[BUG] Bedrock model provider validation error when using conversation manager

[ethan-stone]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

0.7.0

Node.js Version

22.22.1

Operating System

26.3.1

Installation Method

other

Steps to Reproduce

Code snippet: https://gist.github.com/ethan-stone/533a3a3e957257bb760f7ab303c6721a

Run the code snippet. You will see that a ModelError is thrown, indicating that when using Bedrock as the provider the messages sent must start with a user message.

Expected Behavior

The conversation manager should be model agnostic or at minimum any behavior that does differ should be documented.

Actual Behavior

Bedrock model provider fails with sliding window conversation manager if start of window is a non-user message.

Additional Context

No response

Possible Solution

No response

Related Issues

No response

[pgrayy]

I ran the gist provided and it succeeded:

$ AGENT_MODEL_ID=us.anthropic.claude-sonnet-4-20250514-v1:0 AWS_REGION=us-east-1 npx tsx repro.ts
Minimal reproduction for SlidingWindowConversationManager.
Expected: trimmed conversation should still start with a user message.

Original messages
Message count: 9
1. user [text:"message 0"]
2. assistant [text:"message 1"]
3. user [text:"message 2"]
4. assistant [text:"message 3"]
5. user [text:"message 4"]
6. assistant [text:"message 5"]
7. user [text:"message 6"]
8. assistant [text:"message 7"]
9. user [text:"message 8"]
Validation: valid conversation prefix

Trimmed messages (windowSize=8)
Message count: 8
1. assistant [text:"message 1"]
2. user [text:"message 2"]
3. assistant [text:"message 3"]
4. user [text:"message 4"]
5. assistant [text:"message 5"]
6. user [text:"message 6"]
7. assistant [text:"message 7"]
8. user [text:"message 8"]
Validation:
- Conversation does not start with a real user text message.

Calling Bedrock with the trimmed messages...
Bedrock accepted the conversation. stopReason=endTurn

What specifically did you run with to produce the error?

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Update: Tested with nova and encountered the error. Also encountered an error when passing just 1 message with assistant role. With that said, proposed changes still make sense.