synapse/docs/admin_api/user_admin_api.md

User Admin API

Query User Account

This API returns information about a specific user account.

The api is:

GET /_synapse/admin/v2/users/<user_id>

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

It returns a JSON body like the following:

{
    "displayname": "User",
    "threepids": [
        {
            "medium": "email",
            "address": "<user_mail_1>",
            "added_at": 1586458409743,
            "validated_at": 1586458409743
        },
        {
            "medium": "email",
            "address": "<user_mail_2>",
            "added_at": 1586458409743,
            "validated_at": 1586458409743
        }
    ],
    "avatar_url": "<avatar_url>",
    "admin": 0,
    "deactivated": 0,
    "shadow_banned": 0,
    "password_hash": "$2b$12$p9B4GkqYdRTPGD",
    "creation_ts": 1560432506,
    "appservice_id": null,
    "consent_server_notice_sent": null,
    "consent_version": null,
    "external_ids": [
        {
            "auth_provider": "<provider1>",
            "external_id": "<user_id_provider_1>"
        },
        {
            "auth_provider": "<provider2>",
            "external_id": "<user_id_provider_2>"
        }
    ],
    "user_type": null
}

URL parameters:

• user_id: fully-qualified user id: for example, @user:server.com.

Create or modify Account

This API allows an administrator to create or modify a user account with a specific user_id.

This api is:

PUT /_synapse/admin/v2/users/<user_id>

with a body of:

{
    "password": "user_password",
    "displayname": "User",
    "threepids": [
        {
            "medium": "email",
            "address": "<user_mail_1>"
        },
        {
            "medium": "email",
            "address": "<user_mail_2>"
        }
    ],
    "external_ids": [
        {
            "auth_provider": "<provider1>",
            "external_id": "<user_id_provider_1>"
        },
        {
            "auth_provider": "<provider2>",
            "external_id": "<user_id_provider_2>"
        }
    ],
    "avatar_url": "<avatar_url>",
    "admin": false,
    "deactivated": false,
    "user_type": null
}

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

Returns HTTP status code:

• 201 - When a new user object was created.
• 200 - When a user was modified.

URL parameters:

• user_id: fully-qualified user id: for example, @user:server.com.

Body parameters:

• password - string, optional. If provided, the user's password is updated and all devices are logged out.
• displayname - string, optional, defaults to the value of user_id.
• threepids - array, optional, allows setting the third-party IDs (email, msisdn)
  • medium - string. Kind of third-party ID, either email or msisdn.
  • address - string. Value of third-party ID. belonging to a user.
• external_ids - array, optional. Allow setting the identifier of the external identity provider for SSO (Single sign-on). Details in Sample Configuration File section sso and oidc_providers.
  • auth_provider - string. ID of the external identity provider. Value of idp_id in homeserver configuration.
  • external_id - string, user ID in the external identity provider.
• avatar_url - string, optional, must be a MXC URI.
• admin - bool, optional, defaults to false.
• deactivated - bool, optional. If unspecified, deactivation state will be left unchanged on existing accounts and set to false for new accounts. A user cannot be erased by deactivating with this API. For details on deactivating users see Deactivate Account.
• user_type - string or null, optional. If provided, the user type will be adjusted. If null given, the user type will be cleared. Other allowed options are: bot and support.

If the user already exists then optional parameters default to the current value.

In order to re-activate an account deactivated must be set to false. If users do not login via single-sign-on, a new password must be provided.

List Accounts

This API returns all local user accounts. By default, the response is ordered by ascending user ID.

GET /_synapse/admin/v2/users?from=0&limit=10&guests=false

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

A response body like the following is returned:

{
    "users": [
        {
            "name": "<user_id1>",
            "is_guest": 0,
            "admin": 0,
            "user_type": null,
            "deactivated": 0,
            "shadow_banned": 0,
            "displayname": "<User One>",
            "avatar_url": null,
            "creation_ts": 1560432668000
        }, {
            "name": "<user_id2>",
            "is_guest": 0,
            "admin": 1,
            "user_type": null,
            "deactivated": 0,
            "shadow_banned": 0,
            "displayname": "<User Two>",
            "avatar_url": "<avatar_url>",
            "creation_ts": 1561550621000
        }
    ],
    "next_token": "100",
    "total": 200
}

To paginate, check for next_token and if present, call the endpoint again with from set to the value of next_token. This will return a new page.

