HTTPAPI                                                    M. Nottingham
Obsoletes: 7807 (if approved)                                   E. Wilde
Intended status: Standards Track
Expires: 16 April 18 October 2022                                        S. Dalal
                                                         13 October 2021
                                                           16 April 2022

                     Problem Details for HTTP APIs


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

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at

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   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 16 April 18 October 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   4   3
   3.  The Problem Details JSON Object . . . . . . . . . . . . . . .   4
     3.1.  Members of a Problem Details Object . . . . . . . . . . .   5
       3.1.1.  "type"  . . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  "status"  . . . . . . . . . . . . . . . . . . . . . .   6
       3.1.3.  "title" . . . . . . . . . . . . . . . . . . . . . . .   7
       3.1.4.  "detail"  . . . . . . . . . . . . . . . . . . . . . .   7
       3.1.5.  "instance"  . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Extension Members . . . . . . . . . . . . . . . . . . . .   8
   4.  Defining New Problem Types  . . . . . . . . . . . . . . . . .   8
     4.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   9  10
     4.2.  Registered Problem Types  . . . . . . . . . . . . . . . .  10
       4.2.1.  about:blank . . . . . . . . . . . . . . . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  JSON Schema for HTTP Problems  . . . . . . . . . . .  14
   Appendix B.  HTTP Problems and XML  . . . . . . . . . . . . . . .  15
   Appendix C.  Using Problem Details with Other Formats . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   HTTP [HTTP] status codes are sometimes not sufficient to (Section 15 of [HTTP]) cannot always convey enough
   information about an error errors to be helpful.  While humans behind using Web
   browsers can be informed about the nature of the problem with often understand an HTML [HTML5] response body, non-human non-
   human consumers of so-called "HTTP
   APIs" are usually not.

   This HTTP APIs have difficulty doing so.

   To address that shortcoming, this specification defines simple JSON
   [RFC8259] and XML [XML] document formats to suit this purpose.  They are designed to be
   reused by HTTP APIs, which can identify distinct "problem types"
   specific to their needs.

   Thus, API clients can be informed of both the high-level error class
   (using the status code) and the finer-grained details of describe the problem
   (using one specifics of these formats).
   problem(s) encountered -- "problem details".

   For example, consider a response that indicates indicating that the client's account
   doesn't have enough credit.  The API's designer might decide to use
   the 403 Forbidden status code
   might be deemed most appropriate to use, as it will inform HTTP-
   generic HTTP-generic software (such
   as client libraries, caches, and proxies) of the response's general semantics of the response.

   However, that doesn't give
   semantics.  API-specific problem details (such as the API client enough information about why the server
   refused the request was forbidden, and the applicable account balance, or how
   to correct the problem.  If these details are included balance) can be
   carried in the response body in a machine-readable format, content, so that the client can treat it
   appropriately; for act upon them
   appropriately (for example, triggering a transfer of more credit into
   the account. account).

   This specification does this by identifying a identifies the specific type of
   problem "problem type" (e.g., "out
   of credit") with a URI [RFC3986]; [RFC3986].  HTTP APIs can
   do this by nominating new use URIs under their control,
   control to identify problems specific to them, or by reusing can reuse existing ones.

   Additionally, problem
   ones to facilitate interoperability and leverage common semantics
   (see Section 4.2).

   Problem details can contain other information, such as a URI that identifies
   identifying the problem's specific occurrence of the problem (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

   The data model for problem details is a JSON [RFC8259] object; when
   serialized as a JSON document, it uses the "application/problem+json"
   media type.  Appendix B defines how to express them in an equivalent XML format, which uses
   the "application/problem+xml" media type.

   Note that problem details are (naturally) not the only way to convey
   the details of a problem in HTTP; if HTTP.  If the response is still a
   representation of a resource, for example, it's often preferable to
   accommodate describing
   describe the relevant details in that application's format.
   Likewise, in many situations, there is an appropriate defined HTTP status code that does not require extra detail codes cover many situations with no
   need to be conveyed.

   Instead, the convey extra detail.

   This specification's aim of this specification is to define common error formats for those
   applications that need one, 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.

2.  Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 [RFC8259] object.

   When serialized as a JSON document, that format is identified with
   the "application/problem+json" media type.

