Empirical Analysis of Prompt Engineering Strategies for Smart Contract Vulnerability Detection: A Multi-model Comparison

Smart contract vulnerabilities have resulted in billions of dollars in losses across Decentralized Finance (DeFi) ecosystems. While recent work explores fine-tuned Large Language Models (LLMs) for vulnerability detection, little research systematically examines prompt engineering strategies with pre-trained models. This paper presents the first comprehensive empirical study comparing five code-understanding LLMs (CodeLlama, CodeBERT, InCoder, DeepSeek-Coder, StarCoder) for smart contract security analysis without fine-tuning. Through 15 experimental iterations testing different prompting approaches across 21 vulnerability types using real DeFi exploit patterns, we discover a strong inverse correlation between prompt complexity and detection success: simple prompts (200–400 characters) achieve 100% response reliability while complex structured prompts (1500+ characters) result in complete failure. Our multi-model comparison reveals dramatic architectural differences: CodeBERT and InCoder achieve 92% accuracy but 0% recall (classifying everything as safe), while CodeLlama demonstrates superior detection with 66.67% recall using few-shot learning. DeepSeek-Coder offers optimal balance with 33.33% recall at 6.1 s inference time. These findings establish baseline performance metrics for prompt-based approaches and provide practical deployment guidelines for security practitioners.