System prompt parameter for ChatBedrockConverse

[Steinkreis]

I think it would be beneficial to add a system_prompt parameter to the ChatBedrockConverse class, similar to how a system prompt can be defined for agents.

In the boto3 implementation, the client call works like this:

client.converse(messages=bedrock_messages, system=system, **params)

Here, the system message is handled separately from the rest of the messages. Allowing a system_prompt parameter would make the developer experience more consistent with using Bedrock directly.

This parameter could be implemented as an optional str, defaulting to None. That way, ChatBedrockConverse could be used in two ways:

1. Default behavior – the system prompt is provided via the messages list.
2. Parameter-based behavior – the system prompt is set using the system_prompt parameter, which is automatically added to every conversation.

I’ve looked into the source code, and with a small change to the _messages_to_bedrock function, this should be straightforward to implement. I’d also be happy to contribute this change.

[michaelnchin]

Moved to issue: #728