Codex Plugin

21st — Codex plugin

Wires up the 21st MCP server and the 21st.dev skills in Codex, so the agent searches, installs and publishes on 21st.dev from the terminal or via MCP, instead of hand-writing UI.

> The official Codex plugin directory is not open yet, so this plugin is > self-hosted only: add the marketplace by its git repo / URL directly, then > install through the in-CLI /plugins browser.

What's inside

• MCP server 21st — the remote endpoint https://21st.dev/api/mcp,

exposing 21 tools: search, get_component, get_theme, bookmarks + lists, teams, get_usage, generate, and edit/delete for components/themes/ templates. Metadata search is free; component code, generation and writes are metered.

• Skills — four focused skills that teach the 21st CLI, auto-activating

when the project has a components.json: 21st-cli-use (search/get/add), 21st-ai (generate/iterate/grab-code with 21st AI), 21st-registry (publish & manage), and 21st-design-sync (publish the project's design tokens as a theme).

Install

1. Set your 21st API key (get one at https://21st.dev/settings/api-keys):

``bash export API_KEY_21ST="sk_..." ``

1. Add this self-hosted marketplace:

``bash codex plugin marketplace add 21st-dev/codex-plugin ``

(Replace 21st-dev/codex-plugin with the actual /, git URL, or local path that hosts this directory.)

1. Open the in-CLI plugin browser and install 21st:

`` /plugins ``

Find 21st in the browser and install it. This registers the skill and the MCP server from this plugin.

Auth

The MCP server is configured (.mcp.json) to read your key from the API_KEY_21ST environment variable and send it as a bearer token. The equivalent Codex config.toml block is:

[mcp_servers.21st]
url = "https://21st.dev/api/mcp"
bearer_token_env_var = "API_KEY_21ST"

If tool calls report "Not authenticated," confirm API_KEY_21ST is exported in the shell that launched Codex.

Layout

.codex-plugin/
  plugin.json     # skills + mcp references
.mcp.json         # remote 21st MCP (bearer via API_KEY_21ST env)
marketplace.json  # self-hosted marketplace listing this plugin
skills/
  21st-cli-use/
    SKILL.md       # bundled skills (shared with the Claude Code plugin
  21st-ai/         # and https://21st.dev/api/skills/)
    SKILL.md
  21st-registry/
    SKILL.md
  21st-design-sync/
    SKILL.md

Source & license

This open-source MCP server is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: 21st-dev
• Source: 21st-dev/codex-plugin
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.