Internet Engineering Task Force (IETF)                     M. Nottingham
Request for Comments: 9457
Obsoletes: 7807                                                 E. Wilde
Category: Standards Track
ISSN: 2070-1721                                                 S. Dalal
                                                              July 2023


                    Problem Details for HTTP APIs

Abstract

  This document defines a "problem detail" to carry machine-readable
  details of errors in HTTP response content to avoid the need to
  define new error response formats for HTTP APIs.

  This document obsoletes RFC 7807.

Status of This Memo

  This is an Internet Standards Track document.

  This document is a product of the Internet Engineering Task Force
  (IETF).  It represents the consensus of the IETF community.  It has
  received public review and has been approved for publication by the
  Internet Engineering Steering Group (IESG).  Further information on
  Internet Standards is available in Section 2 of RFC 7841.

  Information about the current status of this document, any errata,
  and how to provide feedback on it may be obtained at
  https://www.rfc-editor.org/info/rfc9457.

Copyright Notice

  Copyright (c) 2023 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  (https://trustee.ietf.org/license-info) in effect on the date of
  publication of this document.  Please review these documents
  carefully, as they describe your rights and restrictions with respect
  to this document.  Code Components extracted from this document must
  include Revised BSD License text as described in Section 4.e of the
  Trust Legal Provisions and are provided without warranty as described
  in the Revised BSD License.

Table of Contents

  1.  Introduction
  2.  Requirements Language
  3.  The Problem Details JSON Object
    3.1.  Members of a Problem Details Object
      3.1.1.  "type"
      3.1.2.  "status"
      3.1.3.  "title"
      3.1.4.  "detail"
      3.1.5.  "instance"
    3.2.  Extension Members
  4.  Defining New Problem Types
    4.1.  Example
    4.2.  Registered Problem Types
      4.2.1.  about:blank
  5.  Security Considerations
  6.  IANA Considerations
  7.  References
    7.1.  Normative References
    7.2.  Informative References
  Appendix A.  JSON Schema for HTTP Problems
  Appendix B.  HTTP Problems and XML
  Appendix C.  Using Problem Details with Other Formats
  Appendix D.  Changes from RFC 7807
  Acknowledgements
  Authors' Addresses

1.  Introduction

  HTTP status codes (Section 15 of [HTTP]) cannot always convey enough
  information about errors to be helpful.  While humans using web
  browsers can often understand an HTML [HTML5] response content, non-
  human consumers of HTTP APIs have difficulty doing so.

  To address that shortcoming, this specification defines simple JSON
  [JSON] and XML [XML] document formats to describe the specifics of a
  problem encountered -- "problem details".

  For example, consider a response indicating that the client's account
  doesn't have enough credit.  The API's designer might decide to use
  the 403 Forbidden status code to inform generic HTTP software (such
  as client libraries, caches, and proxies) of the response's general
  semantics.  API-specific problem details (such as why the server
  refused the request and the applicable account balance) can be
  carried in the response content so that the client can act upon them
  appropriately (for example, triggering a transfer of more credit into
  the account).

  This specification identifies the specific "problem type" (e.g., "out
  of credit") with a URI [URI].  HTTP APIs can use URIs under their
  control to identify problems specific to them or can reuse existing
  ones to facilitate interoperability and leverage common semantics
  (see Section 4.2).

  Problem details can contain other information, such as a URI
  identifying the problem's specific occurrence (effectively giving an
  identifier to the concept "The time Joe didn't have enough credit
  last Thursday"), which can be useful for support or forensic
  purposes.

  The data model for problem details is a JSON [JSON] object; when
  serialized as a JSON document, it uses the "application/problem+json"
  media type.  Appendix B defines an equivalent XML format, which uses
  the "application/problem+xml" media type.

  When they are conveyed in an HTTP response, the contents of problem
  details can be negotiated using proactive negotiation; see
  Section 12.1 of [HTTP].  In particular, the language used for human-
  readable strings (such as those in title and description) can be
  negotiated using the Accept-Language request header field
  (Section 12.5.4 of [HTTP]), although that negotiation may still
  result in a non-preferred, default representation being returned.

