Scheduler-Boundary Observability in Shared Quantum Clouds: Compilation- and Backend-Conditioned Diagnostics

Shared quantum-cloud runtimes expose service outcomes shaped by compilation, backend conditions, and scheduling. We study whether workload-dependent information remains in tenant-visible outputs after scheduler mediation, while separating such evidence from provider-side boundary traces and internal diagnostics. Experiments use a provider-inspired prototype with synthetic IBM-style backend abstractions rather than IBM’s proprietary scheduler. Across 16 independent workload seeds, leave-one-seed-out evaluation using provider-reported waiting time yields a mean accuracy of 0.6010 and a classification-based signed advantage of 0.1010 over the majority-class baseline. Monotone padding and delayed release reduce the estimated advantage to 0.0324 and 0.0418, with mean added delays of 0.185 and 0.131 in normalized simulator-time units. Controlled workload pairs and compilation- and backend-level diagnostics show configuration-dependent, nonmonotonic variation across internal observability channels, but do not attribute the external signal to a uniquely quantum mechanism. A six-run experiment on one public backend provides only weak and variable in-sample timing separability. These results demonstrate preliminary scheduler-boundary separability in the controlled prototype, but do not establish a practical public-cloud attack.