Prompt example extracted from Anthropic Prompt Library.
Claude Mythos Preview , prefilled responses (providing a partial assistant message for Claude to continue from) on the last assistant turn are no longer supported. Requests with prefilled assistant messages to these models return a 400 error. Model intelligence and instruction following have advanced such that most use cases of prefill no longer require it. Earlier models continue to support prefills, and adding assistant messages elsewhere in the conversation is not affected.
