MSC4369: M_CAPABILITY_NOT_ENABLED error code for when capability is not enabled on an API endpoint

proposals/4369-capability-error-code.md

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+# MSC4369: M_CAPABILITY_NOT_ENABLED error code for when capability is not enabled on an API endpoint
+
+The [capabilities negotiation] feature in the [Client-Server API] provides a mechanism for clients
+to discover whether specific capabilities are available to a user.
+
+However, the endpoints that use capabilities do not currently specify what error response a client
+should expect if they try to use an API that does not have the capability enabled for the user.
+
+I believe this is an omission in the current spec that should be addressed for existing capabilities
+and be checked for future MSCs that use capabilities.
+
+For background: this MSC originates from a
+[spec PR](https://github.com/matrix-org/matrix-spec/pull/2212) that has since been closed.
+
+## Proposal
+
+Define a new [other error code] of `M_CAPABILITY_NOT_ENABLED`.
+
+Define the following behaviour for existing API endpoints that use capabilities:
+
+Capability|Endpoint(s)|Current spec|Proposed behaviour
+-|-|-|-
+[`m.change_password`](https://spec.matrix.org/v1.16/client-server-api/#mchange_password-capability)|[`POST /_matrix/client/v3/account/password`](https://pr2212--matrix-spec-previews.netlify.app/client-server-api/#post_matrixclientv3accountpassword)|Not defined|Return `403` with `M_CAPABILITY_NOT_ENABLED` if `m.change_password`.`enabled` is `false`
+[`m.profile_fields`](https://spec.matrix.org/v1.16/client-server-api/#mprofile_fields-capability)|[`PUT /_matrix/client/v3/profile/{userId}/{keyName}`](https://spec.matrix.org/v1.16/client-server-api/#put_matrixclientv3profileuseridkeyname) [`DELETE /_matrix/client/v3/profile/{userId}/{keyName}`](https://spec.matrix.org/v1.16/client-server-api/#delete_matrixclientv3profileuseridkeyname)|`403` for `PUT`. Maybe `403` for `DELETE`. Error code not specified for either|Return `403` with `M_CAPABILITY_NOT_ENABLED` if: `m.profile_fields`.`enabled` is `false`; or, `m.profile_fields`.`disallowed` contains `{keyName}`;
+[`m.set_displayname`](https://spec.matrix.org/v1.16/client-server-api/#mset_displayname-capability)|As per `m.profile_fields`|As per `m.profile_fields`|Return `403` with `M_CAPABILITY_NOT_ENABLED` if: `m.profile_fields`.`enabled` is `false`; or, `m.profile_fields`.`disallowed` contains `{keyName}`; or, `m.set_displayname`.`enabled` is `false`;
+[`m.set_avatar_url`](https://spec.matrix.org/v1.16/client-server-api/#mset_avatar_url-capability)|As per `m.profile_fields`|As per `m.profile_fields`|Return `403` with `M_CAPABILITY_NOT_ENABLED` if: `m.profile_fields`.`enabled` is `false`; or, `m.profile_fields`.`disallowed` contains `{keyName}`; or, `m.set_avatar_url`.`enabled` is `false`;
+[`m.3pid_changes`](https://spec.matrix.org/v1.16/client-server-api/#m3pid_changes-capability)|[`POST /_matrix/client/v3/account/3pid`](https://spec.matrix.org/v1.16/client-server-api/#post_matrixclientv3account3pid) [`POST /_matrix/client/v3/account/3pid/add`](https://spec.matrix.org/v1.16/client-server-api/#post_matrixclientv3account3pidadd) [`POST /_matrix/client/v3/account/3pid/delete`](https://spec.matrix.org/v1.16/client-server-api/#post_matrixclientv3account3piddelete)|Not defined|Return `400` with `M_CAPABILITY_NOT_ENABLED` if `m.3pid_changes`.`enabled` is `false`
+[`m.get_login_token`](https://spec.matrix.org/v1.16/client-server-api/#mget_login_token-capability)|[`POST /_matrix/client/v1/login/get_token`](https://spec.matrix.org/v1.16/client-server-api/#post_matrixclientv1loginget_token)|Maybe `400` with undefined error code if `m.get_login_token`.`enabled` is `false`|`403` with `M_CAPABILITY_NOT_ENABLED` if `m.get_login_token`.`enabled` is `false`
+
+## Potential issues
+
+TODO: Change in behaviour on `m.get_login_token`.
+
+## Alternatives
+
+Base on existing implementations. This is what was originally in https://github.com/matrix-org/matrix-spec/pull/2212
+
+## Security considerations
+
+None.
+
+## Unstable prefix
+
+Whilst this proposal is under development the `IO.ELEMENT.MSCXXXX_CAPABILITY_NOT_ENABLED` value
+should be used instead of `CAPABILITY_NOT_ENABLED`.
+
+## Dependencies
+
+None.
+
+[capabilities negotiation]: https://spec.matrix.org/v1.16/client-server-api/#capabilities-negotiation
+[Client-Server API]: https://spec.matrix.org/v1.16/client-server-api/
+[other error code]: https://spec.matrix.org/v1.16/client-server-api/#other-error-codes