21st-ai

21st AI — sketch, iterate & grab code from the terminal

21st AI is a fast UI-drafting board. Mental model: draft it here, then finish it — sketch a few variants from a prompt, preview them, refine the one you like with plain-English edits, and pull its code into the repo. The whole loop runs from the 21st CLI (or the matching MCP tools), so you never leave the editor.

Sketch mode produces self-contained HTML + Tailwind drafts (fast, no build). The recommended handoff is the copy-prompt: a spec that tells you to rebuild the design in the target project's real stack — not paste raw HTML verbatim.

Auth

Same credential as the rest of the CLI: 21st login (saved token) or a 21st_sk_ key via --api-key / TWENTYFIRST_TOKEN. 21st usage (MCP: get_usage) shows the account tier, aiGenerationEnabled, and remaining free daily component-code retrieval quota. It does not report the AI credit balance. (See the 21st-cli-use skill for the full auth notes.)

Before generating or iterating, check MCP get_usage.aiGenerationEnabled or the 21st AI generation line from 21st usage. CLI 1.17.1 also supports 21st usage --json. Proceed only when AI is explicitly enabled; missing or unknown status does not grant access. A base Builder subscription does not enable AI. When it is false, use search → get_component and adapt the code with the user's own agent. Do not attempt generate or iterate_generation, including through the CLI as a fallback.

The loop

# 1) Sketch — generate variants ("takes") from a prompt. Opens the preview in
#    the browser and prints the projectId.
21st generate "a pricing section with 3 tiers and a monthly/annual toggle"
 
# 2) List the takes (1-based numbers + whether each has rendered yet).
21st

Open the preview URL (printed by generate, and returned by iterate) in the editor's built-in browser to watch the variants render and update in place.

When to use which

• generate — start fresh from a prompt. Returns a preview URL + projectId.
• generation — list the takes so you know which --take N to act on.
• iterate — refine an EXISTING take (edited in place; a new version is appended, so you can step back in the workspace). Use this to change something already generated instead of starting over.
• take — pull a take's code. Default prints the copy-prompt (adapt-to-my- stack spec); --code prints the raw HTML. Both FREE and unlimited.

Metering

• generate — requires AI enabled on the account and available AI credits. Builder and Team start with AI off; membership alone does not enable it.
• iterate — billed by real token cost per edit (the shared 21st AI credit pool); it does real model work each call, so batch changes into one clear instruction rather than many tiny ones.
• generation (listing) and take (grabbing code) are FREE.

MCP parity

The same loop is exposed as MCP tools for MCP hosts (Cursor, Claude Desktop, Claude Code): generate (with mode: "sketch"), get_generation, iterate_generation and get_take. iterate_generation also returns the freshly-edited html + copyPrompt, so you can grab the code without a second call. Wire up the server with 21st init --client <name> --write.

The MCP tool list depends on the account: generate and iterate_generation are listed only when AI is enabled. get_generation and get_take remain available for reading existing drafts. After enabling AI, refresh the client's tool list or reconnect the MCP server. If a cached tool call returns ai_subscription_required, stop generation attempts and use the component search and retrieval workflow above.

Tips

• A sketch is a draft, not production code — treat the copy-prompt as a design spec and rebuild it in the project's real components; don't paste the HTML as-is.
• Refer to takes by the 1-based number from generation / the "Take N" label in the preview. --take must be a whole number ≥ 1 (a bad value errors rather than silently editing take 1).
• Only sketch generations you created can be listed, edited, or fetched.