Cedar vs OPA for MCP Access Control: Deterministic Policy Verification Beats Rego Expressiveness for Agent Guardrails

Natoma·medium signal

A head-to-head benchmark of OPA/Rego against AWS Cedar for controlling MCP tool access shows Cedar wins on safety-critical properties: mathematically verifiable (Cedar Analysis can formally prove policies are correct), zero runtime exceptions (Rego failed multiple tests due to non-determinism), and fully analyzable by static tools. For agent contexts where a policy bug could allow unintended tool execution, Cedar's constraint model — no unbounded loops, explicit attribute types, and CNCF Sandbox status — makes it the safer choice, while OPA retains an edge for complex operational logic and existing enterprise integrations.