  Problem details can be used with any HTTP status code, but they most
  naturally fit the semantics of 4xx and 5xx responses.  Note that
  problem details are (naturally) not the only way to convey the
  details of a problem in HTTP.  If the response is still a
  representation of a resource, for example, it's often preferable to
  describe the relevant details in that application's format.
  Likewise, defined HTTP status codes cover many situations with no
  need to convey extra detail.

  This specification's aim is to define common error formats for
  applications that need one so that they aren't required to define
  their own or, worse, tempted to redefine the semantics of existing
  HTTP status codes.  Even if an application chooses not to use it to
  convey errors, reviewing its design can help guide the design
  decisions faced when conveying errors in an existing format.

  See Appendix D for a list of changes from [RFC7807].

2.  Requirements Language

  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
  "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
  "OPTIONAL" in this document are to be interpreted as described in
  BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
  capitals, as shown here.

3.  The Problem Details JSON Object

  The canonical model for problem details is a JSON [JSON] object.
  When serialized in a JSON document, that format is identified with
  the "application/problem+json" media type.

  For example:

  POST /purchase HTTP/1.1
  Host: store.example.com
  Content-Type: application/json
  Accept: application/json, application/problem+json

  {
    "item": 123456,
    "quantity": 2
  }

  HTTP/1.1 403 Forbidden
  Content-Type: application/problem+json
  Content-Language: en

  {
   "type": "https://example.com/probs/out-of-credit",
   "title": "You do not have enough credit.",
   "detail": "Your current balance is 30, but that costs 50.",
   "instance": "/account/12345/msgs/abc",
   "balance": 30,
   "accounts": ["/account/12345",
                "/account/67890"]
  }

  Here, the out-of-credit problem (identified by its type) indicates
  the reason for the 403 in "title", identifies the specific problem
  occurrence with "instance", gives occurrence-specific details in
  "detail", and adds two extensions: "balance" conveys the account's
  balance, and "accounts" lists links where the account can be topped
  up.

  When designed to accommodate it, problem-specific extensions can
  convey more than one instance of the same problem type.  For example:

  POST /details HTTP/1.1
  Host: account.example.com
  Accept: application/json

  {
    "age": 42.3,
    "profile": {
      "color": "yellow"
    }
  }

  HTTP/1.1 422 Unprocessable Content
  Content-Type: application/problem+json
  Content-Language: en

  {
   "type": "https://example.net/validation-error",
   "title": "Your request is not valid.",
   "errors": [
               {
                 "detail": "must be a positive integer",
                 "pointer": "#/age"
               },
               {
                 "detail": "must be 'green', 'red' or 'blue'",
                 "pointer": "#/profile/color"
               }
            ]
  }

  The fictional problem type here defines the "errors" extension, an
  array that describes the details of each validation error.  Each
  member is an object containing "detail" to describe the issue and
  "pointer" to locate the problem within the request's content using a
  JSON Pointer [JSON-POINTER].

  When an API encounters multiple problems that do not share the same
  type, it is RECOMMENDED that the most relevant or urgent problem be
  represented in the response.  While it is possible to create generic
  "batch" problem types that convey multiple, disparate types, they do
  not map well into HTTP semantics.

  Note also that the API has responded with the "application/
  problem+json" type, even though the client did not list it in Accept,
  as is allowed by HTTP (see Section 12.5.1 of [HTTP]).

3.1.  Members of a Problem Details Object

  Problem detail objects can have the following members.  If a member's
  value type does not match the specified type, the member MUST be
  ignored -- i.e., processing will continue as if the member had not
  been present.

3.1.1.  "type"

  The "type" member is a JSON string containing a URI reference [URI]
  that identifies the problem type.  Consumers MUST use the "type" URI
  (after resolution, if necessary) as the problem type's primary
  identifier.

  When this member is not present, its value is assumed to be
  "about:blank".

  If the type URI is a locator (e.g., those with an "http" or "https"
  scheme), dereferencing it SHOULD provide human-readable documentation
  for the problem type (e.g., using HTML [HTML5]).  However, consumers
  SHOULD NOT automatically dereference the type URI, unless they do so
  when providing information to developers (e.g., when a debugging tool
  is in use).

