How to connect Grok Build to APIArc and use GPT models

Grok Build can load custom OpenAI-compatible models. If a provider gives you a compatible base URL, a model ID and an API key, you can add that provider without changing the rest of your workflow. This guide uses APIArc and gpt-5.6-luna as a complete example.

grok -m apiarc-luna
grok -p "Describe this project in three sentences" -m apiarc-luna

Install Grok Build

On macOS, install Grok Build with Homebrew, then verify that the command is available:

brew install --cask grok-build
grok --version

Collect the APIArc connection details

You'll use these three values in the configuration:

Base URL   https://apiarc.ai/v1
Model ID   gpt-5.6-luna
Key env    APIARC_API_KEY

APIArc exposes an OpenAI-compatible API. Grok Build uses the Chat Completions protocol, so the base URL should be the APIArc /v1 endpoint rather than a full /chat/completions URL. Keep the real API key in an environment variable; never paste it into a public tutorial or commit it to a repository.

Create the Grok Build configuration

Create the user configuration directory and file if they do not already exist:

mkdir -p ~/.grok
touch ~/.grok/config.toml
nano ~/.grok/config.toml

Add this model definition:

[model.apiarc-luna]
model = "gpt-5.6-luna"
base_url = "https://apiarc.ai/v1"
name = "APIArc GPT-5.6 Luna"
description = "GPT-5.6 Luna through APIArc"
env_key = "APIARC_API_KEY"
api_backend = "chat_completions"
context_window = 200000

apiarc-luna is Grok Build's local alias. You can rename it, but model must remain the real APIArc model ID.env_key tells Grok Build which environment variable to read, while context_window helps it decide when to compact context.

Set the API key safely

Export your APIArc key in the current terminal. Replace the placeholder with your own key:

export APIARC_API_KEY="your_apiarc_api_key"

This only affects the current shell. If you add it to ~/.zshrc, protect that file and your device because the key will be stored locally in plain text. To check that the variable exists without printing the secret, inspect only whether it is non-empty:

test -n "$APIARC_API_KEY" && echo "APIARC_API_KEY is set"

Check that Grok Build loaded the model

List the configured models, and use the inspection command if the alias does not appear:

grok models
grok inspect

The list should contain apiarc-luna. If it does not, check that Grok Build is reading ~/.grok/config.toml and that the TOML section name, quotes and field names are valid.

Run a small end-to-end test

Start with a short, deterministic prompt to make troubleshooting easier:

grok \
  -p "Reply with only APIARC_LUNA_OK" \
  -m apiarc-luna \
  --max-turns 1 \
  --disable-web-search \
  --no-subagents

If the terminal returns APIARC_LUNA_OK, Grok Build has successfully called gpt-5.6-luna through APIArc. Warnings about unrelated local MCP services can be investigated separately; they do not necessarily indicate an APIArc failure.

Add other APIArc GPT models

You can add more model aliases to the same file and reuse the same key variable:

[model.apiarc-sol]
model = "gpt-5.6-sol"
base_url = "https://apiarc.ai/v1"
name = "APIArc GPT-5.6 Sol"
env_key = "APIARC_API_KEY"
api_backend = "chat_completions"
context_window = 200000

[model.apiarc-terra]
model = "gpt-5.6-terra"
base_url = "https://apiarc.ai/v1"
name = "APIArc GPT-5.6 Terra"
env_key = "APIARC_API_KEY"
api_backend = "chat_completions"
context_window = 200000

Launch any alias directly:

grok -m apiarc-sol
grok -m apiarc-terra

Before adding a new model, confirm its exact ID from APIArc:

curl https://apiarc.ai/v1/models \
  -H "Authorization: Bearer $APIARC_API_KEY"

Common errors

Grok Build may also report warnings from local MCP or plugin configuration. For a clean model-only test, keep --disable-web-search and --no-subagents enabled.

Complete configuration

A minimal working ~/.grok/config.toml looks like this:

[model.apiarc-luna]
model = "gpt-5.6-luna"
base_url = "https://apiarc.ai/v1"
name = "APIArc GPT-5.6 Luna"
env_key = "APIARC_API_KEY"
api_backend = "chat_completions"
context_window = 200000

Export the key, then start Grok Build with the alias. From then on, switching APIArc models only requires copying the model block and changing its alias and model ID.

Browse APIArc's model catalog to check current model IDs and pricing.