Independent Test Generation for RESTful APIs

Abstract

Modern REST API testing relies on brittle sequences of calls to build system state. These multi-step tests suffer from non-determinism, poor scalability, and a "reachability tax" where a single failed setup step invalidates the entire test. We introduce TECTON, which breaks this cycle by replacing implicit state construction with explicit state synthesis of both the request payload and mock data it depends on. TECTON achieves this through two complementary mechanisms: it generates diverse, valid payloads directly, and it augments existing test mocks with realistic data so those payloads have valid system state to reference. Both mechanisms apply combinatorial testing to a new domain: the nested property space of Abstract Data Types (ADTs). TECTON decomposes complex API requests into primitive components to unleash LLMs on the more tractable subtasks of identifying equivalence classes of these primitives and generating representative values for them. It then uses LLMs to extract and inject state values via test mocks, enabling payloads to reference valid state. It recomposes these values into covering combinations to directly produce high-coverage test payloads. On standard RESTful benchmarks, TECTON achieves 70% average line coverage - a 20% absolute increase over sequence-based generators. It exposes 2x more runtime errors than any prior tool, including assertions and data constraint failures. TECTON's shift from sequencing API calls to synthesized payloads advances the state of the art in automated API validation.