Manual Prompt Engineering is Not Dead: A Case Study on Large Language Models for Code Vulnerability Detection with DSPy

Automated prompt engineering tools have recently emerged as a promising solution to simplify the traditional man-ual task of crafting prompts for large language models (LLMs). This study investigates whether such tools can fully replace manual prompt engineering for code vulnerability detection. We leverage the DSPy (Declarative Self-improving Python) frame-work, which uses modular “signatures” rather than prompts to specify tasks. In DSPy, a signature defines the expected input-output behavior of a task and can optionally include a description to guide the model's objective. This signature is then auto-matically translated into an optimized prompt through DSPy's modules and optimizers. This study compares the performance of GPT-4o-mini on basic and detailed signatures to determine how DSPy optimizations affect model performance in each case. The basic signature prompts the model to classify code as vulnerable or not, while the detailed signature specifies particular vulnerabilities to identify. For each signature, we used DSPy's modules and optimizers to create three prompt configurations: zero-shot (baseline, where the model performs the task without examples), chain-of-thought (where the model shows step-by-step reasoning), and bootstrap few-shot (where the model is guided through a small set of examples). Results show that DSPy's automated optimizations improve performance for both signature types over the zero-shot baseline; however, detection performance increases significantly with detailed signatures. Specifically, the detailed signatures achieved higher F1 scores, with improvements of approximately 23 % for zero-shot, 13 % for chain-of-thought, and 11 % for bootstrap few-shot techniques. These findings indicate that while automated tools enhance prompt efficiency, optimal results are achieved by combining automated techniques with human-crafted signature details, underscoring the ongoing importance of manual refinement in specialized tasks.