Why: Material uploads need an immediate truthful reply while incomplete tool streams remain blocked and diagnosable.
Evidence: Dev task card records ten browser scenarios, isolated PG/Redis, component suites, and composed test/build; one later governance test replay hit a concurrent fixed-port collision and remains under diagnosis.
Impact: Pins Native, Squad, Backend, Frontend, and Documents; no paid model request or merge.
Refs: U3 r0.5 implementation plan and feature-fast-slow-u2-u3-plan task card
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