If the endpoint does not return a next_token then there are no more users to paginate through.

Parameters

The following parameters should be set in the URL:

• user_id - Is optional and filters to only return users with user IDs that contain this value. This parameter is ignored when using the name parameter.
• name - Is optional and filters to only return users with user ID localparts or displaynames that contain this value.
• guests - string representing a bool - Is optional and if false will exclude guest users. Defaults to true to include guest users.
• deactivated - string representing a bool - Is optional and if true will include deactivated users. Defaults to false to exclude deactivated users.
• limit - string representing a positive integer - Is optional but is used for pagination, denoting the maximum number of items to return in this call. Defaults to 100.
• from - string representing a positive integer - Is optional but used for pagination, denoting the offset in the returned results. This should be treated as an opaque value and not explicitly set to anything other than the return value of next_token from a previous call. Defaults to 0.

• order_by - The method by which to sort the returned list of users. If the ordered field has duplicates, the second order is always by ascending name, which guarantees a stable ordering. Valid values are:

  • name - Users are ordered alphabetically by name. This is the default.
  • is_guest - Users are ordered by is_guest status.
  • admin - Users are ordered by admin status.
  • user_type - Users are ordered alphabetically by user_type.
  • deactivated - Users are ordered by deactivated status.
  • shadow_banned - Users are ordered by shadow_banned status.
  • displayname - Users are ordered alphabetically by displayname.
  • avatar_url - Users are ordered alphabetically by avatar URL.
  • creation_ts - Users are ordered by when the users was created in ms.

• dir - Direction of media order. Either f for forwards or b for backwards. Setting this value to b will reverse the above sort order. Defaults to f.

Caution. The database only has indexes on the columns name and creation_ts. This means that if a different sort order is used (is_guest, admin, user_type, deactivated, shadow_banned, avatar_url or displayname), this can cause a large load on the database, especially for large environments.

Response

The following fields are returned in the JSON response body:

• users - An array of objects, each containing information about an user. User objects contain the following fields:

  • name - string - Fully-qualified user ID (ex. @user:server.com).
  • is_guest - bool - Status if that user is a guest account.
  • admin - bool - Status if that user is a server administrator.
  • user_type - string - Type of the user. Normal users are type None. This allows user type specific behaviour. There are also types support and bot.
  • deactivated - bool - Status if that user has been marked as deactivated.
  • shadow_banned - bool - Status if that user has been marked as shadow banned.
  • displayname - string - The user's display name if they have set one.
  • avatar_url - string - The user's avatar URL if they have set one.
  • creation_ts - integer - The user's creation timestamp in ms.

• next_token: string representing a positive integer - Indication for pagination. See above.

• total - integer - Total number of media.

Query current sessions for a user

This API returns information about the active sessions for a specific user.

The endpoints are:

GET /_synapse/admin/v1/whois/<user_id>

and:

GET /_matrix/client/r0/admin/whois/<userId>

See also: Client Server API Whois.

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

It returns a JSON body like the following:

{
    "user_id": "<user_id>",
    "devices": {
        "": {
            "sessions": [
                {
                    "connections": [
                        {
                            "ip": "1.2.3.4",
                            "last_seen": 1417222374433,
                            "user_agent": "Mozilla/5.0 ..."
                        },
                        {
                            "ip": "1.2.3.10",
                            "last_seen": 1417222374500,
                            "user_agent": "Dalvik/2.1.0 ..."
                        }
                    ]
                }
            ]
        }
    }
}

last_seen is measured in milliseconds since the Unix epoch.

Deactivate Account

This API deactivates an account. It removes active access tokens, resets the password, and deletes third-party IDs (to prevent the user requesting a password reset).

It can also mark the user as GDPR-erased. This means messages sent by the user will still be visible by anyone that was in the room when these messages were sent, but hidden from users joining the room afterwards.

