Mockzilla
An open-source mock server that generates a running API from an OpenAPI specification, validating both the spec and every incoming request against it, with optional latency and error simulation.
Mockzilla turns an OpenAPI specification into a running mock server without writing code: it generates realistic responses from the schema, validates incoming requests against the spec, and can proxy a real backend with automatic fallback to a mock so real and mocked endpoints coexist on the same server. Multiple specifications can run on one server, each under its own URL prefix, which lets a developer combine several third-party APIs, such as Stripe, Twilio and SendGrid, into a single local mock rather than depending on their sandboxes or rate limits.
Beyond a portable mode that needs no code, a codegen mode generates typed Go handlers so custom logic and middleware can be layered onto the generated mock, and static responses can override specific paths outside the spec entirely. The project states it has validated its generation against more than 2,200 real-world OpenAPI specifications with no generation failures.
It is written in Go under an open-source license.