Top accelerator startups

Startups14,448Programs29

14,448 startups · 29 programs

21st

Active · UI building blocks for the agentic internet

WebsiteYC profile

Why it ranks

How the rank works
7signal of 100 · rising 7
  • went through YCProgram · this site+4
  • team of 3Team size · YC's directory+3.1

A sum of public facts, not a judgement and not investment advice.

Competitors

AIInactiveStructured LabsShared: agentic internet, create, developerYC S23San FranciscoStartup →Developer toolsAcquiredVislyShared: react component, building, createYC W19RemoteStartup →Developer toolsAcquiredMoltinShared: building block, application, developerYC W15BostonStartup →AIUnlayerShared: building block, template, agentYC W22San FranciscoStartup →DataAcquiredTremorShared: react componentYC S23ZugStartup →
Companies closest to 21st
Company
Structured LabsShared: agentic internet, create, developerYC S23InactiveAI, Data, Devtools
VislyShared: react component, building, createYC W19AcquiredDevtools
MoltinShared: building block, application, developerYC W15AcquiredDevtools
UnlayerShared: building block, template, agentYC W22ActiveAI, Devtools
TremorShared: react componentYC S23AcquiredData, Devtools

Adjacent

OtherInactiveCovieShared: building blockYC S21AustinStartup →Developer toolsBirdShared: building block, largestYC S16AmsterdamStartup →AIAcquiredVogentShared: building block, agentYC S21San FranciscoStartup →AIRespellShared: building block, powered, applicationPlug and PlayUnited StatesStartup →Developer toolsVoxeetShared: building block, application, developer500 Global GA 18United StatesStartup →
Companies next to 21st
Company
CovieShared: building blockYC S21InactiveOther
BirdShared: building block, largestYC S16ActiveDevtools
VogentShared: building block, agentYC S21AcquiredAI
RespellShared: building block, powered, applicationPlug and Play Not statedAI
VoxeetShared: building block, application, developer500 Global GA 18Not statedDevtools

Also from W26 in AI

All of W26
AI/dev/fast/dev/fast is the AI-native code forgeYC W26San FranciscoStartup →AI10x ScienceThe AI-native platform for next-generation protein characterization.YC W26San FranciscoStartup →AIAemonThe Forward-Deployed AI Research EngineerYC W26San FranciscoStartup →AIAgentic FabriqThe control plane for AI agents.YC W26San FranciscoStartup →AIAlt-XBuilding the best venue for private markets exposure.YC W26Los AngelesStartup →AIAnchorheadEvals for frontier research agentsYC W26San FranciscoStartup →
Company
/dev/fast/dev/fast is the AI-native code forgeteam of 4ActiveAI, Devtools
10x ScienceThe AI-native platform for next-generation protein characterization.team of 5hiringActiveAI
AemonThe Forward-Deployed AI Research Engineerteam of 3hiringActiveAI, Devtools
Agentic FabriqThe control plane for AI agents.team of 2ActiveAI
Alt-XBuilding the best venue for private markets exposure.team of 2ActiveAI, Data
AnchorheadEvals for frontier research agentsActiveAI, Devtools

Founders

  • Serafim KorablevFounder/CEO
  • Sergey BunasFounder/CTO

About 21st

21st.dev is the largest React component registry for AI applications, used by 2M developers We provide the building blocks to create agent-powered apps: UI components, agent templates.

YC batch
W26 · Winter 2026
Status, per YC
Active
Category
AI, Developer tools
Team size
3
Location
San Francisco, CA, USA
Founded
2024
Checked
7h ago

21st in W26: 199 companies, 81% AI

LinkedIn

21st: questions

Which YC batch was 21st in?

21st was in Y Combinator's Winter 2026 batch (W26).

Is 21st still active?

Y Combinator lists 21st as active.

Who founded 21st?

21st's founders, as listed on its YC page: Serafim Korablev (Founder/CEO), Sergey Bunas (Founder/CTO).

Where is 21st based?

21st lists San Francisco, CA, USA as its location on YC's directory.