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mediumAI Engineering

What is the difference between a base model, an instruction-tuned model and a chat model?

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01

Understand the problem

Pretraining vs post-training: why raw next-token predictors need alignment before they are useful assistants.

base-modelinstruction-tuningrlhfpost-training
02

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03

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A base model is the raw next-token predictor from pretraining — it completes text but does not follow instructions. Instruction tuning fine-tunes it on (instruction, response) pairs so it answers requests, and chat models add multi-turn conversation formatting plus preference training (RLHF/RLAIF) for helpfulness and s

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04

Read the code

Same input, different behaviour
# base model — plausible continuation, not an answer
IN:  "What is the capital of France?"
OUT: " What is the capital of Spain? What is..."   # quiz-sheet pattern

# chat model — role-structured, answers the request
IN:  [system] You are a concise assistant.
     [user]   What is the capital of France?
OUT: "Paris."
05

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