  When "type" contains a relative URI, it is resolved relative to the
  document's base URI, as per [URI], Section 5.  However, using
  relative URIs can cause confusion, and they might not be handled
  correctly by all implementations.

  For example, if the two resources "https://api.example.org/foo/
  bar/123" and "https://api.example.org/widget/456" both respond with a
  "type" equal to the relative URI reference "example-problem", when
  resolved they will identify different resources
  ("https://api.example.org/foo/bar/example-problem" and
  "https://api.example.org/widget/example-problem", respectively).  As
  a result, it is RECOMMENDED that absolute URIs be used in "type" when
  possible and that when relative URIs are used, they include the full
  path (e.g., "/types/123").

  The type URI is allowed to be a non-resolvable URI.  For example, the
  tag URI scheme [TAG] can be used to uniquely identify problem types:

  tag:[email protected],2021-09-17:OutOfLuck

  However, resolvable type URIs are encouraged by this specification
  because it might become desirable to resolve the URI in the future.
  For example, if an API designer used the URI above and later adopted
  a tool that resolves type URIs to discover information about the
  error, taking advantage of that capability would require switching to
  a resolvable URI, creating a new identity for the problem type and
  thus introducing a breaking change.

3.1.2.  "status"

  The "status" member is a JSON number indicating the HTTP status code
  ([HTTP], Section 15) generated by the origin server for this
  occurrence of the problem.

  The "status" member, if present, is only advisory; it conveys the
  HTTP status code used for the convenience of the consumer.
  Generators MUST use the same status code in the actual HTTP response,
  to assure that generic HTTP software that does not understand this
  format still behaves correctly.  See Section 5 for further caveats
  regarding its use.

  Consumers can use the status member to determine what the original
  status code used by the generator was when it has been changed (e.g.,
  by an intermediary or cache) and when a message's content is
  persisted without HTTP information.  Generic HTTP software will still
  use the HTTP status code.

3.1.3.  "title"

  The "title" member is a JSON string containing a short, human-
  readable summary of the problem type.

  It SHOULD NOT change from occurrence to occurrence of the problem,
  except for localization (e.g., using proactive content negotiation;
  see [HTTP], Section 12.1).

  The "title" string is advisory and is included only for users who are
  unaware of and cannot discover the semantics of the type URI (e.g.,
  during offline log analysis).

3.1.4.  "detail"

  The "detail" member is a JSON string containing a human-readable
  explanation specific to this occurrence of the problem.

  The "detail" string, if present, ought to focus on helping the client
  correct the problem, rather than giving debugging information.

  Consumers SHOULD NOT parse the "detail" member for information;
  extensions are more suitable and less error-prone ways to obtain such
  information.

3.1.5.  "instance"

  The "instance" member is a JSON string containing a URI reference
  that identifies the specific occurrence of the problem.

  When the "instance" URI is dereferenceable, the problem details
  object can be fetched from it.  It might also return information
  about the problem occurrence in other formats through use of
  proactive content negotiation (see [HTTP], Section 12.5.1).

  When the "instance" URI is not dereferenceable, it serves as a unique
  identifier for the problem occurrence that may be of significance to
  the server but is opaque to the client.

  When "instance" contains a relative URI, it is resolved relative to
  the document's base URI, as per [URI], Section 5.  However, using
  relative URIs can cause confusion, and they might not be handled
  correctly by all implementations.

  For example, if the two resources "https://api.example.org/foo/
  bar/123" and "https://api.example.org/widget/456" both respond with
  an "instance" equal to the relative URI reference "example-instance",
  when resolved they will identify different resources
  ("https://api.example.org/foo/bar/example-instance" and
  "https://api.example.org/widget/example-instance", respectively).  As
  a result, it is RECOMMENDED that absolute URIs be used in "instance"
  when possible, and that when relative URIs are used, they include the
  full path (e.g., "/instances/123").

3.2.  Extension Members

  Problem type definitions MAY extend the problem details object with
  additional members that are specific to that problem type.

