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21st-ui-build

21st-ui-build is a skill for Codex from 21st-dev/skill. It costs 111 tokens per session (655 once invoked), scanned A, original, Apache-2.0.

A workflow for building or substantially changing production user interfaces using the project's existing design context and components. It also uses 21st design references when they fit the work.

In plain words
What is it for?
Use it to implement or redesign pages, sections, and components, search for suitable design patterns, generate alternatives, and review the changed UI.
Why use it?
It helps keep new screens consistent with the product while covering practical requirements such as responsive layouts, keyboard use, focus states, reduced motion, loading, empty, and error states.

Skill for Codex

Written for Codex: agents/openai.yaml present.

not rated 34repo +1 changed 1mo ago A scan Socket: passSnyk: pass 111 tokens original Apache-2.0

Good fit Use it to implement or redesign pages, sections, and components, search for suitable design patterns, generate alternatives, and review the changed UI.

Compare 6 skills from other repositories ↓
Install with agentmods
npx agentmods add skills/21st-dev/skill/21st-ui-build
View source ↗ 21st-dev/skill
Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

Any agent
npx skills add 21st-dev/skill --skill 21st-ui-build
Clone the repo
git clone --depth 1 https://github.com/21st-dev/skill

Made for: Codex.

Wrote this? Show the measurements

A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.

agentmods badge for 21st-ui-build

README.md
[![agentmods](https://agentmods.dev/badge/skills/21st-dev/skill/21st-ui-build/github.svg)](https://agentmods.dev/skills/21st-dev/skill/21st-ui-build)
Your own site
<a href="https://agentmods.dev/skills/21st-dev/skill/21st-ui-build"><img src="https://agentmods.dev/badge/skills/21st-dev/skill/21st-ui-build/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

agentmods 80×15 button for 21st-ui-build

Your own site · 80×15
<a href="https://agentmods.dev/skills/21st-dev/skill/21st-ui-build"><img src="https://agentmods.dev/badge/skills/21st-dev/skill/21st-ui-build.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 111 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 655 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe. Third-party audits
  • Socket pass 10 Sept 2026
  • Snyk pass 10 Sept 2026
How audits are shown
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5.1 $0.00111 $0.00655
Opus 5.5 $0.00044 $0.00262
Sonnet 5.5 $0.00022 $0.00131
Haiku 4.5 $0.00011 $0.00065

Measured 1mo ago against content hash 151dbc3e9728, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

21st-ui-build scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

Origin

Copies of this mod

2 near-identical copies found in the catalogue:

skills/21st-ui-build/SKILL.md · 61 lines

How it starts

The opening of the file, as written. The whole thing — 61 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Build project-aware UI

Preserve the product's identity while using 21st supply to accelerate implementation.

Workflow

  1. Read .21st/design.json and .21st/DESIGN.md. If neither exists, run 21st init --design-context.

  2. Inspect the target route, its closest components, project instructions, and existing tokens. Treat explicit project facts as stronger than generic taste.

  3. Search before generating:

    21st search "<specific UI need>" --context auto

  4. Reuse installed project primitives first. Retrieve or install a 21st result only when it reduces duplication or materially improves the result.

  5. If no result fits, check MCP get_usage.aiGenerationEnabled or run 21st usage and read the 21st AI generation line. CLI 1.17.1+ also supports 21st usage --json. Only when AI is explicitly enabled, generate project-aware alternatives using available AI credits:

    21st generate "<goal and constraints>" --context auto --variants 3

    Otherwise, implement alternatives with your own coding agent, grounded in 21st search and 21st get (MCP: search and get_component). Do not call or suggest hosted generate or iterate_generation without AI access.

  6. Implement the selected direction in the project's real framework. Do not paste sketch HTML when the project expects React components, tokens, or established APIs.

  7. Preserve responsive behavior, keyboard interaction, visible focus, semantics, reduced motion, loading states, empty states, and error states.

  8. Run the project's checks and 21st review <changed paths>. Apply only safe, deterministic fixes automatically.

Design rules

  • Prefer project tokens over new arbitrary values.
  • Prefer existing components over parallel lookalikes.
  • Preserve established density, radii, typography, iconography, and layout rhythm unless the user explicitly requests a redesign.
  • Use real product copy and states when available.
  • Avoid decorative gradients, excessive cards, glass effects, huge headlines, and animation unless supported by the project or requested.
  • Never claim a component, token, or capability exists without inspecting it.

Read the full file on GitHub · 61 lines

Files

What ships with it

1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 1mo ago Changed · +7 lines 151dbc3e9728
  2. 2mo ago First seen · 54 lines · 111 tokens per session scan A 746095b0c07e

Subscribe to this mod's changes

21st-ui-build is a skill published in the GitHub repository 21st-dev/skill (34 stars, last pushed 1mo ago), licensed Apache-2.0. It adds 111 tokens to every session and 655 once invoked, about $0.0004 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.

Design and UXFrontend
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