Improving LLM-Based Unit Test Generation Through Root-Cause-Driven Prompt Design

Abstract

This paper addresses automated unit test generation with large language models (LLMs). LLM-based test generation has not yet attained a quality level sufficient for practical use in industry. Although LLMs often reproduce API syntax faithfully, they frequently disregard semantic usage constraints and execution-environment dependencies, leading to assertion failures, mock-related errors, and reference/resolution errors. A prior failure analysis of Java unit test generation using GPT-4o classified 2980 trials into eight failure patterns and identified three root-cause mechanisms: external context ignorance, internal context ignorance, and a syntax–semantics gap. Building on that analysis, this paper proposes a prompt design comprising three strategies: (1) making the execution state explicit in the generated test, (2) stating semantic constraints explicitly, and (3) injecting environment constraints prior to generation. In contrast to generic techniques such as few-shot learning or chain-of-thought prompting, each proposed strategy is tied to a specific root-cause mechanism, yielding a systematic design in which each rule is explicitly justified by its correspondence to a specific root-cause mechanism. Experiments on 298 methods with five models (GPT-4o, GPT-5, GPT-5.1-Codex, Claude Sonnet 4.5, and Gemini 2.5 Pro) show improved test execution success rates for every model, with absolute gains ranging from 1.1 to 21.1 percentage points (pp). Mock-related errors were reduced by 61.9%–99.2% relative to the baseline prompt, demonstrating effectiveness against the targeted failure patterns. Finally, conditions under which the strategies transfer to other code-generation tasks are discussed, along with limitations on their scope.