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For the stated pure product inputs, each local classical output is uniformly $c({\psi }_X)$ or $c({\psi }_X^{\perp })$, independently of the remote bit and state. Those cases alone are therefore non-signalling.

The behavior on entangled inputs is not fixed by the specification, because a qubit entangled with another system has an improper mixed state rather than its own pure state vector. A naive extension that reports a remotely steered pure-state decomposition would permit signalling: one party could choose a measurement basis on half of an entangled pair, and the other party's infinite-precision descriptions would distinguish the resulting ensembles even though they have the same reduced density matrix.

That extension is not forced. For example, the boxes may base outputs only on the local quantum state under objective collapse, or use a fixed ensemble determined solely by the local reduced density matrix; they may also reject inputs that are not pure local states. If one or both input qubits are entangled, such a local rule can output a description of the same local mixed state, a fixed basis ensemble for it, or a designated invalid-input result, all independently of spacelike-separated choices. The device therefore does not necessarily allow superluminal signalling, although nonlocal pure-state-readout extensions would do so.

Assume an objective-collapse theory in which collapses occur at localized spacetime events. At an event $x$, define the local quantum state under objective collapse by taking the reduced density operator on spacelike hypersurfaces that approach the past light cone of $x$. A quantum state readout device at $x$ prints a complete classical description of this local density operator, in a fixed basis and here with infinite precision, without disturbing it.

For a localized pure state whose relevant preparation lies in the past light cone, the output is its rank-one projector $|\psi ⟩⟨\psi |$. For an improper mixed state produced by entanglement with an uncollapsed exterior system, it is the reduced density matrix. For a proper mixed state, once the selecting preparation or collapse event is in the past light cone, it is the actually selected component; before that causal information arrives, it is the corresponding local mixture.

A spacelike-separated collapse is excluded from the hypersurface limit and cannot change the output. Its effect appears only on and inside its future light cone. The definition therefore prevents superluminal signalling while allowing light-speed changes in the readout.