As large language model (LLM) agents gain the ability to take real-world actions on behalf of humans, the question of how to bound their authority becomes urgent. We present the Agent Passport System, an open protocol that applies monotonic narrowing, the principle that delegated capabilities can only be attenuated, never amplified, as a unifying design invariant for autonomous AI systems. The protocol provides Ed25519 cryptographic identity, scoped delegation chains with cascade revocation, Merkle-tree beneficiary attribution, signed agent communication, a three-signature policy chain, coordination primitives, and agentic commerce gates. We specify the protocol using mathematically stated invariants over an abstract state model and validate the implementation with unit and adversarial tests. We do not claim machine-checked proof of implementation correctness. The system is implemented in TypeScript (359 tests, 105 suites) and Python (86 tests), published as open-source SDKs with a 44-tool MCP server. We map the protocol against the OWASP AIVSS risk taxonomy, report honest coverage (5 strong, 3 partial, 2 weak), present 10 adversarial evaluation scenarios including 2 expected failures, and identify 15 known limitations. We propose Bounded Escalation as a formally designed extension for cases where strict narrowing is insufficient, and identify runtime attestation (proposed Layer 9) as the critical missing layer.
Pidlisnyi Tymofii (Tue,) studied this question.