CastKit is a security-first marketplace for AI Agent Skills — the universal standard adopted by OpenAI, Google, Microsoft, and Anthropic. Browse and download from 1,369 verified skills, each presented as a unique trading card with AI-generated artwork. Our Threat Grimoire exposes 341 malicious skills found on GitHub, so you know exactly what to avoid. Free to use, built for developers and AI builders who want powerful agent capabilities without the security risk.