# 21st — WebMCP observations

Canonical: https://mudpie.ai/directory/webmcp/accelerators-2026-09-20/yc-31123/

- Roster: Y Combinator / Winter 2026
- Roster observed: 2026-09-18
- Official profile: <https://www.ycombinator.com/companies/21st>
- Listed website: <https://21st.dev>
- Browser: 153.0.8010.50

## Default Chrome

Not observed in this window. Final URL: <https://21st.dev/>

Window: 2026-09-20T21:22:05.759Z to 2026-09-20T21:22:08.465Z.

Tools: Not observed in this window.

Source SHA-256: a62c3282eea712d00c4bc20522914285c870dcdedbed11047d3ad021769b8d10

## Experimental features enabled

Registration observed. Final URL: <https://21st.dev/>

Window: 2026-09-20T21:45:45.104Z to 2026-09-20T21:45:47.670Z.

Tools: get\_21st\_agent\_setup, read\_21st\_page\_markdown, search\_21st\_components.

Source SHA-256: 5845c6ee83821f80bb276ce32dbe4a23bf3f9c329affb6a6d246a0eb32d69dea

## Limits

No tool was invoked. Registration does not establish execution, product quality, task success or adoption. Absence in one window is not website-wide absence.

[Study and methods](https://mudpie.ai/agent-readiness/measure-agent-distribution-without-counting-registrations/) · [Full dataset](https://mudpie.ai/research/native-webmcp-census-2026-09-20.json) · [Census directory](https://mudpie.ai/directory/webmcp/accelerators-2026-09-20/)