Prompt Hardener - Automatically Evaluating and Securing LLM System Prompts
Prompt injection remains one of the most critical and under-addressed vulnerabilities in LLM applications. Despite its growing impact, most developers still rely on ad hoc, manual methods to evaluate and secure system prompts, often missing subtle weaknesses that attackers can exploit. Prompt Hardener is an open source toolkit that automates the evaluation, hardening, and adversarial testing of system prompts using the LLM itself. It applies modern prompt hardening techniques such as spotlighting, signed prompts, rule reinforcement, and structured output to improve prompt resilience. The tool also performs injection testing with categorized payloads that simulate real world threats, including system prompt leaking and improper output handling based on OWASP Top 10 for LLM Applications 2025. It is mainly intended for use by LLM application developers and security engineers at business companies for evaluating, improving, and testing system prompts for their LLM applications. In this talk, we will also give a live demo of how to strengthen system prompts using the Prompt Hardener CLI mode and Web UI. Join us to learn how to strengthen your system prompts.