   For example, an HTTP response carrying JSON problem details:

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

    "type": "",
    "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",

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

   The ability

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

   HTTP/1.1 400 Bad Request
   Content-Type: application/problem+json
   Content-Language: en

    "type": "",
    "title": "Your request parameters didn't validate.",
   "invalid_params": is not valid.",
    "causes": [
                         "name": "age",
                  "detail": "must be a positive integer" integer",
                  "problem-pointer": "#/age"
                         "name": "color",
                  "detail": "must be 'green', 'red' or 'blue'"} 'blue'",
                  "problem-pointer": "#/profile/color"


   The fictional problem type here defines the "causes" extension, an
   array that this requires each of describes the subproblems details of multiple occurrences.  Each
   member is an object containing "detail" to be similar enough describe the issue, and
   "problem-pointer" to use locate the same HTTP status code.  If they problem within the request's content
   using a JSON Pointer [RFC6901].

   When an API encounters multiple problems that do not, not share the 207 (Multi-
   Status) code [RFC4918] could same
   type, it is RECOMMENDED that the most relevant or urgent problem be used
   represented in the response.  While it is possible to encapsulate multiple status
   messages. create generic
   "batch" problem types that convey multiple, disparate types, they do
   not map well into HTTP semantics.

3.1.  Members of a Problem Details Object

   Problem detail objects can have the following members.  If the type
   of 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
   [RFC3986] that identifies the problem type.  Consumers MUST use the
   "type" URI (after resolution, if necessary) as the problem's primary identifier
   for the problem type.

   When this member is not present, its value is assumed to be

   If the type URI is a locator (e.g., those with a "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
   in the course of
   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 [RFC3986], 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 "
   bar/123" and "" both respond with a
   "type" equal to the relative URI reference "example-problem", when
   resolved they will identify different resources
   ("" and
   "" 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 can also be a non-resolvable URI.  For example, the tag
   URI scheme [RFC4151] can be used to uniquely identify problem types:,2021-09-17:OutOfLuck

   Non-resolvable URIs ought not be used when there is some future
   possibility that it might become desireable desirable to do so.  For example, if
   an API designer used the URI above were used in an API and later adopted a tool was adopted that
   resolves type URIs to discover information about the error, taking
   advantage of that capability would require switching to a resolvable
   URI, thereby 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, in cases where it has been
   changed (e.g., by an intermediary or cache), and when message bodies
   persist 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 purposes of localization (e.g., using proactive content negotiation;
   see [HTTP], Section 12.1).

   The "title" string is advisory and included only for users who are
   not aware of the semantics of the URI and do can not have the ability to discover them
   (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" member, 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

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 [RFC3986], 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 "
   bar/123" and "" both respond with
   an "instance" equal to the relative URI reference "example-instance",
   when resolved they will identify different resources
   ("" and
   "" 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.

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

   Similarly, the "Multi-Status" example defines two extensions --
   "causes" and "problem-pointer".  Extensions like "problem-pointer"
   are more appropriate to use for problems associated with client side
   errors 4xx only.

   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.

   Note that because extensions are effectively put into a namespace by
   the problem type, it is not possible to define new "standard" members
   without defining a new media type.

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

   Before doing so, it's important to understand what they are good for,
   and what's 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 consider 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 could
   potentially 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

   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), and

   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

   A problem's 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 [RFC8288] to another resource that can be used by machines can use to resolve
   the problem.

   If such additional members are defined, their names SHOULD start with
   a letter (ALPHA, as per [RFC5234], Appendix B.1) and SHOULD consist
   of comprise
   characters from ALPHA, DIGIT ([RFC5234], 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

   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 consider using use one of the problem details
   formats -- JSON if your API is JSON-based, or XML if it uses that

   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.

   In summary: an instance URI will always identify a specific
   occurrence of a problem.  On the other hand, type URIs can be reused
   if an appropriate description of a problem type is already available
   someplace else, or they can be created for new problem types.

4.2.  Registered Problem Types

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

   Registration requests are reviewed and approved by a Designated
   Expert, as

   The policy for this registry is Specification Required, per
   [RFC8126], Section 4.5.  A specification document is
   appreciated, but not required.

   When evaluating requests requests, the 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 not registrable.  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 "
   problem-types#", and are encouraged to do so when a stable, neutral
   URI is desirable.
   problem-types#" for the type URI.

   Registration requests should use the following template:

   *  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
   ( for details on
   where to send registration requests.

