FastChat
Open platform for training, serving, and evaluating LLM chatbots
About FastChat
FastChat is an open-source framework by LMSYS Org for training, deploying, and benchmarking large language model chatbots. It powers Chatbot Arena with 10M+ chat requests across 70+ LLMs and provides OpenAI-compatible APIs, distributed multi-model serving, and evaluation tools like MT-Bench.
Pros & Cons
Pros
- Fully open-source with Apache 2.0 license for complete transparency
- OpenAI-compatible API enables seamless migration from proprietary to self-hosted models
- Supports 70+ LLMs with broad hardware flexibility including GPU, CPU, and Apple Silicon
- Battle-tested at scale powering Chatbot Arena with 10M+ requests
- Comprehensive evaluation tools including MT-Bench and LLM-as-a-judge
- Backed by UC Berkeley, Stanford, UCSD, and CMU researchers
Cons
- Not optimized for ultra-low-latency production — may need pairing with vLLM
- Setup requires significant ML and infrastructure expertise
- Documentation can be fragmented across multiple repository files
- No managed or hosted service available
- Vicuna model weights carry separate LLaMA license restrictions
Key Features
Distributed Multi-Model Serving
Controller/worker architecture for horizontal scaling and mixed-model deployments
OpenAI-Compatible API
Drop-in replacement for OpenAI APIs, compatible with the openai-python library
Chatbot Arena
Crowdsourced evaluation platform where users compare anonymous models and vote on quality
MT-Bench Evaluation
Multi-turn benchmark with LLM-as-a-judge for automated quality assessment
Vicuna Model Training
Training and fine-tuning pipeline for conversational models built on LLaMA
Broad Hardware Support
Deploy on single GPU, multi-GPU, CPU-only, Apple Silicon, Intel XPU, or Huawei NPU
Quantized Inference
Supports ExLlama V2, GPTQ 4-bit, and AWQ 4-bit inference for consumer hardware
Web UI
Built-in web interface for testing and demoing chatbot interactions
70+ Model Support
Compatible with Llama 2, Alpaca, ChatGLM, Falcon, and dozens of other open-source LLMs
Pricing
- Full platform
- Training + serving + evaluation
- Web UI
- OpenAI-compatible API
- All model support
- Apache 2.0 license
Best For
Self-Hosted LLM API Server
Organizations run an OpenAI-compatible API backed by open-source models on their own infrastructure
Model Evaluation & Benchmarking
ML teams compare multiple LLMs using MT-Bench and human preference voting
LLM Fine-Tuning
Researchers fine-tune conversational models on custom datasets for domain-specific chatbots
Internal Chatbot Prototyping
Companies deploy a web UI chatbot demo without relying on external SaaS providers
Academic Research
University labs study LLM behavior, alignment, and evaluation methodologies
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