MLX Server

OpenAI-compatible API server for running local LLMs on Apple Silicon via MLX. Supports vision and tool use with automatic model swapping — only one model is loaded in memory at a time, switched on demand based on the request's model field.

Supported Models

Alias	Model	Context	Capabilities
gemma	mlx-community/gemma-3-4b-it-4bit	128k	Vision, tool use (tool_code blocks)
qwen	mlx-community/Qwen3-VL-4B-Instruct-4bit	256k	Vision, tool use (<tool_call> tags)

Quick Start

source .venv/bin/activate

# Start with Gemma 3 (default)
./run.sh

# Start with Qwen3
./run.sh qwen

# Or directly
python -m mlx_server.main --model mlx-community/gemma-3-4b-it-4bit --port 1234

The server starts at http://127.0.0.1:1234.

API

Standard OpenAI-compatible endpoints:

• GET /v1/models — lists all available models with context_window sizes
• POST /v1/chat/completions — chat completions (streaming and non-streaming)
• GET /health — health check

Model Swapping

Send any available model ID (or alias) in the model field. If it differs from the currently loaded model, the server unloads the old one and loads the new one automatically:

# Uses Gemma
curl http://localhost:1234/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{"model": "mlx-community/gemma-3-4b-it-4bit", "messages": [{"role": "user", "content": "Hello"}]}'

# Swaps to Qwen
curl http://localhost:1234/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{"model": "mlx-community/Qwen3-VL-4B-Instruct-4bit", "messages": [{"role": "user", "content": "Hello"}]}'

Vision

Pass images as base64 data URIs or URLs in the image_url content part:

{
  "model": "mlx-community/gemma-3-4b-it-4bit",
  "messages": [{
    "role": "user",
    "content": [
      {"type": "text", "text": "What's in this image?"},
      {"type": "image_url", "image_url": {"url": "data:image/png;base64,..."}}
    ]
  }]
}

Context Window Management

Each model's context window is read from its HuggingFace config (max_position_embeddings) and reported in /v1/models via the context_window field. Clients can use this to manage conversation length proactively.

If a request exceeds the context window, the server:

1. Automatically summarizes older messages (keeping system messages and the last 6 messages intact)
2. Retries with the compressed conversation
3. Returns an OpenAI-compatible context_length_exceeded error if it still doesn't fit

Tool Use

Pass tools in the tools field (OpenAI format). The server handles model-specific formatting and parses tool calls from the output automatically.

Installation

Requires Python 3.11+ and Apple Silicon.

uv pip install -e "."

Project Structure

mlx_server/
  main.py    — FastAPI server, endpoints, CLI entrypoint
  engine.py  — Model loading, prompt building, generation (mlx_vlm)
  models.py  — Pydantic models for OpenAI API types

Design Notes

• Uses mlx_vlm (not mlx_lm) as the backend — supports both text and vision in a single model load
• Offline-first: if the model is cached locally (~/.cache/huggingface/hub/), no network requests are made
• Thread lock on generation — MLX models aren't safe for concurrent generation
• KV prefix caching for multi-turn conversations
• Context window read from each model's config (Gemma 3 4B: 128k, Qwen3-VL 4B: 256k) with automatic summarization fallback