Llama 2

Open-weight release: Released pretrained and RLHF-tuned chat models under a permissive license that allowed commercial use, reshaping the open-source LLM landscape.

Training recipe: Pretrained on 2T tokens with 4K context; chat models use SFT followed by iterative RLHF with Ghost Attention (GAtt) for multi-turn consistency.

Safety investment: Extensive red-teaming, safety reward models, and context distillation produce chat models with strong helpfulness-safety trade-offs.

Ecosystem catalyst: Llama 2 became the base for hundreds of community fine-tunes (Vicuna, WizardLM, CodeLlama) and catalyzed the open-weight movement that 2024's Llama 3 and Mistral would extend.