The api is:

POST /_synapse/admin/v1/deactivate/<user_id>

with a body of:

{
    "erase": true
}

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

The erase parameter is optional and defaults to false. An empty body may be passed for backwards compatibility.

The following actions are performed when deactivating an user:

• Try to unpind 3PIDs from the identity server
• Remove all 3PIDs from the homeserver
• Delete all devices and E2EE keys
• Delete all access tokens
• Delete all pushers
• Delete the password hash
• Removal from all rooms the user is a member of
• Remove the user from the user directory
• Reject all pending invites
• Remove all account validity information related to the user

The following additional actions are performed during deactivation if erase is set to true:

• Remove the user's display name
• Remove the user's avatar URL
• Mark the user as erased

The following actions are NOT performed. The list may be incomplete.

• Remove mappings of SSO IDs
• Delete media uploaded by user (included avatar images)
• Delete sent and received messages
• Delete E2E cross-signing keys
• Remove the user's creation (registration) timestamp
• Remove rate limit overrides
• Remove from monthly active users

Reset password

Changes the password of another user. This will automatically log the user out of all their devices.

The api is:

POST /_synapse/admin/v1/reset_password/<user_id>

with a body of:

{
   "new_password": "<secret>",
   "logout_devices": true
}

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

The parameter new_password is required. The parameter logout_devices is optional and defaults to true.

Get whether a user is a server administrator or not

The api is:

GET /_synapse/admin/v1/users/<user_id>/admin

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

A response body like the following is returned:

{
    "admin": true
}

Change whether a user is a server administrator or not

Note that you cannot demote yourself.

The api is:

PUT /_synapse/admin/v1/users/<user_id>/admin

with a body of:

{
    "admin": true
}

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

List room memberships of a user

Gets a list of all room_id that a specific user_id is member.

The API is:

GET /_synapse/admin/v1/users/<user_id>/joined_rooms

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

A response body like the following is returned:

    {
        "joined_rooms": [
            "!DuGcnbhHGaSZQoNQR:matrix.org",
            "!ZtSaPCawyWtxfWiIy:matrix.org"
        ],
        "total": 2
    }

The server returns the list of rooms of which the user and the server are member. If the user is local, all the rooms of which the user is member are returned.

Parameters

The following parameters should be set in the URL:

• user_id - fully qualified: for example, @user:server.com.

Response

The following fields are returned in the JSON response body:

• joined_rooms - An array of room_id.
• total - Number of rooms.

User media

List media uploaded by a user

Gets a list of all local media that a specific user_id has created. By default, the response is ordered by descending creation date and ascending media ID. The newest media is on top. You can change the order with parameters order_by and dir.

The API is:

GET /_synapse/admin/v1/users/<user_id>/media

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

A response body like the following is returned:

{
  "media": [
    {
      "created_ts": 100400,
      "last_access_ts": null,
      "media_id": "qXhyRzulkwLsNHTbpHreuEgo",
      "media_length": 67,
      "media_type": "image/png",
      "quarantined_by": null,
      "safe_from_quarantine": false,
      "upload_name": "test1.png"
    },
    {
      "created_ts": 200400,
      "last_access_ts": null,
      "media_id": "FHfiSnzoINDatrXHQIXBtahw",
      "media_length": 67,
      "media_type": "image/png",
      "quarantined_by": null,
      "safe_from_quarantine": false,
      "upload_name": "test2.png"
    }
  ],
  "next_token": 3,
  "total": 2
}

To paginate, check for next_token and if present, call the endpoint again with from set to the value of next_token. This will return a new page.

If the endpoint does not return a next_token then there are no more reports to paginate through.

Parameters

The following parameters should be set in the URL:

• user_id - string - fully qualified: for example, @user:server.com.
• limit: string representing a positive integer - Is optional but is used for pagination, denoting the maximum number of items to return in this call. Defaults to 100.
• from: string representing a positive integer - Is optional but used for pagination, denoting the offset in the returned results. This should be treated as an opaque value and not explicitly set to anything other than the return value of next_token from a previous call. Defaults to 0.

