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21st

YC Winter 2026 Active

Infrastructure and UI building blocks for the agentic internet

https://21st.dev ↗
Thesis fit
Off-thesis

Outside your stated focus; easy to skip unless something exceptional.

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Below your usual filter (40+)
Idea match Moderate

How close the company’s idea is to your thesis statement

Sector None

Overlap with sectors you care about

Geography Unknown

Location unknown — scores 0

Your thesis: “We back exceptional technical founders building AI-first products and infrastructure, deploying $100K checks within 24 hours.”

Founder → stable Traction → stable Idea vs market → stable
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Correct facts used on the next screening or memo.

People

No founders linked yet.

YC company pages list founders in page JSON, but we only scrape a capped slice of the directory. Pull this slug with .venv/bin/python run_sourcing.py --sources yc_founders --slugs 21st then re-ingest. Open on YC ↗

Activity & evidence

Y Combinator Y Combinator · in batch signal ▸

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