The Anthropic library now accepts SystemMessages at any position in the message list.

The official source reports this update: @langchain/anthropic@1.5.11. Patch Changes #11669 fb2a9f3 Thanks @richardscarrott ! - feat(anthropic): accept a SystemMessage anywhere in the message list. Previously only the first message could be a system message. Anything else threw. A system message that isn't first is now sent at its own position. It applies from that point in the conversation onwards, and adding one leaves the prompt cache for the earlier turns intact. Two smaller changes come with it. A leading run of two or more SystemMessage s now merges into the top-level system field instead of being rejected.