[BUG] Claude Opus 4.6 doesn't support assistant prefill messages

[jsgsamzn]

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

1.26

Python Version

3.13.9

Operating System

15.7.3

Installation Method

pip

Steps to Reproduce

Error: This model does not support assistant message prefill. The conversation must end with a user message

I am seeing this when using AgentCoreMemorySessionManager, which appends an Assistant role message before sending the conversation history to the LLM.

Model ID: us.anthropic.claude-opus-4-6-v1

Works fine with Sonnet 4.5.

Expected Behavior

Should pass summary and other memory records to model as context, in a way that Claude 4.6+ will allow.

Actual Behavior

Error: This model does not support assistant message prefill. The conversation must end with a user message

Additional Context

Prefill support removed in Claude 4.6:

https://platform.claude.com/docs/en/about-claude/models/whats-new-claude-4-6#prefill-removal

Possible Solution

No response

Related Issues

No response

[notgitika]

This issue has been addressed in the above linked PR and should be released as a patch in the next release, tentatively scheduled for Monday. Thank you for raising this!

[jsgsamzn]

This was fixed in bedrock-agentcore 1.3.2

As far as we know, this issue was not seen elsewhere in the Strands library.

[notgitika]

This bug was fixed in this PR: aws/bedrock-agentcore-sdk-python#271

Thank you!