Idempotency-Key header

Act An Idempotency-Key is a unique client-generated token attached to an HTTP POST/PATCH request or agent payment invocation so that if an agent retries after a network timeout, the server recognizes the duplicate and returns the original result rather than executing the action or charging the user twice. It is standard across Agentic Commerce Protocol, x402 / MPP payment retries, and non-idempotent MCP tool annotations (idempotentHint: false). Used by: agentic-commerce-protocol, mpp.