  For example, our out-of-credit problem above defines two such
  extensions -- "balance" and "accounts" to convey additional, problem-
  specific information.

  Similarly, the "validation error" example defines an "errors"
  extension that contains a list of individual error occurrences found,
  with details and a pointer to the location of each.

  Clients consuming problem details MUST ignore any such extensions
  that they don't recognize; this allows problem types to evolve and
  include additional information in the future.

  When creating extensions, problem type authors should choose their
  names carefully.  To be used in the XML format (see Appendix B), they
  will need to conform to the Name rule in Section 2.3 of [XML].

4.  Defining New Problem Types

  When an HTTP API needs to define a response that indicates an error
  condition, it might be appropriate to do so by defining a new problem
  type.

  Before doing so, it's important to understand what they are good for
  and what is better left to other mechanisms.

  Problem details are not a debugging tool for the underlying
  implementation; rather, they are a way to expose greater detail about
  the HTTP interface itself.  Designers of new problem types need to
  carefully take into account the Security Considerations (Section 5),
  in particular, the risk of exposing attack vectors by exposing
  implementation internals through error messages.

  Likewise, truly generic problems -- i.e., conditions that might apply
  to any resource on the Web -- are usually better expressed as plain
  status codes.  For example, a "write access disallowed" problem is
  probably unnecessary, since a 403 Forbidden status code in response
  to a PUT request is self-explanatory.

  Finally, an application might have a more appropriate way to carry an
  error in a format that it already defines.  Problem details are
  intended to avoid the necessity of establishing new "fault" or
  "error" document formats, not to replace existing domain-specific
  formats.

  That said, it is possible to add support for problem details to
  existing HTTP APIs using HTTP content negotiation (e.g., using the
  Accept request header to indicate a preference for this format; see
  [HTTP], Section 12.5.1).

  New problem type definitions MUST document:

  1.  a type URI (typically, with the "http" or "https" scheme)

  2.  a title that appropriately describes it (think short)

  3.  the HTTP status code for it to be used with

  Problem type definitions MAY specify the use of the Retry-After
  response header ([HTTP], Section 10.2.3) in appropriate
  circumstances.

  A problem type URI SHOULD resolve to HTML [HTML5] documentation that
  explains how to resolve the problem.

  A problem type definition MAY specify additional members on the
  problem details object.  For example, an extension might use typed
  links [WEB-LINKING] to another resource that machines can use to
  resolve the problem.

  If such additional members are defined, their names SHOULD start with
  a letter (ALPHA, as per [ABNF], Appendix B.1) and SHOULD comprise
  characters from ALPHA, DIGIT ([ABNF], Appendix B.1), and "_" (so that
  it can be serialized in formats other than JSON), and they SHOULD be
  three characters or longer.

4.1.  Example

  For example, if you are publishing an HTTP API to your online
  shopping cart, you might need to indicate that the user is out of
  credit (our example from above) and therefore cannot make the
  purchase.

  If you already have an application-specific format that can
  accommodate this information, it's probably best to do that.
  However, if you don't, you might use one of the problem detail
  formats -- JSON if your API is JSON-based or XML if it uses that
  format.

  To do so, you might look in the registry (Section 4.2) for an
  already-defined type URI that suits your purposes.  If one is
  available, you can reuse that URI.

  If one isn't available, you could mint and document a new type URI
  (which ought to be under your control and stable over time), an
  appropriate title and the HTTP status code that it will be used with,
  along with what it means and how it should be handled.

4.2.  Registered Problem Types

  This specification defines the "HTTP Problem Types" registry for
  common, widely used problem type URIs, to promote reuse.

  The policy for this registry is Specification Required, per
  [RFC8126], Section 4.6.

  When evaluating requests, the designated expert(s) should consider
  community feedback, how well-defined the problem type is, and this
  specification's requirements.  Vendor-specific, application-specific,
  and deployment-specific values are unable to be registered.
  Specification documents should be published in a stable, freely
  available manner (ideally located with a URL) but need not be
  standards.

  Registrations MAY use the prefix "https://iana.org/assignments/http-
  problem-types#" for the type URI.  Note that those URIs may not be
  able to be resolved.

