alt Hacker NewsI'm disappointed that they're removing the prefill option: https://platform.claude.com/docs/en/about-claude/models/what...
> Prefilling assistant messages (last-assistant-turn prefills) is not supported on Opus 4.6. Requests with prefilled assistant messages return a 400 error.
That was a really cool feature of the Claude API where you could force it to begin its response with e.g. `<svg` - it was a great way of forcing the model into certain output patterns.
They suggest structured outputs or system prompting as the alternative but I really liked the prefill method, it felt more reliable to me.
Replies
So what exactly is the input to Claude for a multi-turn conversation? I assume delimiters are being added to distinguish the user vs Claude turns (else a prefill would be the same as just ending your input with the prefill text)?
A bit of historical trivia: OpenAI disabled prefill in 2023 as a safety precaution (e.g., potential jailbreaks like " genocide is good because"), but Anthropic kept prefill around partly because they had greater confidence in their safety classifiers. (https://www.lesswrong.com/posts/HE3Styo9vpk7m8zi4/evhub-s-sh...).
It is too easy to jailbreak the models with prefill, which was probably the reason why it was removed. But I like that this pushes people towards open source models. llama.cpp supports prefill and even GBNF grammars [1], which is useful if you are working with a custom programming language for example.
[1] https://github.com/ggml-org/llama.cpp/blob/master/grammars/R...