# Sundial — The Open Registry for Agent Skills

> The largest open registry of agent skills. Browse, install, and publish reusable AI skills for Claude Code, Cursor, Gemini, Antigravity, ChatGPT, GitHub Copilot, Codex CLI, Windsurf, and any agent that reads markdown.

## What Are Agent Skills?

Agent skills are reusable capabilities defined in the open **SKILL.md format**. Each skill is a folder containing a SKILL.md file — YAML frontmatter for metadata (name, description) and a Markdown body with instructions that tell an AI agent how to perform a specific task.

Skills use **progressive disclosure** to manage context efficiently. At startup, agents load only each skill's name and description (~200 bytes). When a user's task matches a skill, the agent reads the full SKILL.md instructions on demand. This keeps context windows lean while giving agents access to deep, specialized knowledge.

The SKILL.md specification is an **open standard** anyone can implement. The same skill works across every compatible agent platform. **Skill authors** publish once and reach every platform.

### Key Properties of Agent Skills

- **Portable**: Skills are just files. They work with any agent that reads markdown — Claude Code, Cursor, Gemini, Antigravity, ChatGPT, Codex, GitHub Copilot, Windsurf, Cline, Aider, Amp, and more.
- **Composable**: Use multiple skills together. A research workflow might combine web-search, pdf-to-text, and citations skills.
- **Self-documenting**: Any skill author or user can read a SKILL.md and understand what it does. Easy to audit and improve.
- **Reusable**: Build once, deploy everywhere. The same skill works in Claude Code, Cursor, Codex, and any compatible agent.

### The SKILL.md Format

```
my-skill/
├── SKILL.md            # Required: instructions + metadata
├── references/         # Optional: documentation
├── scripts/            # Optional: executable code
└── assets/             # Optional: templates, resources
```

Frontmatter fields:
- `name` (required): Unique skill identifier
- `description` (required): One-line summary — agents use this to decide when to activate the skill
- `license`: SPDX identifier (MIT, Apache-2.0, etc.)
- `metadata`: Freeform key-value map (author, version, emoji)
- `allowed-tools`: Space-delimited tool names the agent may use
- `compatibility`: Environment requirements

Full specification: https://sundialhub.com/docs/specification

## Compatible Agent Platforms

Agent skills work with any AI agent that reads markdown files:

- **Claude Code** (Anthropic) — installs to `.claude/skills/`
- **Cursor** — installs to `.cursor/skills/`
- **Gemini / Antigravity** (Google) — installs to `.gemini/skills/`
- **Codex CLI** (OpenAI) — installs to `.codex/skills/`
- **ChatGPT** — via paste or file upload
- **GitHub Copilot** — via workspace configuration
- **Windsurf** (Codeium)
- **Cline**
- **Aider**
- **Amp**
- **OpenCode**
- Any agent that reads markdown

## Install Skills

Install any skill with one command:

```
npx sundial-hub add <skill-name>
```

Or point any AI agent to:

```
Read https://sundialhub.com/<skill-name>.md
```

## Main Pages

- [Home](https://sundialhub.com): Agent skills registry overview
- [Browse Agent Skills](https://sundialhub.com/explore): Search and discover all public skills and assistants
- [What Are Agent Skills?](https://sundialhub.com/docs/skills): In-depth guide to the skill format
- [SKILL.md Specification](https://sundialhub.com/docs/specification): Complete format reference
- [Documentation](https://sundialhub.com/docs): Getting started, CLI guides, workspace features

## Documentation

- [Getting Started](https://sundialhub.com/docs): Quick start guide for skill authors
- [What Are Agent Skills?](https://sundialhub.com/docs/skills): How skills work, progressive disclosure, anatomy
- [SKILL.md Specification](https://sundialhub.com/docs/specification): Complete format reference for skill authors
- [Assistants Guide](https://sundialhub.com/docs/assistants): How assistants bundle skills
- [CLI Reference](https://sundialhub.com/docs/cli/reference): `sun push`, `sun pull`, `sun add` commands
- [Workspace](https://sundialhub.com/docs/workspace): Collaborative workspace features

## Agent Skills Directory

No public skills available.

## Assistants Directory

No public assistants available.

## For Skill Authors

1. Create a folder with a SKILL.md file
2. Add name and description in YAML frontmatter
3. Write instructions in Markdown
4. Publish: `npx sundial-hub push`

Guide: https://sundialhub.com/docs/create

## Contact

- Website: https://sundialhub.com
- Open standard: https://agentskills.io