Program Structure-aware Language Models: Targeted Software Testing beyond Textual Semantics

Traditional approaches to test case generation often involve manual effort and incur significant computational overhead. Additionally, these approaches are not scalable, and hence, unsuitable for complex software systems. Recently, Large Language Models (LLMs) have been applied to software testing. However, single-shot prompt engineering-based approaches tend to hallucinate and generate redundant test cases, resulting in fewer branches. To handle the above-mentioned limitations, in this paper, we propose FeedbackLLM, a novel automated language-agnostic test case generation framework based on tightly coupled two-stage approach. In the first stage, FeedbackLLM extracts the input constraints by parsing source code and generates the possible test cases. The quality of the test cases is evaluated in the second stage by the following two specialized LLM feedback agents: (i) Line Feedback Agent: extracts the metadata related to missed line executions and (ii) Branch Feedback Agent: extracts the metadata of the unexecuted branch conditions. The above agents operate in a two-stage process, communicating in tandem, and this procedure is repeated for k-steps. Further, we also introduced a redundancy prevention cache to avoid duplicate API requests and avoid unnecessary execution cycles. The performance of the proposed architecture is evaluated on the standard benchmark programs related to C and Python programs. FeedbackLLM demonstrated more line and branch coverage than baseline tools while scaling linearly in execution time.

Large Language Models (LLMs) have shown tremendous promise in automated software engineering. In this paper, we investigate LLMs for just-in-time regression test generation for programs, like parsers, interpreters, or compilers, that take highly structured, human-readable inputs. When a bug fix or code change is committed, the repository (as part of CI/CD) runs an LLM for a few minutes to generate regression tests that exercise the changed code and potentially trigger bugs. We frame LLM-based regression test generation as a machine translation task that takes the developer-provided commit message, the code change, and the input format name (e.g., XML), and produces regression tests for the described change in that format. Testing 72 commits to Mujs, Libxml2, Poppler, JerryScript, Z3, PHP, JQ, and MicroPython, our feedback-directed, zero-shot prototype Cleverest performed well, even without the code change. In under 2 minutes on average, Cleverest found as many bugs as the state-of-the-art directed greybox fuzzer WAFLGo did in 24 hours, even though WAFLGo started with a commit-reaching seed corpus in most cases. Using the Cleverest-generated tests as a seed corpus in coverage-guided greybox fuzzing doubles the number of bugs found; we call this integration ClevFuzz. We also find that some commit messages are more expressive than others, thus we ask how it impacts Cleverest's effectiveness. Cleverest picks up on the change intention: e.g., given a commit message that a patch changes how floating point variables are treated in the Mujs JavaScript interpreter, Cleverest generates JavaScript programs that contain floating point variables. Minimally changing commit messages to reduce or increase their information substantially impacts effectiveness: adding 17 words on average (max. 43) to make ineffective commit messages more expressive significantly increased the number of bugs found.

Evaluating software engineering capabilities has become a core component of modern large language models (LLMs); however, the key bottleneck hindering further scaling lies not in the scarcity of high-quality solutions, but in the lack of high-quality test suites. Test suites are indispensable both for synthesizing program repair trajectories and for providing precise feedback signals in reinforcement learning. Unfortunately, due to the high cost and difficulty of annotation, high-quality test suites have long been hard to obtain, while those automatically generated by LLMs tend to be superficial and lack sufficient discriminative power. As a first step toward constructing high-quality test suites, we introduce SWE-Mutation, a benchmark for evaluating LLM-generated test suites. The benchmark characterizes test suites by introducing systematically mutated solutions that attempt to ``fool'' the test suites and pass validation. We further propose an agentic, language-agnostic framework for automatically generating complex mutants. Our benchmark consists of 2,636 mutated variants derived from 800 original instances and includes a multilingual subset spanning nine programming languages. Experiments on seven LLMs reveal that even DeepSeek-V3.1 achieves only 10.20% verification and 36.15% detection rates, highlighting the inadequacy of current LLMs. Additionally, our agentic mutation strategy enhances realism, reducing average detection rates from 71.04% to 39.81% compared to conventional methods. These findings expose persistent deficiencies in the ability of current LLMs to generate reliable and discriminative test suites.