System Prompt for Bedrock Agent being ignored

Your interpretation of the statement is correct, but there's an important distinction to understand about what's happening.

When all the conditions you listed are true (single knowledge base, no action groups, default prompts, disabled user input, etc.), the agent enters an optimized performance mode. In this mode, the agent instructions (system prompt) are technically not "honored" or fully applied in the traditional sense, but they aren't completely ignored either.

What's happening is that the agent is using a streamlined flow optimized for simple knowledge base queries. You may still see your system prompt appearing in the traces because the system is still processing it as part of the request, but the agent's behavior is primarily driven by the optimized flow rather than following all the nuances of your custom instructions.

This optimization is specifically designed for simple use cases with a single knowledge base to improve latency. If you need the agent to strictly follow your custom instructions, you would need to change at least one of the conditions - for example, by overriding an advanced prompt, adding another knowledge base, or enabling an action group.

If you need precise control over the agent's behavior, including how it responds when information isn't found in the knowledge base, you should consider disabling this optimization by changing one of the conditions.
Sources
Customize agent for your use case - Amazon Bedrock
How to modify or disable the fall back message of the bedrock agent. | AWS re:Post
Optimize performance for Amazon Bedrock agents using a single knowledge base - Amazon Bedrock

Hi There,

Thank you for reaching out to AWS Premium Support,

With regards to System Prompting / Agent Instructions,

• The system isn't completely ignoring the instructions.
• Rather, it's using a streamlined flow where the instructions are processed but not fully "honored" in the traditional sense.
• This is done to optimize performance for simple knowledge base queries.

Should you want the Agent to follow strict instruction, you may do the following:

• Change at least one condition (e.g. add action group, enable another knowledge base).
• This will disable the optimization and ensure full instruction adherence.

For additional support, kindly log a support case and we will have a closer look at your environment and review accordingly.