Twinbay is a hosted API simulation platform for developers testing AI agents and applications that call third-party services. Its stateful API twins can return configurable responses and model conditions such as payment declines, rate limits, expired credentials, email bounces, and paginated results. Teams direct compatible SDK requests to a twin, create isolated environments for test runs, inspect request logs, and use the environments in development or CI. The catalog includes twins for services such as GitHub, HubSpot, Plaid, Resend, Klaviyo, Linear, NetSuite, and QuickBooks. Twinbay offers a free plan with limits on twin usage and environment duration, pay-as-you-go twin-hour billing, and an Enterprise option. It is designed for engineers who need repeatable tests of agent behavior without relying on live production accounts or shared provider sandboxes.