  The following template should be used for registration requests:

  Type URI:  [a URI for the problem type]
  Title:  [a short description of the problem type]
  Recommended HTTP status code:  [what status code is most appropriate
     to use with the type]
  Reference:  [to a specification defining the type]

  See the registry at <https://iana.org/assignments/http-problem-types>
  for details on where to send registration requests.

4.2.1.  about:blank

  This specification registers one Problem Type, "about:blank", as
  follows.

  Type URI:  about:blank
  Title:  See HTTP Status Code
  Recommended HTTP status code:  N/A
  Reference:  RFC 9457

  The "about:blank" URI [ABOUT], when used as a problem type, indicates
  that the problem has no additional semantics beyond that of the HTTP
  status code.

  When "about:blank" is used, the title SHOULD be the same as the
  recommended HTTP status phrase for that code (e.g., "Not Found" for
  404, and so on), although it MAY be localized to suit client
  preferences (expressed with the Accept-Language request header).

  Please note that according to how the "type" member is defined
  (Section 3.1), the "about:blank" URI is the default value for that
  member.  Consequently, any problem details object not carrying an
  explicit "type" member implicitly uses this URI.

5.  Security Considerations

  When defining a new problem type, the information included must be
  carefully vetted.  Likewise, when actually generating a problem --
  however it is serialized -- the details given must also be
  scrutinized.

  Risks include leaking information that can be exploited to compromise
  the system, access to the system, or the privacy of users of the
  system.

  Generators providing links to occurrence information are encouraged
  to avoid making implementation details such as a stack dump available
  through the HTTP interface, since this can expose sensitive details
  of the server implementation, its data, and so on.

  The "status" member duplicates the information available in the HTTP
  status code itself, bringing the possibility of disagreement between
  the two.  Their relative precedence is not clear, since a
  disagreement might indicate that (for example) an intermediary has
  changed the HTTP status code in transit (e.g., by a proxy or cache).
  Generic HTTP software (such as proxies, load balancers, firewalls,
  and virus scanners) are unlikely to know of or respect the status
  code conveyed in this member.

6.  IANA Considerations

  In the "application" registry under the "Media Types" registry, IANA
  has updated the "application/problem+json" and "application/
  problem+xml" registrations to refer to this document.

  IANA has created the "HTTP Problem Types" registry as specified in
  Section 4.2 and populated it with "about:blank" as per Section 4.2.1.

7.  References

7.1.  Normative References

  [ABNF]     Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
             Specifications: ABNF", STD 68, RFC 5234,
             DOI 10.17487/RFC5234, January 2008,
             <https://www.rfc-editor.org/info/rfc5234>.

  [HTTP]     Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
             Ed., "HTTP Semantics", STD 97, RFC 9110,
             DOI 10.17487/RFC9110, June 2022,
             <https://www.rfc-editor.org/info/rfc9110>.

  [JSON]     Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
             Interchange Format", STD 90, RFC 8259,
             DOI 10.17487/RFC8259, December 2017,
             <https://www.rfc-editor.org/info/rfc8259>.

  [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119,
             DOI 10.17487/RFC2119, March 1997,
             <https://www.rfc-editor.org/info/rfc2119>.

  [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
             Writing an IANA Considerations Section in RFCs", BCP 26,
             RFC 8126, DOI 10.17487/RFC8126, June 2017,
             <https://www.rfc-editor.org/info/rfc8126>.

  [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
             2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
             May 2017, <https://www.rfc-editor.org/info/rfc8174>.

  [URI]      Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
             Resource Identifier (URI): Generic Syntax", STD 66,
             RFC 3986, DOI 10.17487/RFC3986, January 2005,
             <https://www.rfc-editor.org/info/rfc3986>.

  [XML]      Bray, T., Paoli, J., Sperberg-McQueen, C. M., Maler, E.,
             and F. Yergeau, "Extensible Markup Language (XML) 1.0
             (Fifth Edition)", W3C Recommendation REC-xml-20081126,
             November 2008,
             <https://www.w3.org/TR/2008/REC-xml-20081126/>.

