"Open weights, not open source" is the distinction most coverage gets wrong. Models like DeepSeek, Qwen, and Kimi K2 release downloadable weights, often under permissive licenses, but that is a different thing from the open-source software definition, which also expects the source and the ability to rebuild from scratch.
Why it matters: open weights let a company self-host a capable model, fine-tune it on private data, and run it without sending anything to a vendor. The license still governs commercial use, and some are "modified" (Kimi's K2-family license adds an attribution requirement at scale, and the custom Kimi K3 License that shipped with its 2026 flagship goes further, requiring a separate agreement once a model-as-a-service operator passes $20 million in revenue over any 12 months). Always read the model card rather than trusting the "open source" label, which is applied loosely across the industry.