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About
FastChat is a comprehensive open-source framework developed by LMSYS that enables researchers and developers to train, serve, and evaluate large language model (LLM)-based chatbots. Originally the release repository for the Vicuna model and Chatbot Arena, FastChat has grown into a full-featured LLM platform used at scale. At its core, FastChat powers Chatbot Arena (lmarena.ai), a crowdsourced evaluation platform that has served over 10 million chat requests for more than 70 LLMs and collected over 1.5 million human preference votes through side-by-side model comparisons to produce an online Elo leaderboard. Key capabilities include state-of-the-art model training and fine-tuning pipelines (supporting Vicuna and other instruction-tuned models), a distributed multi-model serving system capable of handling concurrent requests efficiently, and a fully OpenAI-compatible RESTful API layer that makes it straightforward to swap in open-source models for OpenAI-dependent applications. A built-in web UI allows interactive chatting and evaluation. FastChat also includes MT-Bench evaluation tooling for rigorous model assessment. FastChat is ideal for AI researchers benchmarking model quality, ML engineers building production LLM applications with open-source models, and organizations looking to self-host and evaluate LLMs without vendor lock-in. Licensed under Apache 2.0, it is freely available and actively maintained on GitHub.
Key Features
- Distributed Multi-Model Serving: Serve multiple LLMs concurrently with a scalable distributed architecture, capable of handling millions of chat requests.
- OpenAI-Compatible RESTful API: Drop-in replacement API layer compatible with OpenAI's interface, enabling seamless migration from proprietary to open-source models.
- Model Training & Fine-Tuning: Includes training code for state-of-the-art instruction-tuned models like Vicuna, with support for fine-tuning on custom datasets.
- Chatbot Arena & LLM Evaluation: Integrated evaluation tooling including MT-Bench and Chatbot Arena for rigorous, human-preference-based model benchmarking.
- Built-in Web UI: Interactive web interface for chatting with and comparing multiple LLMs side by side, as seen in the public Chatbot Arena.
Use Cases
- Researchers benchmarking and comparing open-source LLMs using MT-Bench or Chatbot Arena's human preference evaluation.
- ML engineers deploying self-hosted LLM APIs with OpenAI-compatible endpoints to avoid vendor lock-in.
- Organizations fine-tuning open-source base models (e.g., LLaMA) on proprietary data to create custom instruction-following chatbots.
- Developers building and prototyping LLM-powered applications locally or on-premise with a full serving infrastructure.
- AI teams running large-scale model evaluation experiments to select the best model for a specific task or domain.
Pros
- Fully Open Source (Apache 2.0): Free to use, modify, and deploy commercially with a permissive license and a large active community on GitHub.
- OpenAI API Compatibility: Plug-and-play compatibility with OpenAI's API format makes it easy to integrate open-source LLMs into existing applications.
- Battle-Tested at Scale: Powers Chatbot Arena serving 10M+ requests for 70+ models, proving its reliability and performance in production environments.
- End-to-End LLM Workflow: Covers the full lifecycle — training, serving, and evaluation — eliminating the need for multiple separate tools.
Cons
- Requires Technical Expertise: Setup and configuration demand solid ML engineering knowledge; not suitable for non-technical users without assistance.
- Infrastructure Overhead: Running large models locally or on-premise requires significant GPU/compute resources which can be costly to provision.
- Self-Managed Deployment: Unlike managed SaaS offerings, users are responsible for scaling, security, and maintenance of their own deployments.
Frequently Asked Questions
What is FastChat?
FastChat is an open-source platform by LMSYS for training, serving, and evaluating large language model chatbots. It is the foundation behind Chatbot Arena and the Vicuna model series.
Is FastChat free to use?
Yes. FastChat is fully open-source and released under the Apache 2.0 license, making it free for personal and commercial use.
Does FastChat support OpenAI-compatible APIs?
Yes. FastChat provides an OpenAI-compatible RESTful API server, allowing you to use it as a drop-in replacement for OpenAI's API with open-source models.
What models does FastChat support?
FastChat supports 70+ LLMs including Vicuna, LLaMA-based models, and many others. You can also fine-tune and serve custom models using the provided training pipelines.
What is Chatbot Arena?
Chatbot Arena (lmarena.ai) is a crowdsourced LLM evaluation platform powered by FastChat. Users chat with two anonymous models side by side and vote on which is better, generating a live Elo-based leaderboard with over 1.5M human votes collected.
