The streaming progress seam grows from a bare output callback to
ToolProgress: output chunks as before, plus an optional ToolStatus —
the phase a tool's inner work is in and, when the tool runs model
calls of its own, their cumulative spend and the model they ran on.
The engine stamps the call's id on it and the sink receives it as an
ephemeral ToolStatus event, replacing any earlier one for the call;
status is screen-facing, never history.
The subagent tool is the first emitter: its inner turn's lifecycle
becomes phase transitions (thinking, running, writing output) and a
running token total. Every sink decides what a status is worth: the
cloud wire forwards it, ACP and the claw have no slot and drop it.
vhaudiquet
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