Agent Skills is an open-source, MIT-licensed library of twenty markdown-based skills for AI coding agents, published by its author on GitHub, reported to have crossed 27,000 stars. Each skill is a workflow file with checkpoints and a defined exit criterion, rather than reference documentation, meant to bolt senior-engineer discipline (writing a spec, writing tests first, sizing a change for review) back onto an agent that would otherwise take the shortest path to a finished-looking diff.
Capabilities
The library organizes its twenty skills around six SDLC phases — define, plan, build, verify, review, ship — surfaced through seven slash commands: /spec, /plan, /build, /test, /review, /ship, and /code-simplify. A router skill, using-agent-skills, decides which of the twenty skills apply to a given task, so a small bug fix activates only a few while a complex feature can activate around eleven in sequence. Each skill embeds Anti-Rationalization Tables pairing common excuses for skipping the workflow with a written rebuttal, and skills are loaded via progressive disclosure rather than all at once.
Three ways of using it are described: installing it as a Claude Code plugin (/plugin marketplace add addyosmani/agent-skills, then /plugin install agent-skills@addy-agent-skills), dropping the plain markdown files into another tool’s own system-prompt mechanism (e.g. Cursor’s .cursor/rules/, Gemini CLI, Codex, Aider, Windsurf, OpenCode), or reading the skill files themselves as a specification of practice without installing anything.
Adoption & Ecosystem
The skills are described as saturated with practices drawn from Software Engineering at Google and Google’s public engineering culture, including Hyrum’s Law, the test pyramid and Beyoncé Rule, DAMP-over-DRY testing, ~100-line PR sizing with Critical/Nit/Optional/FYI severity labels, Chesterton’s Fence, trunk-based development, shift-left CI/CD, and treating code as a liability in deprecation decisions. The project is presented as one layer of the author’s broader Harness Engineering framing, sitting alongside AGENTS.md, hooks, and tools.