
2. A NixAPI API Key
3. Permission to edit hidden files in your user directory

If you do not have a key, create one in the NixAPI console. NixAPI currently has a limited-time top-up exchange rate: RMB 0.80 = USD 1.00, which is useful to know before running long coding tasks.

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Step 1: Locate the .codex Directory

Codex Desktop uses the same user-level directory as Codex CLI:

• Windows: C:\Users\your-name\.codex\
• macOS: ~/.codex/

If you cannot see .codex:

• Windows: enable “Show hidden items” in File Explorer
• macOS: press Command + Shift + . in Finder

If the folder does not exist, create it. On macOS:

mkdir -p ~/.codex
touch ~/.codex/auth.json ~/.codex/config.toml

On Windows, make sure the files are not saved with .txt extensions.

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Step 2: Write auth.json

Open .codex/auth.json and add:

{
  "OPENAI_API_KEY": "nix-your-api-key"
}

Replace nix-your-api-key with your real NixAPI API Key.

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Step 3: Write config.toml

Open .codex/config.toml and add:

model = "gpt-5.4"
model_provider = "nixapi"
model_reasoning_effort = "medium"
disable_response_storage = true
preferred_auth_method = "apikey"

[model_providers.nixapi]
name = "NixAPI"
base_url = "https://nixapi.com/v1"
wire_api = "responses"
requires_openai_auth = true

Two fields matter most:

• base_url must be https://nixapi.com/v1
• wire_api must be responses

Codex Desktop uses the responses protocol. If wire_api is set to chat, or /v1 is missing from the Base URL, you may see 404, unknown endpoint, or empty response errors.

You can change the model to:

• gpt-5.4
• claude-sonnet-4-20250514
• claude-opus-4-7
• gemini-3-pro-preview
• deepseek-v4

Use the NixAPI supported models page as the source of truth.

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Step 4: Restart Codex Desktop

Save both files and fully quit Codex Desktop:

• macOS: use Command + Q
• Windows: also check the system tray

Open the app again and send:

Hello. Explain how you can help me write code.

If you get a normal reply, Codex Desktop is using the configured third-party API.

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Troubleshooting

The app still asks for an official login

The config files were probably not loaded. Check the directory path, file names, and file extensions.

404 or Unknown endpoint

Make sure both are present:

base_url = "https://nixapi.com/v1"
wire_api = "responses"

Model not found

Copy the model ID from the NixAPI pricing page, then update the model field.

Windows sandbox permission issue

If command execution keeps failing because of permissions, add:

[windows]
sandbox = "elevated"

Then fully restart the app.

Changes do not apply

Closing the window may not quit the app. Make sure no Codex Desktop process remains in the Dock, tray, or background.

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Summary

Codex Desktop requires OPENAI_API_KEY in auth.json, plus base_url = "https://nixapi.com/v1" and wire_api = "responses" in config.toml. Once set up, you can switch models by changing the model ID.