21st
21st.dev is the largest React component registry for AI applications, used by 2M developers (370K MAU). We provide the building blocks to create AI agents and agent-powered apps: UI components, agent templates, and an SDK with built-in UI, tools, memory, observability, and managed infrastructure.
Y Combinator
yc-company-directory #597421st.dev is the largest React component registry for AI applications, used by 2M developers (370K MAU). We provide the building blocks to create AI agents and agent-powered apps: UI components, agent templates, and an SDK with built-in UI, tools, memory, observability, and managed infrastructure.
source21st is classified as AI agents from source descriptions and fund-directory context. Similar companies below are ranked from offline product facets and meaningful description overlap; category and source affiliation can only strengthen an existing product match. For lookup-created rows, the profile starts as pending enrichment until deeper source collection runs.
Local clusters rerank extracted product concepts and description terms. The generated graph remains the fallback for sparse descriptions: offline reciprocal product-similarity graph: extracted concepts + description terms + bounded category/affiliation boosts. Minimum fallback score: 0; max competitors: 6.
Companies similar to 21st
A deterministic local cluster built from offline product facets and meaningful description terms. Category and selected-institution affiliation provide bounded tie-breaks. Open any peer to continue exploring its cluster.
assistant-ui
match 69assistant-ui helps frontend developers add AI chat to their apps. Our product is used by several dozen companies, like LangChain, Stack AI, Browser Use, and Athena Intelligence. We're building an open source Typescript/React library and a backend as a service, with first class integrations into popular agent frameworks like LangGraph and Vercel AI SDK.
shared extracted concepts: react; shared product terms: react, used, developer, apps; shared product theme: developer infrastructure; same category: AI agents; shared affiliation: Y Combinator
a0.dev
match 66a0.dev is a platform that helps mobile developers build high quality apps in days instead of weeks by generating custom react native projects. We've launched top ranking apps that have been used by hundreds of thousands, and now we're creating the tools we wish we had to build faster.
shared extracted concept terms: react, developer; shared product terms: dev, react, used, developer; shared product theme: developer infrastructure; shared affiliation: Y Combinator
Trigger.dev
match 59Trigger.dev is the open source platform for building AI agents and workflows in TypeScript. Long-running tasks with retries, queues, full observability, and elastic scaling. Previously, the founders built JSON Hero, an open source JSON viewer that is currently used by more than 35k developers a month. Eric was CTO of Code School, an early code learning platform that was acquired in 2015. Matt created two iPad apps that won Apple’s App of the Year.
shared extracted concept terms: observability; shared product terms: dev, used, developer, apps; shared product theme: developer infrastructure; same category: AI agents; shared affiliation: Y Combinator
Papr
match 57Papr is a context intelligence infrastructure for AI agent developers that believe their AI agent's memory should get better at scale, not worse. We help you build the brain for your AI-native agents; safe, compliant, and works at scale. 92% Stanford STaRK hit@5 retrieval accuracy, <150ms retrieval.
shared extracted concept terms: infrastructure, memory; shared product terms: developer, memory, infrastructure; shared product theme: context and memory, developer infrastructure; same category: AI agents
Webvizio
match 55Webvizio accelerates the web development process by generating one-click bug reports enriched with visual context and technical data. This data gathered for each bug is used to create AI prompts for AI dev copilots like Cursor, Tabnine, and Codeium, enabling developers to implement code fixes in seconds rather than days.
shared extracted concept terms: developer; shared product terms: dev, used, developer, create; shared product theme: context and memory, developer infrastructure; same category: AI agents
Prism
match 52Prism is the easiest way to build and deploy a production AI Agent. Stop harness-engineering. Bring your system prompt, tools, MCP, and skills — get infrastructure, orchestration, memory, self learning, and more out of the box.
shared extracted concepts: memory; shared product terms: memory, infrastructure; shared product theme: context and memory, developer infrastructure; same category: AI agents; shared affiliation: Y Combinator