Measured 2026-09-07. Catalogue source: weekly scan 2026-09-06.
What x401 is. x401 is an emerging protocol layer — "Proof / Notarize" — proposed on top of x402, adding response headers that let a paid endpoint attach a verifiable proof or notarization to its answer. It is specified in the open and its contributors are publicly named. Two header generations exist so far: v0.1.0 (PROOF-REQUIRED, PROOF-PRESENTATION, PROOF-RESPONSE) and v0.2.0 (PROOF-REQUEST, PROOF-RESPONSE, PROOF-RESULT).
What we measured. A response-header survey of every resource in the current weekly x402 catalogue — 15,686 endpoints — checking, case-insensitively, for any x401 header of either generation. Zero endpoints emit an x401 header today (0 of 15,686; 0 in v0.1.0, 0 in v0.2.0). Everything is still answering with plain x402 or an ordinary HTTP status.
| HTTP | count |
|---|---|
| 402 (x402 challenge) | 10,603 |
| 404 | 1,991 |
| 405 | 1,021 |
| unreachable (timeout / conn error) | 917 |
| 400 | 550 |
| 200 | 268 |
| 401 | 89 |
| 503 | 63 |
| 429 | 52 |
| 410 | 37 |
Day-zero baseline — we will re-measure weekly. This is the first reading; adoption starts at zero, and the point of publishing it now is to have a fixed reference line to measure the first emitters against.
Machine-readable: /proof/x401-adoption-2026-09-07.json.