FastChat
LMSYS Open Platform for Training, Serving & Benchmarking LLMs
About FastChat
FastChat is an open-source platform developed by LMSYS (Large Model Systems Organization) for training, serving, and evaluating large language model-based chatbots. As the technology powering the popular Chatbot Arena leaderboard, FastChat provides state-of-the-art serving infrastructure with OpenAI-compatible REST APIs, distributed worker orchestration, and Web UI interfaces. Machine learning engineers and enterprise developers use FastChat to self-host open-weights models (like Llama 3, Mistral, Vicuna, and DeepSeek) with multi-GPU acceleration and vLLM integration.
FastChat provides an end-to-end stack: high-throughput model serving workers, a central controller for load balancing across GPU nodes, and an OpenAI-compatible API server. It also includes comprehensive fine-tuning recipes using Hugging Face Transformers, DeepSpeed, and FlashAttention-2. FastChat is the gold standard foundation for organizations establishing sovereign, on-premise AI chat and API infrastructure.
Install FastChat via pip with `pip3 install "fschat[model_worker,webui]"`.
Launch the FastChat Controller and Model Worker with your chosen model weights.
Start the OpenAI-compatible REST API server.
Connect existing client applications and chat UIs directly to the FastChat endpoint.
Deploy the multi-model Gradio Web UI for side-by-side model comparison and benchmarking.
Capabilities & Features
Common Use Cases
Private LLM hosting
Enterprise OpenAI API replacement
Model fine-tuning
Side-by-side model evaluation
Frequently Asked Questions
Can I use OpenAI SDKs with FastChat?
Yes. FastChat exposes a fully compliant OpenAI REST API server, allowing you to use `openai-python` or `openai-node` by simply changing the `base_url`.
Does FastChat support model quantization?
Yes. FastChat supports AWQ, GPTQ, and GGUF quantization formats for efficient inference on consumer and enterprise GPUs.
Free Plan
100% Free and open-source under Apache 2.0 License.
Paid Plan
No commercial licensing fees.
Direct link · Verified & reader-supported
Pros & Cons
Powers the official LMSYS Chatbot Arena evaluation platform
Provides 100% drop-in OpenAI-compatible API server endpoints
Supports distributed multi-GPU serving with vLLM and SGLang backends
100% open-source with extensive community fine-tuning recipes
Ideal for hosting sovereign on-premise LLMs
Requires GPU hardware and Linux command line familiarity
Does not provide cloud-managed hosting directly
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