Please read this first

• Have you read the custom model provider docs, including the 'Common issues' section?Model provider docs Yes
• Have you searched for related issues? Others may have faced similar issues. Yes

Describe the question

Althoughpreserve_thinking_tokens is a param forConverter.items_to_messages, it doesn't attach the thinking tokens unless there was a tool call.

I have a fix for this in a local branch with tests but I don't have permission to push the branch up.

Debug information

• Agents SDK version: v0.6.3
• Python version: 3.12

Repro steps

Enable thinking for Anthropic models using LitellmModel.

Expected behavior

Thinking tokens are attached to the assistant message.