I've lost entire weeks writing mock Salesforce endpoints just to test one agent flow.

because most API mocks are fine for testing an HTTP request, but not useful for testing an AI agent.

State Machines seems to solve for that.
its local enterprise API replicas preserve state across requests, so an agent can modify something and depend on that new state later in the same workflow.

Its environments are designed to begin from the same scenario each time, making multi-step agent behavior repeatable enough to score rather than merely demonstrate.