What’s the difference between proprietary, open weight, and open source AI?

Most people know that when it comes to large language models (LLMs), not all are created equal. Some models are clearly more “intelligent,” while others offer benefits like being significantly cheaper to use. Yet all LLMs fall into one of three categories: proprietary, open weight, or open source. Understanding these distinctions is the key to understanding the benefits and drawbacks of any individual LLM. Proprietary LLMs Let’s start with the first category, proprietary LLMs. As its name suggests, proprietary LLMs are owned by a single entity, usually a large corporation. Even if you don’t know the names of any open source or open weight LLMs, you almost certainly know the proprietary ones. These include the biggest names in AI, such as OpenAI’s GPT and o-series that powers ChatGPT (GPT-5.6 and o3-mini, for example), Anthropic’s Claude, and Google’s Gemini. Not only are proprietary LLMs privately owned by some of the largest tech giants, but they are also incredibly closed-off and opaque. Their code and training data are highly protected and largely a mystery to those outside the company, and if you want to use them, you need to do so through the cloud. Of course, there are benef…