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The two hard constraints from plan §1.1 are enforced structurally rather than checked after the fact: there is no code path that can combine nodes from two fabrics, and only GPUs of the profile's class are ever considered. Getting either wrong produces a replica that starts, passes health checks, and then hangs or crawls under load — which looks like success until traffic arrives. - inventory: nodes, hardware, fabric attachments, allocations. A re-registering agent cannot clear an operator's cordon, since a cordon is usually in place because something is wrong with the node. A GPU with an ECC fault or another process on it is not free in any useful sense. Replicas are released whole, because they fail whole. - placement: highest-priority profile that fits, with every rejected candidate explained. "Unschedulable" on its own tells an operator nothing about which of six constraints bit. - bin packing fills partly used nodes first so whole nodes stay free for large models that cannot be split; multi-node replicas get the emptiest machines instead, being the least movable thing in the cluster. - NVLink-aware GPU selection keeps a tensor-parallel group inside one domain where the topology is known, falling back to lowest free indices otherwise. Property-based tests generate 400 random clusters and assert the invariants on every placement produced: one fabric per multi-node replica, only that fabric's HCAs per shard, one GPU class, no unhealthy or already-allocated GPU. The table-driven tests cover the cases someone thought of; these cover the rest. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>