Claude Code and the Anthropic SDKs asking for extended thinking got the
answer alone: a reasoning model's reasoning_content was dropped. Asked for,
it comes back as a thinking block before the text — streamed as
thinking_delta events and closed with a signature before the text begins —
and the model is told to think through its chat template
(enable_thinking); disabled tells it not to. Not asked for, it is left out
as before.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>