MockServer

MockServer mocks the APIs an application depends on so a team can develop and test against systems that are unavailable, incomplete or hard to reproduce; proxies real traffic to record, inspect and modify requests and responses in flight, including interactive breakpoints for stepping through, editing or aborting an exchange; and injects latency, dropped connections and errors for chaos engineering. HTTP/1.1, HTTPS, HTTP/2, gRPC/gRPC-Web, WebSockets and raw TCP are auto-detected from the first bytes of a connection, so one port handles all of them.

Beyond that core, it adds experimental HTTP/3 (QUIC) on its own UDP port, JSON-RPC for MCP and A2A mocking, AsyncAPI-driven message-broker testing against external Kafka and MQTT brokers, and mocked AI and LLM chat-completion APIs for OpenAI, Anthropic, Gemini, Bedrock, Azure OpenAI and Ollama. It matches requests by method, path, query, headers, cookies and body, including JSON, XML, JSONPath, XPath, regex and OpenAPI, supports response templating and callbacks, generates expectations from an OpenAPI or Swagger spec, verifies which requests were received and how often, and shows a live dashboard of requests and expectations.

It ships Java, JavaScript/Node, Python and Ruby clients with JUnit and Spring integrations; it is written in Java and licensed under Apache-2.0.