AsyncAPI is an open specification for describing event-driven and asynchronous APIs, the kind built on message brokers, event streams, and webhooks rather than on request-and-response HTTP calls. Its specification repository states that AsyncAPI lets you create machine-readable definitions of your asynchronous APIs, mirroring what the OpenAPI Specification does for synchronous REST APIs.
The project frames itself, in its own documentation, as an open-source initiative that seeks to improve the state of event-driven architectures and to make working with them as straightforward as working with REST APIs has become. That ambition is deliberately parallel to OpenAPI: where OpenAPI describes endpoints, methods, and response bodies, AsyncAPI describes channels, the messages that flow over them, and the applications that publish or subscribe.
An AsyncAPI document captures the moving parts of a messaging system in a structured form. It records the channels or topics a system uses, the schema of the messages exchanged on them, which operations send or receive those messages, and the servers and protocols involved, such as a broker speaking AMQP, MQTT, or Kafka. This gives a contract for asynchronous communication that both humans and tools can read.
Because the definition is machine-readable, it drives the same kinds of tooling that grew up around OpenAPI. The AsyncAPI ecosystem, as its documentation describes, spans documentation generation, code generation, discovery, and event management, letting teams produce human-readable docs, generate code for producers and consumers, and validate that systems conform to the agreed contract.
AsyncAPI emerged in the latter half of the 2010s as event-driven and streaming architectures became mainstream and teams found that REST-focused description formats could not capture publish-subscribe and message-queue interactions. By providing a shared vocabulary for those patterns, it has become the common standard for documenting asynchronous APIs across an industry that had no equivalent to OpenAPI for messaging.