In live stream-json sessions a sub-agent's spawning prompt is tagged
with parent_tool_use_id, not the transcript JSONL's isSidechain +
parentUuid. The parser ignored that field, so the prompt parsed as a
main-thread user turn and rendered as a blue "you" card above the
Activity Agent card instead of folding into it.
Carry parent_tool_use_id onto ConversationItem; its presence now marks
the item as a sidechain message. The sidechain fold resolves ownership
directly by tool-use id (no transcript-only uuid chain to walk), so the
prompt folds into its Agent card and the run nests under it as before.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
jpmschweitzerandClaude Opus 4.8
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