422 Unprocessable Entity Explained Updated: March 6, 2017 What Is 422 Unprocessable Entity? A 422 status code occurs when a request is well-formed, however, due to semantic errors it is unable to be processed. This HTTP status was introduced in RFC 4918 and is more specifically geared toward HTTP extensions for Web Distributed Authoring and Versioning (WebDAV). There is some controversy out there on whether or not developers should return a 400 vs 422 error to clients (more on the differences between both statuses below). However, in most cases, it is agreed upon that the 422 status should only be returned if you support WebDAV capabilities. A word-for-word definition of the 422 status code taken from section 11.2 in RFC 4918 can be read below. The 422 (Unprocessable Entity) status code means the server understands the content type of the request entity (hence a 415(Unsupported Media Type) status code is inappropriate), and the syntax of the request entity is correct (thus a 400 (Bad Request) status code is inappropriate) but was unable to process the contained instructions. The definition goes on to say: For example, this error condition may occur if an XML request body contains well-formed (i.e., syntactically correct), but semantically erroneous, XML instructions. 400 vs 422 Status Codes Bad request errors make use of the 400 status code and should be returned to the client if the request syntax is malformed, contains invalid request message framing, or has deceptive request routing. This status code may seem pretty similar to the 422 unprocessable entity status, however, one small piece of information that distinguishes them is the fact that the syntax of a request entity for a 422 error is correct whereas the syntax of a request that generates a 400 error is incorrect. The use of the 422 status should be reserved only for very particular use-cases. In most other cases where a client error has occurred due to malformed syntax, the 400 Bad Request status should be used.

https://www.keycdn.com/support/422-unprocessable-entity/

400 Bad Request现在似乎是用例中最好的HTTP/1.1状态代码。

在你提出问题(以及我最初的回答)的时候，RFC 7231还不存在;在这一点上，我反对400坏请求，因为RFC 2616说(强调我的):

由于语法错误，服务器无法理解请求。

并且您描述的请求是语法上有效的JSON，封装在语法上有效的HTTP中，因此服务器对请求的语法没有任何问题。

然而，正如Lee Saferite在评论中指出的那样，RFC 7231取代了RFC 2616，不包括这个限制:

400(坏请求)状态码表示服务器不能或不愿处理请求，原因是客户机错误(例如，格式错误的请求语法、无效的请求消息框架或欺骗性的请求路由)。

然而，在重新措辞之前(或者如果你想争论RFC 7231现在只是一个提议的标准)，422不可处理实体对于你的用例来说似乎不是一个不正确的HTTP状态代码，因为RFC 4918的介绍说:

而HTTP/1.1提供的状态码足以 描述WebDAV方法遇到的大多数错误条件 是一些不能完全归入现有类别的错误。 该规范定义了为WebDAV开发的额外状态代码 方法(第11节)

422页的描述说

422(不可处理实体)状态码表示服务器 理解请求实体的内容类型(因此是 415(不支持的媒体类型)状态码不合适) 请求实体的语法是正确的(因此是400(坏请求) 状态码不合适)，但无法处理包含的 指令。

(注意对语法的引用;我怀疑7231也部分淘汰了4918)

这听起来和你的情况一模一样，但以防你有任何疑问，它接着说:

例如，如果XML 请求主体包含格式良好的(即语法正确)，但是 语义错误的XML指令。

(将“XML”替换为“JSON”，我想我们可以同意这是你的情况)

现在，有些人会反对RFC 4918是关于“Web分布式编写和版本控制(WebDAV)的HTTP扩展”，你(大概)没有做任何涉及WebDAV的事情，所以不应该使用它。

如果要在使用原始标准中明确不包含该情况的错误代码和使用来自扩展的准确描述该情况的错误代码之间做出选择，我会选择后者。

此外，RFC 4918第21.4节引用了IANA超文本传输协议(HTTP)状态码注册表，其中可以找到422。

我建议HTTP客户端或服务器使用来自该注册中心的任何状态代码是完全合理的，只要它们正确地这样做。

但是对于HTTP/1.1, RFC 7231有吸引力，所以只使用400个坏请求!

