Large Language Models for Fuzz Testing in Microservices: A Systematic Literature Review
Microservice systems (MSS) increasingly rely on heterogeneous APIs whose combinatorial input space and stateful dependencies challenge traditional fuzz testing. Meanwhile, Large Language Models (LLMs) have recently been introduced to enhance fuzzing with semantic reasoning over specifications, inputs, and runtime feedback. This paper presents a systematic literature review (SLR) of LLM-assisted fuzz testing for microservices to synthesise how LLMs are applied, evaluated, and what challenges remain. Following established SLR guidelines, we analyze 20 primary studies published between 2024 and 2026. Results show LLMs are mainly used as semantic input generators in black-box fuzzing, with a growing shift towards agent-based and retrieval-augmented architectures, improving valid input generation and modestly increasing coverage and vulnerability detection. However, evaluation remains heterogeneous, with limited benchmark standardization, scarce cost reporting, and a bias toward single-service experiments, highlighting a gap with real-world multi-service systems. This review provides a taxonomy of LLM roles and integrations, a consolidated view of evaluation practices, and a mapping of open challenges to research directions, supporting the design and deployment of LLM-driven fuzzing in microservices.