Internet-Draft REST API Media Types December 2025
Polli & Andrews Expires 17 June 2026 [Page]
Workgroup:
HTTPAPI
Internet-Draft:
draft-ietf-httpapi-rest-api-mediatypes-latest
Published:
Intended Status:
Informational
Expires:
Authors:
R. Polli
Par-Tec
H. Andrews

REST API Media Types

Abstract

This document registers the following media types used in APIs on the IANA Media Types registry: application/openapi+json, and application/openapi+yaml.

About This Document

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

Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-httpapi-rest-api-mediatypes/.

Discussion of this document takes place on the HTTPAPI Working Group mailing list (mailto:httpapi@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/httpapi/. Subscribe at https://www.ietf.org/mailman/listinfo/httpapi/. Working Group information can be found at https://datatracker.ietf.org/wg/httpapi/about/.

Source for this draft and an issue tracker can be found at https://github.com/ietf-wg-httpapi/mediatypes/labels/rest-api.

Status of This Memo

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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 17 June 2026.

▲

Table of Contents

1. Introduction

OpenAPI Specification [OAS] is a consolidated standard for describing HTTP APIs using the JSON [JSON] and YAML [YAML] data format.

To increase interoperability and leverage content negotiation mechanisms when exchanging API descriptions this specification registers the following media types: application/openapi+json and application/openapi+yaml.

1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. These words may also appear in this document in lower case as plain English words, absent their normative meanings.

The terms "content", "content negotiation", "resource", and "user agent" in this document are to be interpreted as in [HTTP].

2. Media Type registrations

This section describes the information required for IANA to register the above media types per [MEDIATYPE].

2.1. The OpenAPI Media Types

The OpenAPI Specification Media Types convey semantics for OpenAPI resources as defined in [OAS].

Those resources can be represented in [JSON] or [YAML]. Since there are multiple OpenAPI Specification versions, those media types support the version parameter.

The following example conveys the desire of a client to receive an OpenAPI Specification resource based on the stated preferences:

  1. openapi 3.1 in YAML

  2. openapi 3.0 in YAML

  3. any openapi version in JSON

Accept: application/openapi+yaml;version=3.1,
        application/openapi+yaml;version=3.0;q=0.5,
        application/openapi+json;q=0.3

2.1.1. Media Type application/openapi+json

The following information serves as the registration form for the application/openapi+json media type.

Type name:

application

Subtype name:

openapi+json

Required parameters:

None

Optional parameters:

version: its value is a string representing the OpenAPI Specification version. ; unrecognized parameters should be ignored

Encoding considerations:

Same as "application/json"

Security considerations:

See Section 4 of this document, "application/json" and [OAS]

Interoperability considerations:

See Section 3 of this document, "application/json" and [OAS]

Published specification:

this document, [OAS]

Applications that use this media type:

HTTP

Fragment identifier considerations:

[OAS] or the specific version of the OpenAPI document.

Additional information:

  • Deprecated alias names for this type: "application/vnd.oai.openapi+json". This name is used, but not registered.

  • Magic number(s): N/A

  • File extension(s): json

  • Macintosh file type code(s): N/A

Person and email address to contact for further information:

See Authors' Addresses section.

Intended usage:

COMMON

Restrictions on usage:

None.

Author:

See Authors' Addresses section.

Change controller:

IETF

2.1.2. Media Type application/openapi+yaml

The following information serves as the registration form for the application/openapi+yaml media type.

Type name:

application

Subtype name:

openapi+yaml

Required parameters:

N/A

Optional parameters:

version: its value is a string representing the OpenAPI Specification version. ; unrecognized parameters should be ignored

Encoding considerations:

Same as "+yaml" Structured Syntax Suffix

Security considerations:

See Section 4 of this document, "+yaml" Structured Syntax Suffix and [OAS]

Interoperability considerations:

See Section 3 of this document, "+yaml" Structured Syntax Suffix and [OAS]

Published specification:

[OAS]

Applications that use this media type:

HTTP

Fragment identifier considerations:

[OAS] or the specific version of the OpenAPI document.

Additional information:

  • Deprecated alias names for this type: "application/vnd.oai.openapi". This name is used, but not registered.

  • Magic number(s): N/A

  • File extension(s): Same as "application/yaml"

  • Macintosh file type code(s): N/A

Person and email address to contact for further information:

See Authors' Addresses section

Intended usage:

COMMON

Restrictions on usage:

None.

