What is an OpenAPI Specification?

Originally known as Swagger before being donated to the Linux Foundation's OpenAPI Initiative, an OpenAPI document fully describes an API's surface: every path and HTTP method it supports, the shape of the data it expects and returns, which fields are required, and how a client should authenticate. Because the description is machine-readable, tooling can generate interactive API documentation, client SDKs in multiple languages, mock servers for testing, and validation middleware directly from the spec, rather than each of those being hand-written and prone to drifting out of sync with the actual API. For AI agents specifically, an OpenAPI spec is often the fastest way to give a model reliable, structured knowledge of how to call an unfamiliar API: an agent framework can ingest the spec and derive callable tools automatically, with the schema guiding both which parameters are valid and what shape of response to expect back, cutting down on hallucinated API calls.