• order_by - The method by which to sort the returned list of media. If the ordered field has duplicates, the second order is always by ascending media_id, which guarantees a stable ordering. Valid values are:

  • media_id - Media are ordered alphabetically by media_id.
  • upload_name - Media are ordered alphabetically by name the media was uploaded with.
  • created_ts - Media are ordered by when the content was uploaded in ms. Smallest to largest. This is the default.
  • last_access_ts - Media are ordered by when the content was last accessed in ms. Smallest to largest.
  • media_length - Media are ordered by length of the media in bytes. Smallest to largest.
  • media_type - Media are ordered alphabetically by MIME-type.
  • quarantined_by - Media are ordered alphabetically by the user ID that initiated the quarantine request for this media.
  • safe_from_quarantine - Media are ordered by the status if this media is safe from quarantining.

• dir - Direction of media order. Either f for forwards or b for backwards. Setting this value to b will reverse the above sort order. Defaults to f.

If neither order_by nor dir is set, the default order is newest media on top (corresponds to order_by = created_ts and dir = b).

Caution. The database only has indexes on the columns media_id, user_id and created_ts. This means that if a different sort order is used (upload_name, last_access_ts, media_length, media_type, quarantined_by or safe_from_quarantine), this can cause a large load on the database, especially for large environments.

Response

The following fields are returned in the JSON response body:

• media - An array of objects, each containing information about a media. Media objects contain the following fields:
  • created_ts - integer - Timestamp when the content was uploaded in ms.
  • last_access_ts - integer - Timestamp when the content was last accessed in ms.
  • media_id - string - The id used to refer to the media.
  • media_length - integer - Length of the media in bytes.
  • media_type - string - The MIME-type of the media.
  • quarantined_by - string - The user ID that initiated the quarantine request for this media.
  • safe_from_quarantine - bool - Status if this media is safe from quarantining.
  • upload_name - string - The name the media was uploaded with.
• next_token: integer - Indication for pagination. See above.
• total - integer - Total number of media.

Delete media uploaded by a user

This API deletes the local media from the disk of your own server that a specific user_id has created. This includes any local thumbnails.

This API will not affect media that has been uploaded to external media repositories (e.g https://github.com/turt2live/matrix-media-repo/).

By default, the API deletes media ordered by descending creation date and ascending media ID. The newest media is deleted first. You can change the order with parameters order_by and dir. If no limit is set the API deletes 100 files per request.

The API is:

DELETE /_synapse/admin/v1/users/<user_id>/media

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

A response body like the following is returned:

{
  "deleted_media": [
    "abcdefghijklmnopqrstuvwx"
  ],
  "total": 1
}

The following fields are returned in the JSON response body:

• deleted_media: an array of strings - List of deleted media_id
• total: integer - Total number of deleted media_id

Note: There is no next_token. This is not useful for deleting media, because after deleting media the remaining media have a new order.

Parameters

This API has the same parameters as List media uploaded by a user. With the parameters you can for example limit the number of files to delete at once or delete largest/smallest or newest/oldest files first.

Login as a user

Get an access token that can be used to authenticate as that user. Useful for when admins wish to do actions on behalf of a user.

The API is:

POST /_synapse/admin/v1/users/<user_id>/login
{}

An optional valid_until_ms field can be specified in the request body as an integer timestamp that specifies when the token should expire. By default tokens do not expire.

A response body like the following is returned:

{
    "access_token": "<opaque_access_token_string>"
}

This API does not generate a new device for the user, and so will not appear their /devices list, and in general the target user should not be able to tell they have been logged in as.

To expire the token call the standard /logout API with the token.

Note: The token will expire if the admin user calls /logout/all from any of their devices, but the token will not expire if the target user does the same.

User devices

List all devices

Gets information about all devices for a specific user_id.

