# Prefill

> Prefill is the technique of writing the first part of the model's response yourself, so that the model continues from that text and is steered into a specific format or direction.

- Identifier: PTL-0015
- Category: Foundations
- Canonical URL: https://protologue.com/t/prefill/
- Also known as: response prefilling, output priming, assistant prefill

## Description

Starting the assistant turn with an opening brace pushes the model toward JSON output, and starting with a heading or a character's name can enforce structure or persona. Support is model-specific and declining. Anthropic's documentation states that prefilling the final assistant turn is not supported on Claude 4.6 models and later, and recommends explicit instructions or structured outputs instead.

## Related terms

- [Structured Outputs](https://protologue.com/t/structured-outputs/)
- [Delimiters](https://protologue.com/t/delimiters/)

## Sources

- Anthropic (2026). Prompting best practices. https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/claude-prompting-best-practices

## Cite this entry

Protologue. (2026). Prefill. In Protologue: A Taxonomy of Prompting and LLM Techniques (v1.0.0, PTL-0015). https://protologue.com/t/prefill/

License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/)