Author:

See Authors' Addresses section

Change controller:

IETF

3. Interoperability Considerations

Interoperability requirements for media type registrations are discussed in Section 4.6 of [MEDIATYPE] and in the Interoperability Considerations of the "+yaml" Structured Syntax Suffix.

3.1. Using the version parameter

The version parameter ought to be processed according with the associated OpenAPI Specification.

For example, when an OpenAPI 3.1 resource uses the patch version version=3.1.1, its value is expected to be ignored by tooling (see https://spec.openapis.org/oas/v3.1.0.html#versions).

While it is possible to use the version parameter to identify OpenAPI resources prior to version 3.0, the terminology used in this document was formally introduced in OAS 3.0.4 and OAS 3.1.1.

4. Security Considerations

Security requirements for media type registrations are discussed in Section 4.6 of [MEDIATYPE]. and in the Security Considerations of the "+yaml" Structured Syntax Suffix.

4.1. General Considerations

OpenAPI documents are processed by a wide variety of tooling for numerous different purposes, such as client code generation, documentation generation, server side routing, and API testing. OpenAPI document authors must consider the risks of the scenarios where the OpenAPI document may be used.

An OpenAPI document describes the security schemes used to protect the resources it defines. The security schemes available offer varying degrees of protection. Factors such as the sensitivity of the data and the potential impact of a security breach should guide the selection of security schemes for the API resources. Some security schemes, such as basic auth and OAuth Implicit flow, are supported for compatibility with existing APIs. However, their inclusion in OpenAPI does not constitute an endorsement of their use, particularly for highly sensitive data or operations.

OpenAPI documents may contain references to external resources that may be dereferenced automatically by consuming tools. External resources may be hosted on different domains that may be untrusted. References in an OpenAPI document, or across OpenAPI documents may cause a cycle. Tooling must detect and handle cycles to prevent resource exhaustion.

Certain properties allow the use of Markdown which can contain HTML including script. It is the responsibility of tooling to appropriately sanitize the Markdown.

OpenAPI documents use [jsonschema] therefore share the security consideration of JSON Schema.

5. IANA Considerations

This specification defines the following new Internet media types [MEDIATYPE].

IANA is asked to update the "Media Types" registry at https://www.iana.org/assignments/media-types with the registration information provided in the sections below.

Table 1
Media Type Registration information section
application/openapi+json Section 2.1.1 of this document
application/openapi+yaml Section 2.1.2 of this document

6. References

6.1. Normative References

[HTTP]
Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke, Ed., "HTTP Semantics", STD 97, RFC 9110, DOI 10.17487/RFC9110, , <https://www.rfc-editor.org/rfc/rfc9110>.
[JSON]
Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, , <https://www.rfc-editor.org/rfc/rfc8259>.
[jsonschema]
Wright, A., Andrews, H., Hutton, B., and G. Dennis, "JSON Schema", , <https://json-schema.org/specification.html>.
[MEDIATYPE]
Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, , <https://www.rfc-editor.org/rfc/rfc6838>.
[OAS]
Darrel Miller, Jeremy Whitlock, Marsh Gardiner, Mike Ralphson, Ron Ratovsky, and Uri Sarid, "OpenAPI Specification 3.1.0", , <https://spec.openapis.org/oas/latest>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.
[YAML]
Oren Ben-Kiki, Clark Evans, and Ingy dot Net, "YAML Ain't Markup Language Version 1.2", , <https://yaml.org/spec/1.2/spec.html>.

6.2. Informative References

[RFC9205]
Nottingham, M., "Building Protocols with HTTP", BCP 56, RFC 9205, DOI 10.17487/RFC9205, , <https://www.rfc-editor.org/rfc/rfc9205>.

Acknowledgements

Thanks to Erik Wilde and David Biesack for being the initial contributors of this specification, and to Darrel Miller and Rich Salz for their support during the adoption phase.

In addition to the people above, this document owes a lot to the extensive discussion inside and outside the HTTPAPI workgroup. The following contributors have helped improve this specification by opening pull requests, reporting bugs, asking smart questions, drafting or reviewing text, and evaluating open issues:

Austin Wright, Ben Hutton and Jason Desrosiers.

FAQ

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

Q: Why this document?

After all these years, we still lack a proper media type for REST related document types. This has some security implications too (eg. wrt on identifying parsers or treat downloads)

Examples of content negotiation

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

This section explores the possibilities for User Agents to retrieve information about API specifications using their preferred version of OpenAPI.