4.2.1.  about:blank

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

   *  Type URI: about:blank

   *  Title: See HTTP Status Code

   *  Recommended HTTP status code: N/A

   *  Reference: [this document]

   The "about:blank" URI [RFC6694], 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

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

   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, thereby 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).

   As such, those defining problem types as well as generators and
   consumers of problems need to be aware that generic
   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

   Please update the "application/problem+json" and "application/
   problem+xml" registrations in the Internet media types registry
   [RFC6838]. to refer to this document.

   Please create the HTTP Problem Types Registry, as specified in
   Section 4.2, and populate it with "about:blank" as per Section 4.2.1.

7.  References

7.1.  Normative References

   [HTTP]     Fielding, R. T., Nottingham, M., and J. Reschke, "HTTP
              Semantics", Work in Progress, Internet-Draft, draft-ietf-
              httpbis-semantics-19, 12 September 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,

   [XML]      Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
              F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
              Edition)", World Wide Web Consortium Recommendation REC-
              xml-20081126, 26 November 2008,

7.2.  Informative References

   [HTML5]    WHATWG, "HTML - Living Standard", n.d.,

              Wright, A., Andrews, H., Hutton, B., and G. Dennis, "JSON
              Schema: A Media Type for Describing JSON Documents", Work
              in Progress, Internet-Draft, draft-bhutton-json-schema-00,
              8 December 2020, <

              International Organization for Standardization,
              "Information Technology -- Document Schema Definition
              Languages (DSDL) -- Part 2: Grammar-based Validation --
              RELAX NG", ISO/IEC 19757-2, 2003.

   [RDFA]     Adida, B., Birbeck, M., McCarron, S., and I. Herman, "RDFa
              Core 1.1 - Third Edition", World Wide Web Consortium
              Recommendation REC-rdfa-core-20150317, 17 March 2015,

   [RFC4151]  Kindberg, T. and S. Hawke, "The 'tag' URI Scheme",
              RFC 4151, DOI 10.17487/RFC4151, October 2005,

   [RFC4918]  Dusseault, L., Ed., "HTTP Extensions for Web Distributed
              Authoring and Versioning (WebDAV)", RFC 4918,
              DOI 10.17487/RFC4918, June 2007,

   [RFC6694]  Moonesamy, S., Ed., "The "about" URI Scheme", RFC 6694,
              DOI 10.17487/RFC6694, August 2012,

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

   [RFC6901]  Bryan, P., Ed., Zyp, K., and M. Nottingham, Ed.,
              "JavaScript Object Notation (JSON) Pointer", RFC 6901,
              DOI 10.17487/RFC6901, April 2013,

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,

   [XSLT]     Clark, J., Pieters, S., and H. Thompson, "Associating
              Style Sheets with XML documents 1.0 (Second Edition)",
              World Wide Web Consortium Recommendation REC-xml-
              stylesheet-20101028, 28 October 2010,

Appendix A.  JSON Schema for HTTP Problems

   This section presents a non-normative JSON Schema
   [I-D.draft-bhutton-json-schema-00] 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": "",
     "title": "A problem object RFC 7807bis",
     "type": "object",
     "properties": {
       "type": {
         "type": "string",
         "format": "uri-reference",
         "description": "A URI reference RFC3986 that identifies the \
   problem type."
       "title": {
         "type": "string",
         "description": "A short, human-readable summary of the \
   problem type. It SHOULD NOT change from occurrence to occurrence \
   of the problem, except for purposes of localization (e.g., using \
   proactive content negotiation; see RFC7231, Section 3.4)"
       "status": {
         "type": "integer",
         "description": "The HTTP status code (RFC7231, Section 6) \
   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] as their primary format
   convention.  Such APIs can express problem details using
   the format defined in this appendix.

   The RELAX NG schema [ISO-19757-2] for the XML format is as follows.
   Keep in mind that this schema is only meant as documentation, and not
   as a normative schema that captures all constraints of the XML
   format.  Also, it would be possible to use other XML schema languages
   to define a similar set of constraints (depending on the features of
   the chosen schema language). 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 =
        (  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

   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 that contain only child element(s) 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">
     <title>You do not have enough credit.</title>
     <detail>Your current balance is 30, but that costs 50.</detail>

   Note that this
   This format uses an XML namespace.  This is 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 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

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": "",
      "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",

   or by inventing a mapping into 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.


   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 review of this specification.

Authors' Addresses

   Mark Nottingham

   Erik Wilde

   Sanjay Dalal
   United States of America