400 Bad Request是用例的HTTP状态代码。该代码由HTTP/0.9-1.1 RFC定义。

由于格式错误，服务器无法理解请求 语法。客户端不应该重复请求 修改。

https://www.rfc-editor.org/rfc/rfc2616#section-10.4.1

422不可处理实体由RFC 4918 - WebDav定义。请注意，与400相比有轻微的差异，见下面引用的文本。

如果XML 请求主体包含格式良好的(即语法正确)，但是 语义错误的XML指令。

为了保持统一的接口，你应该只在XML响应的情况下使用422，你还应该支持Webdav扩展定义的所有状态码，而不仅仅是422。

https://www.rfc-editor.org/rfc/rfc4918#page-78

另见Mark Nottingham关于状态代码的帖子:

试图“深入”映射应用程序的每个部分是错误的。 转换成HTTP状态码;在大多数情况下，粒度级别你 想要的目标要粗糙得多。有疑问时，可以使用 通用状态代码200 OK, 400 Bad Request和500 Internal 没有更好的匹配时的服务错误。

如何看待HTTP状态码

案例研究:GitHub API

https://docs.github.com/en/rest/overview/resources-in-the-rest-api#client-errors

也许从著名的api复制是一个明智的想法:

There are three possible types of client errors on API calls that receive request bodies: Sending invalid JSON will result in a 400 Bad Request response: HTTP/1.1 400 Bad Request Content-Length: 35 {"message":"Problems parsing JSON"} Sending the wrong type of JSON values will result in a 400 Bad Request response: HTTP/1.1 400 Bad Request Content-Length: 40 {"message":"Body should be a JSON object"} Sending invalid fields will result in a 422 Unprocessable Entity response: HTTP/1.1 422 Unprocessable Entity Content-Length: 149 { "message": "Validation Failed", "errors": [ { "resource": "Issue", "field": "title", "code": "missing_field" } ] }

422 Unprocessable Entity Explained Updated: March 6, 2017 What Is 422 Unprocessable Entity? A 422 status code occurs when a request is well-formed, however, due to semantic errors it is unable to be processed. This HTTP status was introduced in RFC 4918 and is more specifically geared toward HTTP extensions for Web Distributed Authoring and Versioning (WebDAV). There is some controversy out there on whether or not developers should return a 400 vs 422 error to clients (more on the differences between both statuses below). However, in most cases, it is agreed upon that the 422 status should only be returned if you support WebDAV capabilities. A word-for-word definition of the 422 status code taken from section 11.2 in RFC 4918 can be read below. The 422 (Unprocessable Entity) status code means the server understands the content type of the request entity (hence a 415(Unsupported Media Type) status code is inappropriate), and the syntax of the request entity is correct (thus a 400 (Bad Request) status code is inappropriate) but was unable to process the contained instructions. The definition goes on to say: For example, this error condition may occur if an XML request body contains well-formed (i.e., syntactically correct), but semantically erroneous, XML instructions. 400 vs 422 Status Codes Bad request errors make use of the 400 status code and should be returned to the client if the request syntax is malformed, contains invalid request message framing, or has deceptive request routing. This status code may seem pretty similar to the 422 unprocessable entity status, however, one small piece of information that distinguishes them is the fact that the syntax of a request entity for a 422 error is correct whereas the syntax of a request that generates a 400 error is incorrect. The use of the 422 status should be reserved only for very particular use-cases. In most other cases where a client error has occurred due to malformed syntax, the 400 Bad Request status should be used.

https://www.keycdn.com/support/422-unprocessable-entity/

为反映截至2015年的状况:

从行为上讲，客户端和中介端对400和422响应代码的处理是一样的，因此使用哪一种响应代码并没有什么具体的区别。

然而，我希望看到400目前被更广泛地使用，而且HTTPbis规范提供的说明使它成为两种状态代码中更合适的一个:

HTTPbis规范明确了400的意图，不只是用于语法错误。现在使用了更广泛的短语“表示服务器不能或不愿处理由于客户机错误造成的请求”。 422是一个特定的WebDAV扩展，在RFC 2616或更新的HTTPbis规范中没有引用。

就上下文而言，HTTPbis是HTTP/1.1规范的修订版，试图澄清不清楚或不一致的地方。一旦达到批准状态，它将取代RFC2616。