endjin Plugin Marketplace

A marketplace of endjin's plugins and skills for AI coding agents. It uses the Claude Code plugin marketplace format, which GitHub Copilot CLI also reads natively — so one repo serves both tools. The skills themselves use the cross-tool Agent Skills standard (SKILL.md), which is also understood by VS Code, Cursor, and Codex CLI.

Structure

endjin-marketplace/
├── .claude-plugin/
│   └── marketplace.json          # Marketplace catalog (required)
└── plugins/
    ├── code-review-tools/        # One directory per plugin
    │   ├── .claude-plugin/
    │   │   └── plugin.json       # Plugin manifest
    │   └── skills/
    │       └── explain-diff-html/
    │           └── SKILL.md      # One directory per skill
    └── writing-tools/
        ├── .claude-plugin/
        │   └── plugin.json
        └── skills/
            └── clear-technical-writing/  # Skills can ship references/ and scripts/ alongside SKILL.md
                ├── SKILL.md
                ├── references/
                └── scripts/

Each plugin entry in marketplace.json references its directory with an explicit relative path (e.g. "source": "./plugins/code-review-tools").

Using the marketplace with Claude Code

Add the marketplace (once), then install plugins from it:

# From a local checkout (relative or absolute path)
/plugin marketplace add ./endjin-marketplace

# Or, once pushed to GitHub
/plugin marketplace add endjin/endjin-marketplace

# Install a plugin
/plugin install code-review-tools@endjin

# Activate in the current session (new sessions pick plugins up automatically)
/reload-plugins

To get the latest plugin changes later (versions track git commits, so every push to main is a new version):

# Update this marketplace and its installed plugins, then reload
/plugin marketplace update endjin
/reload-plugins

# Or update all registered marketplaces at once
/plugin marketplace update

Skills are namespaced by plugin: for example, explain-diff-html (which builds a rich, self-contained HTML explanation of a code change, branch, or PR — with background, intuition, a code walkthrough, and an interactive quiz) is invoked as /code-review-tools:explain-diff-html, or Claude invokes it automatically based on its description when you ask for a rich explanation of a diff. Similarly, clear-technical-writing (which writes and rewrites technical text — docs, ADRs, runbooks, changelogs, error messages — so it is plain, consistent, and inclusive) is invoked as /writing-tools:clear-technical-writing, or automatically when you ask for clear, plain, or simplified technical writing.

Non-interactive (CI, scripts):

claude plugin marketplace add endjin/endjin-marketplace
claude plugin install code-review-tools@endjin --scope project
claude plugin marketplace update endjin

Using the marketplace with GitHub Copilot CLI

Copilot CLI's plugin system mirrors Claude Code's and reads .claude-plugin/marketplace.json and plugin.json directly, so this marketplace works from Copilot CLI with no changes.

Register the marketplace and install (docs):

copilot plugin marketplace add endjin/endjin-marketplace
copilot plugin install code-review-tools@endjin

Or declaratively, in ~/.copilot/settings.json (user) or .github/copilot/settings.json (repo):

{
  "extraKnownMarketplaces": {
    "endjin": { "source": { "source": "github", "repo": "endjin/endjin-marketplace" } }
  },
  "enabledPlugins": { "code-review-tools@endjin": true }
}

Portability caveats for future plugins:

• Skills are fully portable; commands/, output styles, and the renames field are Claude Code-only (Copilot ignores them).
• Copilot supports only a subset of hook events (command handlers only) and ignores rich agent frontmatter (model, permissionMode, etc.).
• Never add an explicit skills field to plugin.json — both tools auto-discover skills/, and an explicit field breaks compatibility.

Adding a new plugin

1. Create plugins/<plugin-name>/ (kebab-case) with .claude-plugin/plugin.json:

   {
     "name": "<plugin-name>",
     "description": "What the plugin does",
     "author": { "name": "endjin", "url": "https://endjin.com" }
   }

2. Add components at the plugin root (not inside .claude-plugin/):

   • skills/<skill-name>/SKILL.md — model-invoked skills; frontmatter description is required and tells Claude when to use it
   • commands/<name>.md — flat slash-command style skills
   • agents/<name>.md — subagent definitions
   • hooks/hooks.json — hooks (use ${CLAUDE_PLUGIN_ROOT} for script paths)
   • .mcp.json — MCP server definitions

3. Register it in .claude-plugin/marketplace.json under plugins:

   { "name": "<plugin-name>", "source": "./plugins/<plugin-name>", "description": "..." }

4. Validate before committing:

   claude plugin validate .

Versioning

Plugin version is omitted deliberately: without it, every git commit counts as a new version and users pick up changes on /plugin marketplace update. If you add a version to a plugin, users only receive updates when you bump it.

Gotchas

• Plugins cannot reference files outside their own directory (../shared breaks after install).
• Only plugin.json lives in a plugin's .claude-plugin/ folder; everything else goes at the plugin root.
• Marketplace and plugin names must be kebab-case with no spaces.
• After installing or updating in an active session, run /reload-plugins.