Testing NetLogo model code

Unreliable in Practice? A Comprehensive Study of Errors in LLM-Generated Code

Large Language Models (LLMs) are being widely used for coding, with reports indicating that AI now generates an increasing share of production code. Studies show that LLMs can significantly improve developer productivity, yet they still struggle with more complex coding tasks. Just as understanding error modes in human-written code has been central to improving software quality, identifying and characterizing the errors in LLM-generated code is critical for setting realistic expectations and designing mitigation strategies. Prior research has been limited in scope, often focusing on a single language, a small number of problems, or a limited selection of models. As a result, there is still no comprehensive understanding of which errors are common and which are specific to certain models or languages. To address these gaps and develop a deeper understanding of the quality of LLM-generated code, we analyzed a corpus of 86,726 code samples that contained compilation or runtime errors. These samples were generated by seven LLMs across four compiled languages. We classified errors by their underlying causes using an LLM, manually validated these classifications, and performed a comparative analysis. This labeled data is then used to measure error prevalence by model, language, and problem difficulty, to identify common error patterns. Results show that, although error types vary strongly across languages and models, even the largest models frequently make simple mistakes. We also observe that generated code often omits basic input validation or memory-safety checks, which can lead to overflows, resource exhaustion, or other reliability/security issues.

Specula: Scaling formal specifications for autonomous model checking of system code

Specula is a push-button agentic system that generates high-quality formal specifications for large, complex system code and uses the specifications for highly effective model checking and bug finding. Specula employs large language model (LLM) based coding agents to autonomously develop TLA+ specifications, including invariants that describe correctness properties of the target system and formal models that describe the system implementation with the right level of abstractions. Specula is fully autonomous and thus eliminates the barrier of applying formal methods to real-world system code (as in traditional human-centric approaches). Meanwhile, Specula addresses limitations of LLM-driven techniques like reward hacking and hallucinations through self-evolving loops that iteratively improve specification quality by enabling the agents to deepen their understanding of system code and its behaviors. We have used Specula to check 48 open-source system projects; Specula found 249 bugs including many deep bugs that are hard to find by existing approaches. Specula has been used by several companies and is maintained at https://github.com/specula-org/Specula.

On the risk of coding before testing: An empirical study on LLM-based test generation workflow

Large Language Models (LLMs) are increasingly used in software engineering workflows to generate both source code and test suites. This dual capability has enabled emerging development paradigms, including test-first and agentic workflows, where a single model is producing and validating implementations. However, these approaches assume that generated tests act as independent and reliable oracles - a fundamental requirement for effective software testing. In this paper, we challenge this assumption and investigate whether LLM-generated code biases the generation of subsequent tests. We introduce and empirically study the phenomenon of error propagation, where faults in generated code are systematically replicated in associated test artifacts. This leads to cases where incorrect implementations and tests are mutually consistent, masking defects rather than revealing them. We evaluate this effect across a range of programming tasks and agentic workflows, analyzing the consistency between generated code and test assertions, with particular focus on scenarios of aligned failures. Our study examines (i) whether erroneous code artifacts bias test generation, (ii) whether such bias persists under different prompting strategies, including chain-of-thought reasoning, and (iii) how errors propagate across multi-step workflows in which intermediate outputs are reused as context. The results show that error propagation is prevalent and impactful: generating tests after faulty code significantly reduces fault detection effectiveness compared to generating tests independently (14% vs. 25%). These findings highlight a fundamental limitation of current workflows, where lack of independence between generated artifacts undermines the reliability of automated testing. Furthermore, our results expose a previously underexplored threat to validity in empirical studies relying on coupled generation pipelines.

Quick Bug Detection Through Black-Box Checking: a Systematic Evaluation

Combining Tests and Proofs with Contracts for Better Software Verification

Test or prove? These two approaches to software verification have long been presented as opposites. One is dynamic, the other static: A test executes the program, a proof only analyzes the program text. A different perspective is emerging, in which testing and proving are complementary rather than competing techniques for producing software of verified quality. Work performed over the past few years and reviewed here develops this complementarity by taking advantage of "design by contract," as available in Eiffel, and exploiting a feature of modern program-proving tools based on "satisfiability modulo theories" (SMT): counterexample generation. A counterexample is an input combination that makes the program fail. If we are trying to prove a program correct, we hope not to find any. One can, however, apply counterexample generation to incorrect programs, as a tool for automatic test generation. We can also introduce faults into a correct program and turn the counterexamples into an automatically generated regression test suite with full coverage. Additionally, we can use these mechanisms to help produce program fixes for incorrect programs, with a guarantee that the fixes are correct. All three applications, leveraging on the mechanisms of Eiffel and design by contract, hold significant promise to address some of the challenges of program testing, software maintenance, and automatic program repair. Moving past the traditional opposition between tests and proofs, current work on software verification treats them as complementary.

Specification Grounding Drives Test Effectiveness for LLM Code

Large language models frequently generate code that appears correct on typical inputs yet fails on edge cases, invalid inputs, and other specification-defined corner conditions. A popular fix has the model write its own tests and repair until they pass, but the source of the gain is unclear: does it come from the tests merely existing, or from their grounding in a specification of what the code should do? We isolate this factor. Holding the tester, test budget, and repair loop fixed, we change a single prompt line that controls whether the tester receives the spec as a checklist of rules. The baseline is strong: it is already told to probe invalid inputs and edge cases. Grounding the tests in the spec produces correct code +38 percentage points more often than this baseline across three Claude tiers (Haiku 4.5, Sonnet 4.6, Opus 4.8), and +36 points on a held-out set. Grounding, not test quantity, is the primary driver: doubling the test budget barely helps, and combining eight independent ungrounded suites plateaus far below grounding. An ablation isolates the spec's content, not its format: given the spec as a plain paragraph the tester recovers 27 of 30 bugs, but asked to plan tests without the spec it recovers only 2 of 30. The effect survives stronger baselines: a property-based generator catches 28 of 30 bugs but invents out-of-spec requirements, and an AlphaCodium-style loop only matches the baseline. It replicates across vendors (GPT-5.3-codex +28, Gemini 3.5 Flash +19), with a task-level sign test over 18 tasks significant at p=0.002. Grounding improves both sensitivity and precision: it catches more real bugs and wrongly rejects far less correct code, cutting the false-alarm rate from 33% (68% against a Python standard-library oracle) to 0%. On well-specified algorithmic problems it neither helps nor hurts.