# 21st — WebMCP observations

Canonical: https://www.lazyweb.com/directory/webmcp/accelerators-2026-09-20/yc-31123
Updated: 2026-09-20

Native WebMCP census · September 20, 2026

Tools observed on the website listed for 21st, with default and experimental browser contexts shown separately.

- Roster identity: 21st · yc:31123
- Program and cohort: 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

## Observation windows

The experimental mode enabled WebMCP and WebMCPTesting. Both modes used fresh logged-out contexts and a 1.5-second settled observation window.

| Context | Result | Final URL | Start | End |
| --- | --- | --- | --- | --- |
| Default Chrome | Not observed in this window | https://21st.dev/ | 2026-09-20 21:22 UTC | 2026-09-20 21:22 UTC |
| Experimental features enabled | Registration observed | https://21st.dev/ | 2026-09-20 21:45 UTC | 2026-09-20 21:45 UTC |

## Exact tool names

| Name | Default | Experimental |
| --- | --- | --- |
| `get_21st_agent_setup` | Not observed | Observed |
| `read_21st_page_markdown` | Not observed | Observed |
| `search_21st_components` | Not observed | Observed |

## Evidence limits

Passive homepage observation only. No tool was invoked. Registration does not establish execution, product quality, task success or adoption. A tool absent from one window may appear in another context.

Names and metadata describe exposed interfaces. They do not prove that a tool performs the named action or belongs to the company’s core product rather than a hosting platform.

- Default Chrome: source hash `a62c3282eea712d00c4bc20522914285c870dcdedbed11047d3ad021769b8d10`; tool frame origins: No registration observed
- Experimental features enabled: source hash `5845c6ee83821f80bb276ce32dbe4a23bf3f9c329affb6a6d246a0eb32d69dea`; tool frame origins: https://21st.dev

[Census directory](https://www.lazyweb.com/directory/webmcp/accelerators-2026-09-20) · [Study and methods](https://www.lazyweb.com/agent-readiness/measure-agent-distribution-without-counting-registrations) · [All row-level data](https://www.lazyweb.com/datasets/native-webmcp-census-2026-09-20.json) · [Archived WebMCP directory](https://www.lazyweb.com/webmcp)