After a tool call, the model's thinking was streamed into the chat as if it were the main response, and only jumped into the collapsed Thoughts section once the turn finished. Every further tool call repeated it.
Each tool round appended an empty placeholder message item to the output and sent it to the browser, then dropped it again from the copy used to offset the next round's item indices. The browser therefore held one item more than the backend counted, so the first thinking chunk of the next round was written into that leftover message item and rendered as normal text until the finished output replaced it.
The placeholder is removed. It was never needed: a message item is already created when actual content arrives, and dropping it also stops an empty assistant message being sent back to the model on the follow-up request.
Co-authored-by: Claude <noreply@anthropic.com>
