Fail-closed AI gateway design: why the default failure mode is the security mode
A fail-closed AI gateway returns HTTP 503 when the policy decision point cannot reach a verdict, blocking the request rather than forwarding it. A fail-open gateway returns HTTP 200 with the upstream model response, treating the policy outage as a pass. The choice between the two postures determines whether a policy outage produces a security incident or a contemporaneous deny record. EU AI Act Article 12 and Article 26 expect the deny record. The four failure categories that test the design are policy timeout, identity provider outage, redaction engine outage, and audit write outage.
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