LibreChat

LibreChat is a self-hosted chat interface that works with OpenAI, Anthropic, Google, local models and more. It offers multi-user accounts, conversation history, file uploads, agents and plugin support.

Companies use it to give staff one internal chat app with a choice of models, while keeping control of keys and data. The design will feel familiar to ChatGPT users.

You need some technical ability to deploy it, but it is well documented.