The Problem-Details HTTP Header Field

Introduction Problem details give HTTP APIs a common way to describe an error, but only in a message's content, as JSON or XML. That is not always available. A response to a HEAD request has no content; a response whose content is a representation of the resource cannot also carry problem details; and an intermediary that generates or annotates an error might not be able to alter the content it forwards. This specification defines the "Problem-Details" response header field () to carry problem details as a Structured Field . Structured Fields cannot represent the entire JSON data model, so not every problem details object can be carried in the field. defines a profile of problem details -- header-compatible problem types -- to enable problem types to opt into being carried as HTTP header fields. Problem types that do not opt in to that profile can still be conveyed using the field, but the header field will only carry the members defined in .

Notational Conventions 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 when, and only when, they appear in all capitals, as shown here. This document uses the following terminology from to specify syntax and parsing: Dictionary, Inner List, Integer, Decimal, String, Token, Boolean, Parameters, Byte Sequence, Date, and Display String. It uses "problem details object", "problem type" and "extension member" as defined in .

The Problem-Details Header Field The "Problem-Details" response header field conveys a problem details object . Its value is a Dictionary. Each member of the Dictionary corresponds to the member of the problem details object with the same name, and has the Structured Type shown in . Problem Details Members in the Problem-Details Field

Member	Structured Type	JSON type
type	String	string (a URI reference)
status	Integer	number
title	Display String	string
detail	Display String	string
instance	String	string (a URI reference)

The semantics of the members are as defined in . A member whose value is not of the type shown in is ignored, as in . A sender MAY omit any member, including type, which then has its default value of "about:blank". However, a sender MUST NOT include extension members unless it includes type. HTTP responses already carry a status code, so status is redundant in the field; senders SHOULD omit it. Any problem type can be conveyed with the field, using the members in . Extension members are carried only if the problem type is header-compatible () and identifies them as carried in the field; a sender MUST NOT include any others. Recipients MUST ignore members they do not recognise, and any Parameters on members. For example, this response carries the out-of-credit problem from , which we assume is header-compatible, with balance an Integer and accounts an Inner List of Strings: It is equivalent to this problem details object: A response can carry the field with or without problem details content. When it carries both, the field is a subset of the content. A recipient that finds a member with different values uses the content's value.

Header-Compatible Problem Types A problem type is header-compatible if its definition says so and identifies which of its extension members are carried in the field. Only the party that defines a problem type can make it header-compatible; a recipient does not infer it from the members a response carries. Each of those members conforms to the following:

Its name begins with a lower-case letter, followed by lower-case letters, digits, "_", "-" or ".". This is a subset of the keys that allows for a Dictionary member, chosen so that the name is also valid in the XML format ().
Its value is a boolean, number or string, or an array of them. It is not null or an object, and an array does not contain arrays.
The definition specifies the Structured Type of the member, as in . Where the value is an array, it specifies the type of the array's items.

Mapping Between Structured Types and JSON

Structured Type	JSON type	Constraints
Integer	number	an integer; see
Decimal	number	at most 12 integer digits and 3 fractional digits
String	string	ASCII characters only
Token	string	conforms to the syntax of a Token
Display String	string	any Unicode scalar value
Boolean	boolean
Inner List	array	items are of the types above

Members of other Structured Types (Byte Sequence and Date) are ignored. A sender MUST use the Structured Type that the problem type specifies for each member. A recipient converts a String, Token or Display String to a JSON string, and an Integer or Decimal to a JSON number, without checking which the problem type specifies. A member of any other type, such as a Byte Sequence or Date, is ignored. The language of Display Strings is indicated by the Content-Language header field (), when present.

IANA Considerations IANA is asked to update the "Hypertext Transfer Protocol (HTTP) Field Name Registry" as follows:

Field Name	Status	Structured Type	Reference
Problem-Details	permanent	Dictionary

Security Considerations The considerations in apply to problem details using the field in the same way that they do to problem details in content: it is easy to leak implementation details through error messages. Because a header field is visible to, and often logged by, every intermediary on the path, senders should consider not only the impact of individual disclosures, but also what information might be gathered over time by observing many responses. Display Strings can contain any Unicode scalar value, including control characters. Recipients that display title, detail or a Display String extension member need to treat it as untrusted text, and filter or escape it first (). Recipients SHOULD NOT automatically dereference the type or instance URIs (). Header fields are subject to size limits that content generally is not, and some implementations impose them on the whole header section. Senders that need to convey more than a small amount of information should use content or selectively omit members from the field.