[BUG] Claude Opus 4.6 doesn't support assistant prefill messages #1694
Description
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