Open-Weight vs Closed-Weight Models: What's the Difference?

Open-weight models publish their trained weights so anyone can download, run, fine-tune or serve them, while closed-weight models are only available through their maker or licensed clouds. On OpenRouter, open-weight models grew from 26.8% of top-50 model tokens in October 2025 to 71.7% in late September 2026, and cost about a quarter as much per token, though the best closed-weight models still score about 11 points higher on the Artificial Analysis Intelligence Index.