21st
YC W26 · Active · UI building blocks for the agentic internet
Competitors
| Company | Category | Status | Program |
|---|---|---|---|
 Structured LabsShared: agentic internet, create, developerInactiveAI, Data, Devtools | AI, Data, Devtools | Inactive | YCS23 |
 VislyShared: react component, building, createAcquiredDevtools | Devtools | Acquired | YCW19 |
 TremorShared: react componentAcquiredData, Devtools | Data, Devtools | Acquired | YCS23 |
 MoltinShared: building block, application, developerAcquiredDevtools | Devtools | Acquired | YCW15 |
 BirdShared: building block, largestActiveDevtools | Devtools | Active | YCS16 |
Adjacent
| Company | Category | Status | Program |
|---|---|---|---|
 CovieShared: building blockInactiveOther | Other | Inactive | YCS21 |
 UnlayerShared: building block, template, agentActiveAI, Devtools | AI, Devtools | Active | YCW22 |
 VogentShared: building block, agentAcquiredAI | AI | Acquired | YCS21 |
 VoxeetShared: building block, application, developerNot statedDevtools | Devtools | Not stated | 500 GlobalGA 18 |
 LobShared: building block, developerActiveOther | Other | Active | YCS13 |
Also from W26 in AI
All of W26| Company | Category | Status |
|---|---|---|
 /dev/fast/dev/fast is the AI-native code forgeActiveAI, Devtools | AI, Devtools | Active |
 10x ScienceThe AI-native platform for next-generation protein characterization.ActiveAI | AI | Active |
 AemonThe Forward-Deployed AI Research EngineerActiveAI, Devtools | AI, Devtools | Active |
 Agentic FabriqThe control plane for AI agents.ActiveAI | AI | Active |
 Alt-XBuilding the best venue for private markets exposure.ActiveAI, Data | AI, Data | Active |
 AnchorheadEvals for frontier research agentsActiveAI, Devtools | AI, Devtools | Active |
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
- 31m ago
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.
Sources: each program's public company list, read twice a week. Not affiliated with Y Combinator or any other program. Logos via logo.dev; trademarks belong to their owners.