There is a validation that enforces the system message to only be at position zero.

This is no longer applicable since Anthorpic supports multiple system messages.

The use case you are blocking is very important. We can have the first static system message, which can be wrapped into a prompt cache. We have the second system message, which is dynamic based on the user context.

If we use ChatAnthropic - everything works well, if we use ChatBedrock - it breaks.