AI engineering glossary · Prompt context engineering
What is assistant prefill?
Assistant prefill (also called Response prefill): A technique of supplying the first words of the model's reply yourself, so the model continues from them instead of starting fresh. It steers format and tone, for example forcing the reply to begin with an opening brace so that JSON follows. Support depends on the provider.
Assistant prefill: an example
End the message list with an assistant turn containing '{"sentiment":' so the model completes only the value, such as "negative".
Related terms
More Prompt context engineering terms
- Automatic prompt engineer
- Batch API
- Chain-of-thought faithfulness
- Chain-of-thought prompting
- Chain-of-Verification
- Chat completions API
- Complexity-based prompting
- Context budget
Where you learn it
Assistant prefill is part of the AI Engineering Program, which teaches it with notes, a hands-on lab and an exam. The first lesson is free.