You Can REST Now: Automated REST API Documentation and Testing via LLM-Assisted Request Mutations

REST APIs are prevalent among web service implementations, easing interoperability through the HTTP protocol. API testers and users exploit the widely adopted OpenAPI Specification (OAS), a machine-readable standard to document REST APIs. However, documenting APIs is a time-consuming and error-prone task, and existing documentation is not always complete, publicly accessible, or up-to-date. This situation limits the efficiency of testing tools and hinders human comprehension. Large Language Models (LLMs) offer the potential to automatically infer API documentation, using their colossal training data. In this paper, we present RESTSpecIT, the first automated approach that infers documentation and performs black-box testing of REST APIs by leveraging LLMs. Our approach requires minimal user input compared to state-of-the-art tools; Given an API name and an LLM access key, RESTSpecIT generates API request seeds and mutates them with data returned by the LLM. The tool then analyzes API responses for documentation inference and testing purposes. RESTSpecIT utilizes an in-context prompt masking strategy, requiring no prior model fine-tuning. We evaluate the quality of our tool with three state-of-the-art LLMs: DeepSeek V3, GPT-4.1, and GPT-3.5. Our evaluation demonstrates that RESTSpecIT can (1) infer documentation with 88.62% of routes and 89.25% of query parameters found on average, (2) discover undocumented API data, (3) operate efficiently (in terms of model costs, requests sent, runtime), and (4) assist REST API testing by uncovering server errors and generating valid OpenAPI Specification inputs for testing tools.