Starting the model’s answer for it. Supply the first few characters of the reply and the model continues from there, which is a reliable way to force a format, skip preamble, or ensure an answer begins with a particular structure.
Technical view
Appends partial assistant-role content to the prompt so generation continues it rather than starting fresh. Highly effective for enforcing JSON or a fixed opening, and cheaper than constrained decoding. Note the safety consideration: because it bypasses the model’s natural turn opening, it has been used as a jailbreak technique, so applications should not expose it to untrusted users unrestricted.
Related
Part of AI Terminology Index, section: Language Models Specifically.