7.2.  Informative References

  [ABOUT]    Moonesamy, S., Ed., "The "about" URI Scheme", RFC 6694,
             DOI 10.17487/RFC6694, August 2012,
             <https://www.rfc-editor.org/info/rfc6694>.

  [HTML5]    WHATWG, "HTML: Living Standard",
             <https://html.spec.whatwg.org>.

  [ISO-19757-2]
             ISO, "Information technology -- Document Schema Definition
             Language (DSDL) -- Part 2: Regular-grammar-based
             validation -- RELAX NG", ISO/IEC 19757-2:2008, December
             2008, <https://www.iso.org/standard/52348.html>.

  [JSON-POINTER]
             Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
             "JavaScript Object Notation (JSON) Pointer", RFC 6901,
             DOI 10.17487/RFC6901, April 2013,
             <https://www.rfc-editor.org/info/rfc6901>.

  [JSON-SCHEMA]
             Wright, A., Ed., Andrews, H., Ed., Hutton, B., Ed., and G.
             Dennis, "JSON Schema: A Media Type for Describing JSON
             Documents", Work in Progress, Internet-Draft, draft-
             bhutton-json-schema-01, 10 June 2022,
             <https://datatracker.ietf.org/doc/html/draft-bhutton-json-
             schema-01>.

  [RDFA]     Adida, B., Birbeck, M., McCarron, S., and I. Herman, "RDFa
             Core 1.1 - Third Edition", W3C Recommendation, March 2015,
             <https://www.w3.org/TR/2015/REC-rdfa-core-20150317/>.

  [RFC7807]  Nottingham, M. and E. Wilde, "Problem Details for HTTP
             APIs", RFC 7807, DOI 10.17487/RFC7807, March 2016,
             <https://www.rfc-editor.org/info/rfc7807>.

  [TAG]      Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
             RFC 4151, DOI 10.17487/RFC4151, October 2005,
             <https://www.rfc-editor.org/info/rfc4151>.

  [WEB-LINKING]
             Nottingham, M., "Web Linking", RFC 8288,
             DOI 10.17487/RFC8288, October 2017,
             <https://www.rfc-editor.org/info/rfc8288>.

  [XSLT]     Clark, J., Pieters, S., and H. Thompson, "Associating
             Style Sheets with XML documents 1.0 (Second Edition)", W3C
             Recommendation, October 2010,
             <https://www.w3.org/TR/2010/REC-xml-stylesheet-20101028/>.

Appendix A.  JSON Schema for HTTP Problems

  This section presents a non-normative JSON Schema [JSON-SCHEMA] for
  HTTP problem details.  If there is any disagreement between it and
  the text of the specification, the latter prevails.

  # NOTE: '\' line wrapping per RFC 8792
  {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "title": "An RFC 7807 problem object",
    "type": "object",
    "properties": {
      "type": {
        "type": "string",
        "format": "uri-reference",
        "description": "A URI reference that identifies the \
  problem type."
      },
      "title": {
        "type": "string",
        "description": "A short, human-readable summary of the \
  problem type."
      },
      "status": {
        "type": "integer",
        "description": "The HTTP status code \
  generated by the origin server for this occurrence of the problem.",
        "minimum": 100,
        "maximum": 599
      },
      "detail": {
        "type": "string",
        "description": "A human-readable explanation specific to \
  this occurrence of the problem."
      },
      "instance": {
        "type": "string",
        "format": "uri-reference",
        "description": "A URI reference that identifies the \
  specific occurrence of the problem. It may or may not yield \
  further information if dereferenced."
      }
    }
  }

Appendix B.  HTTP Problems and XML

  HTTP-based APIs that use XML [XML] can express problem details using
  the format defined in this appendix.

  The RELAX NG schema [ISO-19757-2] for the XML format is:

     default namespace ns = "urn:ietf:rfc:7807"

     start = problem

     problem =
       element problem {
         (  element  type            { xsd:anyURI }?
          & element  title           { xsd:string }?
          & element  detail          { xsd:string }?
          & element  status          { xsd:positiveInteger }?
          & element  instance        { xsd:anyURI }? ),
         anyNsElement
       }

     anyNsElement =
       (  element    ns:*  { anyNsElement | text }
        | attribute  *     { text })*

  Note that this schema is only intended as documentation and not as a
  normative schema that captures all constraints of the XML format.  It
  is possible to use other XML schema languages to define a similar set
  of constraints (depending on the features of the chosen schema
  language).

