special tokens

**Special tokens** is the **reserved vocabulary items used to encode control signals such as sequence boundaries, padding, role markers, and task directives** - they provide structural semantics beyond ordinary lexical content. **What Is Special tokens?** - **Definition**: Tokenizer entries with predefined operational meaning in model pipelines. - **Common Types**: BOS, EOS, PAD, SEP, CLS, role tags, and task-specific control tokens. - **Training Role**: Special tokens teach models structural boundaries and interaction protocols. - **Inference Role**: Guide decoding behavior, formatting, and multi-turn conversation framing. **Why Special tokens Matters** - **Protocol Reliability**: Consistent special-token use prevents prompt-format confusion. - **Boundary Control**: Enables clear sequence segmentation and termination. - **Feature Support**: Many serving features depend on correctly interpreted control tokens. - **Interoperability**: Model and tokenizer alignment requires stable special-token mapping. - **Safety**: Control tokens can enforce response mode and policy boundaries. **How It Is Used in Practice** - **Schema Definition**: Document special-token inventory and meaning for every model version. - **Compatibility Tests**: Validate token IDs across training, fine-tuning, and serving stacks. - **Prompt Templates**: Standardize token placement to avoid accidental control-state drift. Special tokens is **the control-language layer of tokenizer and model interaction** - robust special-token governance is critical for stable inference behavior.