Prompts Royale is a prompt engineering tool that helps users evaluate multiple prompt candidates by having them compete against each other. Users provide a task description and test scenarios; the system automatically generates prompt variants and test cases, then runs pairwise comparisons using an LLM as judge. It employs Monte Carlo matchmaking to efficiently determine rankings and ELO scoring to rate candidates based on win patterns. All processing happens client-side with requests sent directly to the LLM API.