This paper introduces the Secret-Use Delegation Protocol (SUDP), a security framework for agentic systems to use user secrets in APIs, messaging platforms, and cloud services without granting reusable authority to untrusted requesters. SUDP proposes a protocol where a requester proposes an operation, the user authorizes it with a fresh grant, and a custodian redeems the grant to perform the operation. This approach satisfies six of seven security properties, including authorization integrity and secret confidentiality, and can be composed with a hardware-rooted runtime to achieve runtime-memory confidentiality. The protocol has been implemented with sub-millisecond overhead for mediated calls.