Agent Skills Explained: How SKILL.md Files Work and Why They're Everywhere

Agent skills are reusable files that provide AI coding agents with specific knowledge and instructions, enhancing their capabilities without occupying unnecessary context space. Released by Anthropic as an open standard in December 2025, these skills follow a format that includes a SKILL.md file containing metadata and detailed instructions, which load only when relevant tasks are identified. This system of "progressive disclosure" allows AI agents to perform tasks according to defined preferences and workflows without requiring continual re-explanation. Skills are compatible across multiple platforms, including GitHub Copilot and OpenAI Codex CLI, and can include executable scripts for tasks requiring deterministic execution. The ecosystem has rapidly expanded, with significant growth in skills aimed at improving AI agent productivity and customization for enterprise and individual workflows. Despite their benefits, challenges remain in skill discovery and safe implementation, particularly for skills involving external actions or sensitive data handling.