  The media type for this format is "application/problem+xml".

  Extension arrays and objects are serialized into the XML format by
  considering an element containing a child or children to represent an
  object, except for elements containing only one or more child
  elements named "i", which are considered arrays.  For example, the
  example above appears in XML as follows:

  HTTP/1.1 403 Forbidden
  Content-Type: application/problem+xml
  Content-Language: en

  <?xml version="1.0" encoding="UTF-8"?>
  <problem xmlns="urn:ietf:rfc:7807">
    <type>https://example.com/probs/out-of-credit</type>
    <title>You do not have enough credit.</title>
    <detail>Your current balance is 30, but that costs 50.</detail>
    <instance>https://example.net/account/12345/msgs/abc</instance>
    <balance>30</balance>
    <accounts>
      <i>https://example.net/account/12345</i>
      <i>https://example.net/account/67890</i>
    </accounts>
  </problem>

  This format uses an XML namespace, primarily to allow embedding it
  into other XML-based formats; it does not imply that it can or should
  be extended with elements or attributes in other namespaces.  The
  RELAX NG schema explicitly only allows elements from the one
  namespace used in the XML format.  Any extension arrays and objects
  MUST be serialized into XML markup using only that namespace.

  When using the XML format, it is possible to embed an XML processing
  instruction in the XML that instructs clients to transform the XML,
  using the referenced XSL Transformations (XSLT) code [XSLT].  If this
  code is transforming the XML into (X)HTML, then it is possible to
  serve the XML format, and yet have clients capable of performing the
  transformation display human-friendly (X)HTML that is rendered and
  displayed at the client.  Note that when using this method, it is
  advisable to use XSLT 1.0 in order to maximize the number of clients
  capable of executing the XSLT code.

Appendix C.  Using Problem Details with Other Formats

  In some situations, it can be advantageous to embed problem details
  in formats other than those described here.  For example, an API that
  uses HTML [HTML5] might want to also use HTML for expressing its
  problem details.

  Problem details can be embedded in other formats either by
  encapsulating one of the existing serializations (JSON or XML) into
  that format or by translating the model of a problem detail (as
  specified in Section 3) into the format's conventions.

  For example, in HTML, a problem could be embedded by encapsulating
  JSON in a script tag:

  <script type="application/problem+json">
    {
     "type": "https://example.com/probs/out-of-credit",
     "title": "You do not have enough credit.",
     "detail": "Your current balance is 30, but that costs 50.",
     "instance": "/account/12345/msgs/abc",
     "balance": 30,
     "accounts": ["/account/12345",
                  "/account/67890"]
    }
  </script>

  or by defining a mapping into a Resource Description Framework in
  Attributes (RDFa) [RDFA].

  This specification does not make specific recommendations regarding
  embedding problem details in other formats; the appropriate way to
  embed them depends both upon the format in use and application of
  that format.

Appendix D.  Changes from RFC 7807

  This revision has made the following changes:

  *  Section 4.2 introduces a registry of common problem type URIs

  *  Section 3 clarifies how multiple problems should be treated

  *  Section 3.1.1 provides guidance for using type URIs that cannot be
     dereferenced

Acknowledgements

  The authors would like to thank Jan Algermissen, Subbu Allamaraju,
  Mike Amundsen, Roy Fielding, Eran Hammer, Sam Johnston, Mike McCall,
  Julian Reschke, and James Snell for their comments and suggestions.

Authors' Addresses

  Mark Nottingham
  Prahran
  Australia
  Email: [email protected]
  URI:   https://www.mnot.net/


  Erik Wilde
  Email: [email protected]
  URI:   http://dret.net/netdret/


  Sanjay Dalal
  United States of America
  Email: [email protected]
  URI:   https://github.com/sdatspun2

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