{
  "id": "prefill",
  "code": "PTL-0015",
  "term": "Prefill",
  "aliases": [
    "response prefilling",
    "output priming",
    "assistant prefill"
  ],
  "category": "foundations",
  "definition": "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.",
  "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.",
  "example": null,
  "broader": [],
  "narrower": [],
  "related": [
    "structured-outputs",
    "delimiters"
  ],
  "introduced": null,
  "sources": [
    {
      "title": "Prompting best practices",
      "authors": "Anthropic",
      "year": 2026,
      "url": "https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/claude-prompting-best-practices"
    }
  ],
  "url": "https://protologue.com/t/prefill/",
  "citation": "Protologue. (2026). Prefill. In Protologue: A Taxonomy of Prompting and LLM Techniques (v1.0.0, PTL-0015). https://protologue.com/t/prefill/"
}