The incremental </think> search missed tags split across tokens, inverting reasoning/content classification. Detect the first </think> on the full buffer and track the content boundary as a byte offset. If the model never closes </think>, flush the buffered text as content so clients always receive the response. Chat UI now renders reasoning_content too. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>