21st (YC W26) — UI building blocks for the agentic internet
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.
21st is a Y Combinator company in the Winter 2026 batch. Its directory entry describes it as: UI building blocks for the agentic internet. The founders listed are Serafim Korablev (Founder/CEO), Sergey Bunas (co-founder) (Y Combinator directory entry, December 11, 2025).
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Example from a real report (our sample on Target): “The Q2 2026 earnings report, published August 19, 2026, showed adjusted EPS of $2.46 against an expected $2.33” Source: CNBC
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.
Serafim Korablev
Founder/CEO
Sergey Bunas
co-founder
Founders, titles and bios as the company published them. Photographs are our own copies of the ones on that record, so they do not break when the original link expires.
One of 643 companies held here in B2B › Engineering, Product and Design.
No investor is drawn from a Form D — the form states how many invested and never who. The one investor on this graph is named because a batch has exactly one.
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Example from a real report (our sample on Target): “The Q2 2026 earnings report, published August 19, 2026, showed adjusted EPS of $2.46 against an expected $2.33” Source: CNBC
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What the Full Report on 21st covers
The Short Answer
Which Company This Is
What They Do, And How The Money Works
Who Runs It, And How To Reach Them
The Money: Funding, Valuation, Runway
Who Pays Them, And Who They Are Up Against
Warning Bells
Sources
1 cited source found
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