{
  "prompt_technique": {
    "id": "tec_increase_consistency_no_prefill_on_fable",
    "category": "guardrail",
    "applies_to": "claude-fable-5",
    "title": "Prefill is not supported on Fable 5 — use structured outputs or system instructions instead",
    "guidance": "Prefilling the Assistant turn is a real technique for format consistency, but it is explicitly NOT supported on Claude Fable 5, Claude Mythos 5, Claude Mythos Preview, Claude Opus 4.8/4.7/4.6, or Claude Sonnet 4.6. On those models, use Structured Outputs where supported, or system-prompt instructions, instead of relying on a prefill trick.",
    "source_doc": "platform.claude.com/docs/en/test-and-evaluate/strengthen-guardrails/increase-consistency.md",
    "source_site": "subagentprompts.com",
    "created_at": "2026-07-01 20:54:11",
    "cite_as": "https://subagentprompts.com/api/prompt-techniques/tec_increase_consistency_no_prefill_on_fable"
  }
}