Ruby gem created by Bartosz Blimke during a one-day company hackathon at New Bamboo in 2009 to address his own needs as a Ruby developer. Two core capabilities: (1) stub HTTP requests — declare fake responses so no real connection is made — and (2) verify that HTTP requests (stubbed or real) have been executed. Offers an HTTP-client-agnostic DSL for stubbing and verification, follows a stub–execute–verify paradigm borrowed from the Java Mockito framework, and separates the stubbing phase from verification (unlike RSpec's single expect for both). Supports stubbing timeouts, basic auth, replaying raw curl-recorded responses, request callbacks (used by VCR under the hood), and prints helpful stubbing instructions when a request doesn't match. Supports all popular Ruby HTTP clients and all major Ruby testing frameworks. Internally uses adapters that subclass each HTTP client and monkey-patch request handling: build request, create a request signature in the request registry, check the global stub registry for a matching request stub, return a fake response or raise with stubbing instructions (real connections blocked by default), run registered callbacks and middleware. Adapter enabling swaps the original constant for the instrumented copy via constant replacement, so it can be cleanly reverted. Downloaded >250M times, ~4,000 GitHub stars, 260+ contributors over 15 years, used by ~8,000 registered gems and thousands of public GitHub projects. Has no hardcoded version restrictions on the HTTP clients it wraps — tests always run against the latest gem versions. Inspired web-mocking libraries in other languages.