An agent's own instruction arbitration swings 47 points between deployment windows, so the trust boundary has to live outside the model
A fleet evaluation across 46 model endpoints from 6 vendors (arXiv 2608.28502, Aug 28) shows a recognition-enforcement gap: source-format features are linearly decodable from activations and models will verbally identify forged authority when asked, yet some configurations still emit the conflicting tool call. Average execution under diverse novel attacks is only 1.21% over 14,294 spoofed trials, but the failures cluster in reproducible cells and the per-fingerprint range moves up to 47 percentage points within a single deployment window. Prompt-layer defenses did not generalize across models. The authors' fix is an external reference monitor combining authenticated source routing with capability-gated tool execution, which deterministically rejected every forged, tampered, replayed and unsigned request tested. Treat self-arbitration as a capability, never as a security boundary.
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