Note:

An API catalog exposes API specifications using OAS 3.0, and it wants to provide migration/upgrade capabilities to future versions of OAS and of the referenced resources.

Github publishes OpenAPI specification files.

Resource: https://example.org/openapi.yaml

is an OpenAPI Specification document that references the Foo schema located at /foo.yaml.

The following representation is used both when the User Agent requests OAS 3.0 and as a default representation (see Section 15.5.7 of [HTTP]). For example, this is the response returned when the User Agent requests OAS 3.1.

HTTP/1.1 200 OK
Content-Location: openapi.yaml
Content-Type: application/openapi+yaml; version=3.0

openapi: 3.0.4
…
components:
  schemas:
    Foo:
      "$ref": "/foo.yaml"

The following representation is used when the User Agent requests OAS 3.2.

HTTP/1.1 200 OK
Content-Location: openapi.yaml
Content-Type: application/openapi+yaml; version=3.2

openapi: 3.2.1
"$self": https://example.com/openapi.yaml
…
components:
  schemas:
    Foo:
      "$ref": /foo.yaml
Resource: https://example.org/foo.yaml

is the schema definition for Foo, represented in two different ways depending on the OpenAPI version requested.

HTTP/1.1 200 OK
Content-Location: foo.yaml
Content-Type: application/whatever+yaml

type: string
enum: [ it, en]

Since OAS 3.2 supports JSON Schema, the same syntax can be described adding a title to each enum value.

HTTP/1.1 200 OK
Content-Location: foo.yaml
Content-Type: application/whatever+yaml; version=https://json-schema.org/draft/2020-12/schema

"$schema": "https://json-schema.org/draft/2020-12/schema"
"$id": "https://example.com/foo.yaml"
oneOf:
- const: it
  title: Italian
- const: en
  title: English

User Agent wants only OAS3.1, gets 406

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

The User Agent wants OAS3.1, and is not willing to accept other versions (see Section 12.4.3 of [HTTP].

GET /openapi.yaml HTTP/1.1
Host: example.com
Accept: \
  application/openapi+yaml; version=3.1,
  */*; q=0

The server does not have the selected representation and responds with a 406 Not Acceptable.

HTTP/1.1 406 Not Acceptable
Accept: \
  application/openapi+yaml; version=3.0, \
  application/openapi+yaml; version=3.2

User Agent prefers OAS3.1, gets OAS3.0

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

The User Agent prefers OAS3.1, and does not impose further constraints.

GET /openapi.yaml HTTP/1.1
Host: example.com
Accept: application/openapi+yaml; version=3.1

The server responds with the default representation.

HTTP/1.1 200 OK
Content-Type: application/openapi+yaml; version=3.0

openapi: 3.0.4
…
components:
  schemas:
    Foo:
      "$ref": /foo.yaml

User Agent requires OAS3.0 for entry document and compatible referenced resources

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

The User Agent only supports OAS3.0 entry documents.

GET /openapi.yaml HTTP/1.1
Host: example.com
Accept: \
  application/openapi+yaml; version=3.0;q=1, \
  */*; q=0

The server responds with the desired resource.

HTTP/1.1 200 OK
Content-Type: application/openapi+yaml; version=3.0

openapi: 3.0.4
…
components:
  schemas:
    Foo:
      "$ref": /foo.yaml

The User Agent processes the content and to resolve the $ref, requests /foo.yaml. Since it is not an entry document, the User Agent is willing to accept any content compatible with OAS3.0.

GET /foo.yaml HTTP/1.1
Host: example.com
Accept: \
  application/openapi+yaml; version=3.0; q=1, \
  application/yaml; q=0.5

The server responds with a YAML serialized content that is compatible with OAS3.0. Since the content only contains a Schema Object, that is not associated with any media type, the server uses the application/yaml media type.

HTTP/1.1 200 OK
Content-Type: application/yaml

type: string
enum: [it, en]

Q: Could the server respond with the same content using the application/whatever+yaml media type?

User Agent prefers OAS3.2, gets JSON Schema

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

The User Agent prefers OAS3.2.

GET /openapi.yaml HTTP/1.1
Host: example.com
Accept: application/openapi+yaml; version=3.2

The server responds with the OAS3.2 document.

HTTP/1.1 200 OK
Content-Type: application/openapi+yaml; version=3.2

openapi: 3.0.4
…
components:
  schemas:
    Foo:
      "$ref": /foo.yaml

The User Agent then requests the /foo.yaml, but it wants to retrieve a content with the most recent OAS version.

