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[4] RFC 2068, RFC 2616, RFC 7231 のいずれも、 402 (Payment Required) は理由句が定義されているだけで、状態符号自体は予約 >>2, >>3 されていて、使われていません。

[2] RFC 2068 & 2616 (HTTP/1.1) 10.4.3 402 Payment Required

This code is reserved for future use.

この符号は将来の使用のために予約しています。

[3] RFC 7231 - Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content (2014-08-07 05:54:02 +09:00 版) https://tools.ietf.org/html/rfc7231#section-6.5.2

[1] 法と技術とクローラと私 - 最速転職研究会 (2010-07-08 07:47:20 +09:00 版) http://d.hatena.ne.jp/mala/20100707/1278514965

[409] RFC 2660 - The Secure HyperText Transfer Protocol ( (2014-11-09 14:12:37 +09:00 版)) http://tools.ietf.org/html/rfc2660#section-5.2.1

[5]

REST API Reference - PayPal Developer (2015-07-02 12:54:14 +09:00 版) https://developer.paypal.com/docs/api/

402 - Failed request

[6]

Stripe API Reference (2015-08-29 17:37:34 +09:00 版) https://stripe.com/docs/api#errors

When a request is valid but does not complete successfully (e.g. a card is declined), we return a 402 error code.
402 - Request Failed Parameters were valid but request failed.

[7]

402 Payment Requiredが出た方へ ( (2012-02-11 05:45:34 +09:00)) http://s1.gptwm.com/402error.html

本サイトは同時接続数制限2ですので、それを超えるとエラーが出るようにしています。

[8]

New HTTP Status Code (402 - Payment Required) - API Announcements — Ecommerce University ( (2016-05-15 18:33:58 +09:00)) https://ecommerce.shopify.com/c/api-announcements/t/new-http-status-code-402-payment-required-215030

I want to let you know the API will now respond with (402 - Payment Required) when a shop does not have access to a specific feature. Prior to this change the API would respond with (404 - Not Found).
In addition to this HTTP Status Code there will be an error included in the JSON response:
This shop's plan does not have access to this feature

[9]

Google APIs - Global domain errors | YouTube Data API | Google Developers ( (2015-05-21 03:44:24 +09:00)) https://developers.google.com/youtube/v3/docs/core_errors#PAYMENT_REQUIRED

dailyLimitExceeded402 A daily budget limit set by the developer has been reached.
quotaExceeded402 The requested operation requires more resources than the quota allows. Payment is required to complete the operation.
user402 The requested operation requires some kind of payment from the authenticated user.

[10] XユーザーのCoinbase Developer Platform🛡️さん: 「First, a little history: HTTP 402 “Payment Required” has been a part of the internet’s blueprint for decades, but it was never used. x402 turns this 402 status code into a real payment layer—letting any server request payment, and any client (human or agent) respond with」 / X, 午前1:04 · 2025年5月7日2025-05-06T16:04:00.000Z, 2025-05-08T02:48:50.000Z https://x.com/CoinbaseDev/status/1919785226936713369

[11] x402.org, 2025-05-08T03:02:38.000Z https://www.x402.org/

[12] x402: The Payment Protocol for Agentic Commerce - x402-whitepaper.pdf, 2025-05-08T01:20:16.000Z, 2025-05-08T03:03:22.564Z https://www.x402.org/x402-whitepaper.pdf

[13] GitHub - coinbase/x402: A payments protocol for the internet. Built on HTTP., 2025-05-08T03:04:39.000Z https://github.com/coinbase/x402/

[14] Introducing x402: a new standard for internet-native payments | Coinbase, 2025-05-08T03:05:17.000Z https://www.coinbase.com/developer-platform/discover/launches/x402

[15] Introducing pay per crawl: enabling content owners to charge AI crawlers for access, 2025-07-03T02:31:59.000Z https://blog.cloudflare.com/introducing-pay-per-crawl/

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