To use it, you will need to authenticate by providing an access_token
for a server admin: see Admin API.
This API returns information about a specific user account.
The api is:
GET /_synapse/admin/v2/users/<user_id>
It returns a JSON body like the following:
{
"name": "@user:example.com",
"displayname": "User", // can be null if not set
"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>", // can be null if not set
"is_guest": 0,
"admin": 0,
"deactivated": 0,
"shadow_banned": 0,
"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
.
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
}
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, unlesslogout_devices
is set tofalse
.logout_devices
- bool, optional, defaults totrue
. If set to false, devices aren't logged out even whenpassword
is provided.displayname
- string, optional, defaults to the value ofuser_id
.threepids
- array, optional, allows setting the third-party IDs (email, msisdn)medium
- string. Kind of third-party ID, eitheremail
ormsisdn
.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 the configuration manual under the sections sso and oidc_providers.auth_provider
- string. ID of the external identity provider. Value ofidp_id
in the homeserver configuration. Note that no error is raised if the provided value is not in the 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 tofalse
.deactivated
- bool, optional. If unspecified, deactivation state will be left unchanged on existing accounts and set tofalse
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. Ifnull
given, the user type will be cleared. Other allowed options are:bot
andsupport
.
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.
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
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 thename
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 iffalse
will exclude guest users. Defaults totrue
to include guest users. -
deactivated
- string representing a bool - Is optional and iftrue
will include deactivated users. Defaults tofalse
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 to100
. -
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 ofnext_token
from a previous call. Defaults to0
. -
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 ascendingname
, which guarantees a stable ordering. Valid values are:name
- Users are ordered alphabetically byname
. This is the default.is_guest
- Users are ordered byis_guest
status.admin
- Users are ordered byadmin
status.user_type
- Users are ordered alphabetically byuser_type
.deactivated
- Users are ordered bydeactivated
status.shadow_banned
- Users are ordered byshadow_banned
status.displayname
- Users are ordered alphabetically bydisplayname
.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. Eitherf
for forwards orb
for backwards. Setting this value tob
will reverse the above sort order. Defaults tof
.
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 typeNone
. This allows user type specific behaviour. There are also typessupport
andbot
.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.
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.
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.
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
}
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 unbind 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
- Remove the arbitrary data store known as account data. For example, this includes:
- list of ignored users;
- push rules;
- secret storage keys; and
- cross-signing keys.
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
- Remove the user's creation (registration) timestamp
- Remove rate limit overrides
- Remove from monthly active users
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
}
The parameter new_password
is required.
The parameter logout_devices
is optional and defaults to true
.
The api is:
GET /_synapse/admin/v1/users/<user_id>/admin
A response body like the following is returned:
{
"admin": true
}
Note that you cannot demote yourself.
The api is:
PUT /_synapse/admin/v1/users/<user_id>/admin
with a body of:
{
"admin": true
}
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
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 ofroom_id
.total
- Number of rooms.
Gets information about account data for a specific user_id
.
The API is:
GET /_synapse/admin/v1/users/<user_id>/accountdata
A response body like the following is returned:
{
"account_data": {
"global": {
"m.secret_storage.key.LmIGHTg5W": {
"algorithm": "m.secret_storage.v1.aes-hmac-sha2",
"iv": "fwjNZatxg==",
"mac": "eWh9kNnLWZUNOgnc="
},
"im.vector.hide_profile": {
"hide_profile": true
},
"org.matrix.preview_urls": {
"disable": false
},
"im.vector.riot.breadcrumb_rooms": {
"rooms": [
"!LxcBDAsDUVAfJDEo:matrix.org",
"!MAhRxqasbItjOqxu:matrix.org"
]
},
"m.accepted_terms": {
"accepted": [
"https://example.org/somewhere/privacy-1.2-en.html",
"https://example.org/somewhere/terms-2.0-en.html"
]
},
"im.vector.setting.breadcrumbs": {
"recent_rooms": [
"!MAhRxqasbItqxuEt:matrix.org",
"!ZtSaPCawyWtxiImy:matrix.org"
]
}
},
"rooms": {
"!GUdfZSHUJibpiVqHYd:matrix.org": {
"m.fully_read": {
"event_id": "$156334540fYIhZ:matrix.org"
}
},
"!tOZwOOiqwCYQkLhV:matrix.org": {
"m.fully_read": {
"event_id": "$xjsIyp4_NaVl2yPvIZs_k1Jl8tsC_Sp23wjqXPno"
}
}
}
}
}
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:
account_data
- A map containing the account data for the userglobal
- A map containing the global account data for the userrooms
- A map containing the account data per room for the user
Gets a list of all local media that a specific user_id
has created.
These are media that the user has uploaded themselves
(local media), as well as
URL preview images requested by the user if the
feature is enabled.
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
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 to100
. -
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 ofnext_token
from a previous call. Defaults to0
. -
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 ascendingmedia_id
, which guarantees a stable ordering. Valid values are:media_id
- Media are ordered alphabetically bymedia_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. Eitherf
for forwards orb
for backwards. Setting this value tob
will reverse the above sort order. Defaults tof
.
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. Details about the format are documented under media repository.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.
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
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 deletedmedia_id
total
: integer - Total number of deletedmedia_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.
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.
Gets information about all devices for a specific user_id
.
The API is:
GET /_synapse/admin/v2/users/<user_id>/devices
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.
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"
]
}
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.
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>
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.
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"
}
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.
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>
{}
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.
Gets information about all pushers for a specific user_id
.
The API is:
GET /_synapse/admin/v1/users/<user_id>/pushers
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":"[email protected]"
}
],
"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 ifkind
ishttp
. 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.
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
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.
This API allows to override or disable ratelimiting for a specific user. There are specific APIs to set, get and delete a ratelimit.
The API is:
GET /_synapse/admin/v1/users/<user_id>/override_ratelimit
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.
{}
The API is:
POST /_synapse/admin/v1/users/<user_id>/override_ratelimit
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 to0
.burst_count
- positive integer, optional. How many actions that can be performed before being limited. Defaults to0
.
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.
The API is:
DELETE /_synapse/admin/v1/users/<user_id>/override_ratelimit
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.
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.