Use Modellix LLM with Grok Build

Configure Grok Build to call the Modellix LLM gateway. Grok Build is xAI’s coding agent (interactive TUI, headless CLI, or ACP). It has no built-in Modellix provider, so add custom models in ~/.grok/config.toml that point at Chat Completions on POST /v1/chat/completions.

This page covers LLM text only (https://llm.modellix.ai).Modellix model IDs are provider/name (for example openai/gpt-5.5). Put that full string in each custom model’s model field so Grok Build sends it unchanged in the request body. See Models & Pricing.

Set Up Grok Build

Install Grok Build

curl -fsSL https://x.ai/cli/install.sh | bash

irm https://x.ai/cli/install.ps1 | iex

Get a Modellix API Key

Create a key in the Modellix console and export it. Grok Build reads the key from the environment variable named in env_key:

export MODELLIX_API_KEY="mdlx-xxxxxxxx"

Add the same line to your shell profile (for example ~/.zshrc), then run source ~/.zshrc.

Use a Modellix API key, not an xAI platform key. A built-in Grok catalog model with XAI_API_KEY still talks to https://api.x.ai/v1, not Modellix.

Add Custom Models

Edit the user config at ~/.grok/config.toml (Windows: %USERPROFILE%\.grok\config.toml). If GROK_HOME is set, the file is $GROK_HOME/config.toml instead.Merge the [model."..."] tables into an existing file. Do not put custom models in a project .grok/config.toml—that file only accepts MCP servers, plugins, and permission rules.

[model."openai/gpt-5.5"]
model = "openai/gpt-5.5"
base_url = "https://llm.modellix.ai/v1"
name = "GPT 5.5"
env_key = "MODELLIX_API_KEY"
api_backend = "chat_completions"

[model."openai/gpt-5.6-sol"]
model = "openai/gpt-5.6-sol"
base_url = "https://llm.modellix.ai/v1"
name = "GPT 5.6 Sol"
env_key = "MODELLIX_API_KEY"
api_backend = "chat_completions"

[model."anthropic/claude-sonnet-5"]
model = "anthropic/claude-sonnet-5"
base_url = "https://llm.modellix.ai/v1"
name = "Claude Sonnet 5"
env_key = "MODELLIX_API_KEY"
api_backend = "chat_completions"

[model."google/gemini-3.6-flash"]
model = "google/gemini-3.6-flash"
base_url = "https://llm.modellix.ai/v1"
name = "Gemini 3.6 Flash"
env_key = "MODELLIX_API_KEY"
api_backend = "chat_completions"

[model."xai/grok-4.6"]
model = "xai/grok-4.6"
base_url = "https://llm.modellix.ai/v1"
name = "Grok 4.6"
env_key = "MODELLIX_API_KEY"
api_backend = "chat_completions"

[models]
default = "openai/gpt-5.5"

Add or remove [model."..."] tables as needed. Quote the table key when the local ID contains / or ..

Field	Value
Table key (`[model."id"]`)	Local picker ID used with `-m` and `/model`
`model`	Exact Modellix Model ID (`provider/name`) sent to the API
`base_url`	`https://llm.modellix.ai/v1` (include `/v1`)
`name`	Label shown in the model picker
`env_key`	`MODELLIX_API_KEY` (prefer this over an inline `api_key`)
`api_backend`	`chat_completions` for `POST /v1/chat/completions`
`[models].default`	Local picker ID of the Modellix model to use for new sessions

Setting models.default to a Modellix ID avoids falling back to a built-in xAI catalog model that requires XAI_API_KEY. Full catalog: Models & Pricing.

You do not need a separate Anthropic base URL. Register anthropic/... and google/... IDs on the same Modellix base_url; traffic goes through OpenAI-compatible Chat Completions. Native Anthropic Messages (api_backend = "messages" without /v1) is for the Anthropic SDK and Claude Code.

If you prefer Responses instead of Chat Completions, set api_backend = "responses" and keep base_url = "https://llm.modellix.ai/v1". See Responses. Most setups should keep chat_completions.

Select a Modellix Model

Confirm Grok Build loaded the user config, then start a session from your project directory:

grok inspect
cd your-project
grok

In the TUI, switch models with /model <id> (for example /model openai/gpt-5.5). Headless:

grok -p "Explain this repo" -m "openai/gpt-5.5"

You can also set GROK_DEFAULT_MODEL for the current session (same idea as -m).

Troubleshooting

Symptom	Check
401 / auth errors	`MODELLIX_API_KEY` is a valid Modellix key, and `env_key` matches that variable name
Model not found / 404	`model` is a full ID like `openai/gpt-5.5`, not bare `gpt-5.5`; ID is in the live catalog
Wrong path / URL errors	`base_url` is `https://llm.modellix.ai/v1` (include `/v1`), not the media API host (`https://api.modellix.ai`)
Traffic hits `api.x.ai`	You selected a custom Modellix table, not a built-in `grok-*` catalog model; `models.default` is a Modellix ID
Custom model missing from picker	Entry is in user `~/.grok/config.toml` (or `$GROK_HOME/config.toml`); run `grok inspect` to see loaded config sources
First launch opens a browser	Built-in xAI auth. Set `models.default` to a Modellix ID and export `MODELLIX_API_KEY` so sessions do not need `XAI_API_KEY`

Grok Build overview — install, custom models, TUI and headless usage
Grok Build settings — config.toml scopes and [model.<id>] fields
Models & Pricing — Model IDs and rates
LLM API guide — protocols and curl examples
OpenAI SDK — same /v1 Chat Completions gateway
OpenCode · Codex — other OpenAI-compatible agent setups

Get Started

API

Agent

SDK

Framework

IDE

Tool

Use Modellix LLM with Grok Build

Set Up Grok Build

Troubleshooting

​Set Up Grok Build

​Troubleshooting

​Related

Set Up Grok Build

Troubleshooting

Related