Claude Sonnet-4.6 LLM not working properly (assistant message prefill error)

[parthiv-middleware]

Description

When using Claude Sonnet-4.6 with Agno agents/teams, the request fails with a 400 error related to assistant message prefilling.

It appears that Claude Sonnet-4.6 no longer supports assistant message prefill and requires the conversation to end with a user message.

This behavior worked correctly when using previous models like Claude Sonnet-4.5.

Error Logs

ERROR Claude API error (status 400): Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'This model does not support assistant message prefill. The conversation must end with a user message.'}, 'request_id': 'req_011CZ6ggheCNFP94eWsCFgfH'}

ERROR Non-retryable model provider error: Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'This model does not support assistant message prefill. The conversation must end with a user message.'}, 'request_id': 'req_011CZ6ggheCNFP94eWsCFgfH'}

ERROR Error in Team run: Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'This model does not support assistant message prefill. The conversation must end with a user message.'}, 'request_id': 'req_011CZ6ggheCNFP94eWsCFgfH'}

Steps to Reproduce

1. Create an Agno agent or team.
2. Configure the model as claude-sonnet-4-6.
3. Run the agent.
4. The request fails with a 400 error.

Expected Behavior

Claude Sonnet-4.6 should work with Agno agents/teams without throwing a 400 error.

Actual Behavior

Agent execution fails with an invalid_request_error related to assistant message prefill.

Environment

• Framework: Agno (version 2.5.5)
• Model: claude-sonnet-4-6
• Provider: Anthropic API
• OS: Linux