E2E caught it: registered=8 (per-layer activated correctly) then the prefill
crashed with "Boolean value of Tensor with no values is ambiguous" — req.prefix_indices
is a tensor and `prefix or []` evaluated its truthiness. Use a None+len check
(safe for None/list/tensor), and wrap each request's registration in try/except so
a setup error degrades to the monolithic transfer instead of crashing the scheduler.
Also guard start_send_idx with getattr.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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