Reasoning-summary models emit one reasoning_content delta per completed
summary part, each a self-contained bold heading. The Responses API delimits
those parts with summary_index; the OpenAI chat wire carries no such field —
verified live against Nous Portal, whose reasoning chunks contain nothing but
delta.reasoning_content — so concatenating them glued every part into one
unspaced, half-bold paragraph.
Re-derive the boundary from the signal the wire does carry: a delta opening a
closed bold heading against a mid-line tail. This matches Hermes own Responses
adapter, which already joins its summary parts with a blank line.
Teknium