Lex + Bedrock: fail to log the correct intent in Cloudwatch

The issue you're experiencing with Lex and Bedrock integration is a known behavior with the BedrockAgentIntent and QnAIntent. When these intents are fulfilled, they are automatically marked as "Fulfilled" and subsequent interactions often default to the FallbackIntent, which explains why your CloudWatch logs show FallbackIntent instead of the QnAIntent.

This behavior occurs because of how Lex handles intent fulfillment and context management when working with Bedrock integrations. Even though Bedrock is correctly providing answers, Lex's intent resolution system is not maintaining the context properly.

There are several approaches to address this issue:

1. Use context management in your Lex bot to maintain conversation state by setting output contexts when the BedrockAgentIntent/QnAIntent is fulfilled.

2. Implement session attributes in Lex to maintain state information between turns. You can set specific attributes when the intent is invoked to track that it's a QnA interaction.

3. Consider using a Lambda function as a code hook for your intent to help manage the conversation flow and properly log the intent type.

4. Adjust confidence thresholds for intent recognition to make the QnA intent more likely to be triggered in follow-up utterances.

Regarding your concern about custom prompts and CloudWatch logging: There is indeed a limitation with certain built-in intents like QnAIntent where custom prompts aren't fully supported in the same way as with custom intents. This is a design constraint in the current implementation of these specialized intents that leverage Bedrock's capabilities.

If custom prompts are critical for your use case, you might consider creating a custom intent with Lambda fulfillment that calls Bedrock services directly, giving you more control over both the prompts and the logging, though this requires more implementation work.
Sources
Lex BedrockAgentIntent controlling Fulfilled | AWS re:Post
AWS Lex Bot QnAIntent available in German? | AWS re:Post
Amazon Lex API Call Fails: "Invalid Bot Configuration: Amazon Lex could not access your Amazon Bedrock Agent" | AWS re:Post