21st: Infrastructure and UI building blocks for the agentic internet | 21st

!function(){try{var d=document.documentElement,c=d.classList;c.remove('light','dark');var e=localStorage.getItem('theme');if('system'===e||(!e&&true)){var t='(prefers-color-scheme: dark)',m=window.matchMedia(t);if(m.media!==t||m.matches){c.add('dark')}else{c.add('light')}}else if(e){c.add(e|| '')}}catch(e){}}() #featurebase-iframe-wrapper.featurebase-messenger-frame-wrapper { --fb-element-bottom: 20px !important; } .featurebase-notification-frame-wrapper { --fb-element-bottom: 4px !important; } 21st K Home Explore Components Agent Templates MCP Build Agents 1Code Magic Chat 21st Agents SDK The fastest way to build and deploy AI agents. Read more Dismiss We're in YC 21st is backed by Y Combinator. Excited for what's ahead! Read more Dismiss Title Description Read more Dismiss Log in Home Search K UI Components View all [data-radix-scroll-area-viewport]{scrollbar-width:none;-ms-overflow-style:none;-webkit-overflow-scrolling:touch;}[data-radix-scroll-area-viewport]::-webkit-scrollbar{display:none} Agents SDK Read docs The fastest way to ship AI agents Built-in UI, chat history, spend limits, sandbox execution, and observability. Works with Claude and OpenAI. Get started Documentation E2B Sandboxes Isolated VM per session Streaming Token-by-token via SSE Auth & Limits Tokens, rate limits, spend caps Observability Session replay & traces Agent Templates View all [data-radix-scroll-area-viewport]{scrollbar-width:none;-ms-overflow-style:none;-webkit-overflow-scrolling:touch;}[data-radix-scroll-area-viewport]::-webkit-scrollbar{display:none} Next.js Starter Next.js chat interface with AI agent powered by 21st Agents SDK Claude Code DuckDuckGo Python Starter Minimal server-side Python chat client using the 21st SDK Python 21st SDK Go Starter Minimal terminal chat client in Go using the 21st SDK Go 21st SDK Lead Research Agent Research people by email online, qualify leads, and send Slack alerts for interesting prospects Claude Code Slack Exa.ai Form Autocomplete AI-powered form filling with tabbed forms and chat interface Claude Code Docs Assistant (llms.txt) Documentation Q&A agent — provide a docs URL and get an instant assistant that searches via llms.txt Claude Code llms.txt Support Agent Docs-powered support agent that answers questions from llms.txt and escalates unanswered ones via email (Resend) Claude Code llms.txt Resend Examples Explain GitHub repos with Nia Next.js repository chat with Nia MCP-backed GitHub analysis Claude Code Nia MCP AgentMail Agent Email operations copilot — send, read inbox, auto-reply via AgentMail Claude Code AgentMail Note Taker with Convex AI notebook assistant with persistent notes and real-time sync via Convex Claude Code Convex Slack Monitor Agent Track service status and send updates directly to Slack Claude Code Slack Atlassian Statuspage BrowserUse Agent Extract structured data from any website using Browser Use Cloud Claude Code Browser Use requestAnimationFrame(function(){$RT=performance.now()}); 21st: Infrastructure and UI building blocks for the agentic internet | 21st document.querySelectorAll('body link[rel="icon"], body link[rel="apple-touch-icon"]').forEach(el => document.head.appendChild(el)) $RB=[];$RV=function(a){$RT=performance.now();for(var b=0;b a&&2E3 $RC("B:1","S:1") (self.__next_f=self.__next_f||[]).push([0]) self.__next_f.push([1,"1:\"$Sreact.fragment\"\n4:I[91100,[],\"\"]\n5:I[28682,[],\"\"]\n"]) self.__next_f.push([1,"6:I[66076,[\"79323\",\"static/chunks/0206212e-8cb87bf36e90ea71.js\",\"22863\",\"static/chunks/22863-2654d12621ddc34c.js\",\"13154\",\"static/chunks/13154-b55330c1841e1f11.js\",\"8329\",\"static/chunks/8329-76be832bb4512cf8.js\",\"37925\",\"static/chunks/37925-6ab0ce5e58a20295.js\",\"62146\",\"static/chunks/62146-272dcd108e05cda7.js\",\"47988\",\"static/chunks/47988-318776eaa52eecbe.js\",\"80329\",\"static/chunks/80329-429ad4dea29bc4d0.js\",\"49550\",\"static/chunks/49550-7961a3fe0cd48add.js\",\"76817\",\"static/chunks/76817-2cf54eff76523b6e.js\",\"45942\",\"static/chunks/45942-1b1cd24a5ef416c5.js\",\"89623\",\"static/chunks/89623-35617fcae8f2e112.js\",\"31668\",\"static/chunks/31668-397e4a12725ceab5.js\",\"41027\",\"static/chunks/41027-9c89a8816702d2ae.js\",\"45109\",\"static/chunks/45109-4edc1501cb3f1b64.js\",\"9181\",\"static/chunks/9181-f5dc0189259d2a07.js\",\"70925\",\"static/chunks/70925-53d4b95d697aa8ca.js\",\"47640\",\"static/chunks/47640-3ec01c52f1a721b2.js\",\"11032\",\"static/chunks/11032-eae040ac4036f6c4.js\",\"65980\",\"static/chunks/65980-7a6db8ad11d7cef1.js\",\"43051\",\"static/chunks/43051-679e77808eef2268.js\",\"23469\",\"static/chunks/23469-793ef353117c92c7.js\",\"27854\",\"static/chunks/27854-7a4a177e034addf3.js\",\"73742\",\"static/chunks/73742-1d4f45a796d81935.js\",\"73861\",\"static/chunks/73861-697cafd3ac347d6e.js\",\"3434\",\"static/chunks/3434-ddb1bff5b334e6d4.js\",\"76304\",\"static/chunks/76304-a8b3a0f72add51fa.js\",\"39645\",\"static/chunks/39645-c444f5370daa6b28.js\",\"63228\",\"static/chunk

21st: Infrastructure and UI building blocks for the agentic internet 21stはAIエージェント開発のためのインフラとUIコンポーネントを提供するプラットフォームである。