GET /foo.yaml HTTP/1.1
Host: example.com
Accept: application/openapi+yaml; version=3.2

The server processes the Accept header, and it realizes that the User Agent prefers an OAS3.2 compatible content. It then choses the JSON Schema representation and responds accordingly.

Since JSON Schema has no associated media type, the server uses the application/yaml media type.

HTTP/1.1 200 OK
Content-Type: application/yaml

"$schema": "https://json-schema.org/draft/2020-12/schema"
"$id": "https://example.com/foo.yaml"
oneOf:
- const: it
  title: Italian
- const: en
  title: English

User Agent prefers OAS3.2, Github responds with default representation

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

In this scenario:

  1. the User Agent express preference for referenced resources compatible with OAS3.2.

  2. The server does not support strict content negotiation, and responds with the default representation;

  3. the User Agent processes the content and retrieves the referenced resources.

See Section 12.1 of [HTTP]

  • A user agent cannot rely on proactive negotiation preferences being consistently honored, since the origin server might not implement proactive negotiation for the requested resource or might decide that sending a response that doesn't conform to the user agent's preferences is better than sending a 406 (Not Acceptable) response.

GET /OpenAPITools/openapi-generator-cli/refs/heads/master/examples/v3.0/petstore.yaml HTTP/1.1
Host: raw.githubusercontent.com
Accept: application/openapi+yaml; version=3.2

The server responds with the default representation.

HTTP/1.1 200 OK
Content-Type: text/plain; charset=utf-8

openapi: 3.0.0
info:
  title: Hotel Booker API
…

User Agent negotiates for a non-OAS media type

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

In this scenario:

  1. the User Agent express preference for referenced resources compatible with OAS3.2.

  2. The server enforces a strict content negotiation, and responds with a 406 Not Acceptable;

  3. the User Agent will have to retry using one of the acceptable media types.

The User Agent prefers OAS3.2; moreover uses the q parameter to decline OAS3.0 documents. If it weren't so, the server could have responded with the older version if the OAS3.2 document were not available.

GET /openapi.yaml HTTP/1.1
Host: example.com
Accept: application/openapi+yaml; version=3.2, application/openapi+yaml; version=3.0;q=0

The server responds with the OAS3.2 document.

HTTP/1.1 200 OK
Content-Type: application/openapi+yaml; version=3.2

openapi: 3.0.0
…
components:
  schemas:
    Foo:
      "$ref": /foo.yaml

The User Agent then requests the /foo.yaml, but it wants to retrieve a content with the most recent OAS version.

GET /foo.yaml HTTP/1.1
Host: example.com
Accept: application/openapi+yaml; version=3.2

The server processes the Accept header, and it realizes that foo.yaml is a JSON Schema document and not an OpenAPI document.

It then replies with the acceptable content.

HTTP/1.1 406 Not Acceptable
Accept: \
  application/whatever+yaml; version=https://json-schema.org/draft/2020-12/schema, \
  application/whatever+yaml; version=https://spec.openapis.org/oas/v3.0.4#schema-object

The User Agent decides to retry the request using one of the acceptable media types. It could have also decided to give up.

GET /foo.yaml HTTP/1.1
Host: example.com
Accept: \
  application/whatever+yaml; version=https://json-schema.org/draft/2020-12/schema

Now the server can respond with the JSON Schema representation.

HTTP/1.1 200 OK
Content-Type: application/whatever+yaml; version=https://json-schema.org/draft/2020-12/schema

"$schema": "https://json-schema.org/draft/2020-12/schema"
"$id": "https://example.com/foo.yaml"
oneOf:
- const: it
  title: Italian
- const: en
  title: English

Pros:

  • The returned content is exactly what the User Agent requested.

Cons:

  • Two requests were needed to retrieve the resource.

  • Some User Agents may not be able to handle the 406 Not Acceptable response and give up.

  • Requires a specific Accept value for each possible referenced resource, which may be difficult to manage.

Change Log

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

RFC EDITOR PLEASE DELETE THIS SECTION.

Since -00

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

  • Split YAML registrations in a separate I-D.

Since -04

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

  • Split JSONSCHEMA registrations in a separate I-D.

Since -07

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

  • Support OAS 2.0

Authors' Addresses

Roberto Polli
Par-Tec
Italy
Henry Andrews