What is the difference between a base model and a chat model?

A base model and a chat model usually share the same underlying language-model architecture, but they are trained for different stages and behaviors.

A base model is primarily trained to predict the next token from large amounts of text.

A chat model starts from a base model and is further post-trained to follow instructions, respond conversationally, and handle multi-turn dialogue more effectively.

Key differences

| Aspect | Base model | Chat model | |---|---|---| | Main training stage | Pretraining | Pretraining + post-training | | Primary behavior | Continue or model text | Respond to user instructions | | Conversation handling | Not specifically optimized | Optimized for dialogue | | Instruction following | May be inconsistent | Usually much stronger | | Role-based messages | Usually not a core training format | Commonly trained with system/user/assistant roles | | Typical use | Research, fine-tuning, model development | Assistants, chatbots, interactive applications |

Example

Suppose the prompt is:

Explain photosynthesis in two sentences.

A base model may continue the text according to patterns it learned during pretraining, but it may not reliably respect the instruction or requested format.

A chat model is more likely to interpret the prompt as a task and return a direct two-sentence explanation.

Why does the behavior change?

The difference mainly comes from post-training.

A chat model may undergo:

• Instruction tuning
• Conversation fine-tuning
• Preference or alignment training
• Safety tuning

These stages teach the model how to respond as an assistant rather than simply continue text.

Does a chat model have different knowledge?

Usually, the chat model inherits most of its broad knowledge from the base model.

Post-training mainly changes how that knowledge is used and how the model behaves. However, additional fine-tuning data can also add or reinforce certain capabilities and patterns.

For the individual concepts, see “What is a base LLM?” and “What is a chat LLM?”

In short: a base model is the pretrained language model before assistant-specific tuning, while a chat model is that model after additional training designed to improve instruction following and conversational behavior.