A chat could hard-fail in the browser with "TypeError: can't access property content" and stop rendering the assistant message until a reload.
Streamed response items and content parts were placed at the index the provider reports. That index is not bounded by the length of the array the client has built up, so an entry could land past the end and leave a gap behind. Spreading the array on the next event turned that gap into a real empty entry, and the renderer then dereferenced it while looking for message text.
Output items that would land past the end are now appended, since later events locate them by id anyway. Content and summary parts are padded up to the index instead, because a part carries no id and the text streamed for it is addressed by that same index. The renderer and the structured editor now skip an empty entry as well, so chats already saved in the broken state still display and edit.
Fixes#29244
* fix: stop streaming responses breaking on a duplicate output key
With reasoning-capable models the chat froze mid-stream: the first chunk of the answer appeared, nothing followed, and the whole message only showed up once generation finished. The browser console showed a Svelte each_key_duplicate error.
When a stream event addresses an output slot past the end of the array, the missing slots were filled with the event's own item, id included, so a gap of two left two entries claiming the same id. The next chunk for that item was matched by id, landed in the first of the two, and the rendered list ended up with two items sharing a key, which Svelte refuses to update.
Only the addressed slot now takes the event's item, and the slots before it are anonymous placeholders. Replayed the reported event sequence against the real code: keys are unique again and the chunks stay in order instead of being split across the copies.
* fix: stream reasoning deltas when the provider also sends reasoning_details
Providers such as OpenRouter emit reasoning_details alongside the reasoning
text on the same delta. Merging those details cleared the pending event
unconditionally, discarding the response.reasoning_text.delta that had just
been built, so the client received no reasoning until the response completed
and the thinking block only appeared after generation finished.
The event is now only dropped when the details were all there was to report.
Details persistence is unchanged.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014uuEg4AXPs9zE3vVUfN1Fj
---------
Co-authored-by: Claude <noreply@anthropic.com>
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