What is a chat LLM?

A chat LLM is a language model that has been adapted to interact with users in a conversational format.

It usually starts from a pretrained base model and then undergoes additional post-training so it can follow instructions, respond to questions, maintain conversational context, and behave more consistently as an assistant.

How is a chat LLM different from a base LLM?

A base LLM is mainly trained to predict the next token in text.

A chat LLM is additionally trained to interpret structured conversations such as:

System:
You are a helpful assistant.

User:
Explain gravity simply.

Assistant:
Gravity is the force that pulls objects toward each other...

This conversational structure helps the model learn how to respond appropriately to different roles and turns.

What training is used?

A chat LLM may use several post-training stages, including:

• Instruction tuning, where the model learns from instruction-response examples
• Conversation fine-tuning, where it learns from multi-turn dialogues
• Preference or alignment training, where responses are optimized toward desired behavior
• Safety tuning, where the model is trained to avoid or handle certain unsafe requests appropriately

The exact post-training pipeline differs between model families.

How does a chat LLM handle a conversation?

The model receives the available conversation history as part of its context.

For example:

User: Who discovered penicillin?
Assistant: Alexander Fleming.

User: When did he discover it?

The second question depends on the earlier conversation. The model uses that previous context to understand that he refers to Alexander Fleming.

A chat LLM does not permanently remember every conversation by default. It can only use information included in its current context unless the application provides an external memory system.

Is every instruction-tuned model a chat model?

Not necessarily.

An instruction-tuned model may be optimized to follow individual tasks, while a chat model is usually further adapted for conversational interaction, role-based message formats, and multi-turn dialogue.

In short: a chat LLM is a pretrained language model that has been further tuned to follow instructions and participate effectively in multi-turn conversations.