Live rendering merged reasoning chunks by scanning backward to the latest
assistant row. That fixed late reasoning, but the scan skipped trace rows,
so reasoning after a tool call crossed the Used tools block and attached to
the previous assistant iteration. Refresh looked correct because persisted
history reconstructs assistant/tool boundaries.
Treat trace rows as hard phase boundaries, just like user messages. A
reasoning_delta after Used tools now starts a fresh assistant placeholder,
so live rendering matches replay: Thinking -> Used tools -> Thinking ->
Used tools / answer.
Add a regression for reasoning_delta -> reasoning_end -> tool_hint ->
reasoning_delta.
Co-authored-by: Cursor <cursoragent@cursor.com>
Xubin RenandCursor
521aaa5ecf