[benjamincburns]

IIRC Anthropic's API produces cryptographic signatures for thinking blocks. If you try to submit a set of messages that include thinking blocks with missing/invalid signatures, it'll refuse.

They do this to mitigate jailbreak attempts that rely on fabricated message history (e.g. making it look like the model was compliant in previous messages, increasing the likelihood that it'll continue to be compliant in future messages).