The API is:

GET /_synapse/admin/v2/users/<user_id>/devices

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

A response body like the following is returned:

{
  "devices": [
    {
      "device_id": "QBUAZIFURK",
      "display_name": "android",
      "last_seen_ip": "1.2.3.4",
      "last_seen_ts": 1474491775024,
      "user_id": "<user_id>"
    },
    {
      "device_id": "AUIECTSRND",
      "display_name": "ios",
      "last_seen_ip": "1.2.3.5",
      "last_seen_ts": 1474491775025,
      "user_id": "<user_id>"
    }
  ],
  "total": 2
}

Parameters

The following parameters should be set in the URL:

• user_id - fully qualified: for example, @user:server.com.

Response

The following fields are returned in the JSON response body:

• devices - An array of objects, each containing information about a device. Device objects contain the following fields:

  • device_id - Identifier of device.
  • display_name - Display name set by the user for this device. Absent if no name has been set.
  • last_seen_ip - The IP address where this device was last seen. (May be a few minutes out of date, for efficiency reasons).
  • last_seen_ts - The timestamp (in milliseconds since the unix epoch) when this devices was last seen. (May be a few minutes out of date, for efficiency reasons).
  • user_id - Owner of device.

• total - Total number of user's devices.

Delete multiple devices

Deletes the given devices for a specific user_id, and invalidates any access token associated with them.

The API is:

POST /_synapse/admin/v2/users/<user_id>/delete_devices

{
  "devices": [
    "QBUAZIFURK",
    "AUIECTSRND"
  ],
}

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

An empty JSON dict is returned.

Parameters

The following parameters should be set in the URL:

• user_id - fully qualified: for example, @user:server.com.

The following fields are required in the JSON request body:

• devices - The list of device IDs to delete.

Show a device

Gets information on a single device, by device_id for a specific user_id.

The API is:

GET /_synapse/admin/v2/users/<user_id>/devices/<device_id>

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

A response body like the following is returned:

{
  "device_id": "<device_id>",
  "display_name": "android",
  "last_seen_ip": "1.2.3.4",
  "last_seen_ts": 1474491775024,
  "user_id": "<user_id>"
}

Parameters

The following parameters should be set in the URL:

• user_id - fully qualified: for example, @user:server.com.
• device_id - The device to retrieve.

Response

The following fields are returned in the JSON response body:

• device_id - Identifier of device.
• display_name - Display name set by the user for this device. Absent if no name has been set.
• last_seen_ip - The IP address where this device was last seen. (May be a few minutes out of date, for efficiency reasons).
• last_seen_ts - The timestamp (in milliseconds since the unix epoch) when this devices was last seen. (May be a few minutes out of date, for efficiency reasons).
• user_id - Owner of device.

Update a device

Updates the metadata on the given device_id for a specific user_id.

The API is:

PUT /_synapse/admin/v2/users/<user_id>/devices/<device_id>

{
  "display_name": "My other phone"
}

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

An empty JSON dict is returned.

Parameters

The following parameters should be set in the URL:

• user_id - fully qualified: for example, @user:server.com.
• device_id - The device to update.

The following fields are required in the JSON request body:

• display_name - The new display name for this device. If not given, the display name is unchanged.

Delete a device

Deletes the given device_id for a specific user_id, and invalidates any access token associated with it.

The API is:

DELETE /_synapse/admin/v2/users/<user_id>/devices/<device_id>

{}

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

An empty JSON dict is returned.

Parameters

The following parameters should be set in the URL:

• user_id - fully qualified: for example, @user:server.com.
• device_id - The device to delete.

List all pushers

Gets information about all pushers for a specific user_id.

The API is:

GET /_synapse/admin/v1/users/<user_id>/pushers

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

A response body like the following is returned:

{
  "pushers": [
    {
      "app_display_name":"HTTP Push Notifications",
      "app_id":"m.http",
      "data": {
        "url":"example.com"
      },
      "device_display_name":"pushy push",
      "kind":"http",
      "lang":"None",
      "profile_tag":"",
      "pushkey":"a@example.com"
    }
  ],
  "total": 1
}

Parameters

The following parameters should be set in the URL:

• user_id - fully qualified: for example, @user:server.com.

Response

The following fields are returned in the JSON response body:

• pushers - An array containing the current pushers for the user

  • app_display_name - string - A string that will allow the user to identify what application owns this pusher.

