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
platform.claude.com/docs/en/test-and-evaluate/strengthen-guardrails/increase-consistency.md
provenance
source_site subagentprompts.com · created 2026-07-01 20:54:11 · JSON