  • app_id - string - This is a reverse-DNS style identifier for the application. Max length, 64 chars.

  • data - A dictionary of information for the pusher implementation itself.

  • url - string - Required if kind is http. The URL to use to send notifications to.

  • format - string - The format to use when sending notifications to the Push Gateway.

  • device_display_name - string - A string that will allow the user to identify what device owns this pusher.

  • profile_tag - string - This string determines which set of device specific rules this pusher executes.

  • kind - string - The kind of pusher. "http" is a pusher that sends HTTP pokes.

  • lang - string - The preferred language for receiving notifications (e.g. 'en' or 'en-US')

  • profile_tag - string - This string determines which set of device specific rules this pusher executes.

  • pushkey - string - This is a unique identifier for this pusher. Max length, 512 bytes.

• total - integer - Number of pushers.

See also the Client-Server API Spec on pushers.

Controlling whether a user is shadow-banned

Shadow-banning is a useful tool for moderating malicious or egregiously abusive users. A shadow-banned users receives successful responses to their client-server API requests, but the events are not propagated into rooms. This can be an effective tool as it (hopefully) takes longer for the user to realise they are being moderated before pivoting to another account.

Shadow-banning a user should be used as a tool of last resort and may lead to confusing or broken behaviour for the client. A shadow-banned user will not receive any notification and it is generally more appropriate to ban or kick abusive users. A shadow-banned user will be unable to contact anyone on the server.

To shadow-ban a user the API is:

POST /_synapse/admin/v1/users/<user_id>/shadow_ban

To un-shadow-ban a user the API is:

DELETE /_synapse/admin/v1/users/<user_id>/shadow_ban

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

An empty JSON dict is returned in both cases.

Parameters

The following parameters should be set in the URL:

• user_id - The fully qualified MXID: for example, @user:server.com. The user must be local.

Override ratelimiting for users

This API allows to override or disable ratelimiting for a specific user. There are specific APIs to set, get and delete a ratelimit.

Get status of ratelimit

The API is:

GET /_synapse/admin/v1/users/<user_id>/override_ratelimit

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

A response body like the following is returned:

{
  "messages_per_second": 0,
  "burst_count": 0
}

Parameters

The following parameters should be set in the URL:

• user_id - The fully qualified MXID: for example, @user:server.com. The user must be local.

Response

The following fields are returned in the JSON response body:

• messages_per_second - integer - The number of actions that can be performed in a second. 0 mean that ratelimiting is disabled for this user.
• burst_count - integer - How many actions that can be performed before being limited.

If no custom ratelimit is set, an empty JSON dict is returned.

{}

Set ratelimit

The API is:

POST /_synapse/admin/v1/users/<user_id>/override_ratelimit

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

A response body like the following is returned:

{
  "messages_per_second": 0,
  "burst_count": 0
}

Parameters

The following parameters should be set in the URL:

• user_id - The fully qualified MXID: for example, @user:server.com. The user must be local.

Body parameters:

• messages_per_second - positive integer, optional. The number of actions that can be performed in a second. Defaults to 0.
• burst_count - positive integer, optional. How many actions that can be performed before being limited. Defaults to 0.

To disable users' ratelimit set both values to 0.

Response

The following fields are returned in the JSON response body:

• messages_per_second - integer - The number of actions that can be performed in a second.
• burst_count - integer - How many actions that can be performed before being limited.

Delete ratelimit

The API is:

DELETE /_synapse/admin/v1/users/<user_id>/override_ratelimit

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API

An empty JSON dict is returned.

{}

Parameters

The following parameters should be set in the URL:

• user_id - The fully qualified MXID: for example, @user:server.com. The user must be local.

Check username availability

Checks to see if a username is available, and valid, for the server. See the client-server API for more information.

This endpoint will work even if registration is disabled on the server, unlike /_matrix/client/r0/register/available.

The API is:

GET /_synapse/admin/v1/username_available?username=$localpart

The request and response format is the same as the /_matrix/client/r0/register/available API.

To use it, you will need to authenticate by providing an access_token for a server admin: Admin API