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# MSC1753: client-server capabilities API
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A mechanism is needed for clients to interrogate servers to establish whether
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particular operations can be performed.
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For example, users may not be able to change their password if a server is
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configured to authenticate against a separate system, in which case it is
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nonsensical to offer the user such an option.
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## Proposal
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### `GET /_matrix/client/r0/capabilities`
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We will add a new endpoint to the client-server API: `GET
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/_matrix/client/r0/capabilities`. The endpoint will be authenticated as normal
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via an access token.
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The server should reply with a list of supported features, as shown:
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```json
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{
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"capabilities": {
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"m.capability_one": {}
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}
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}
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```
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The keys of the `capabilities` object are capability identifiers. As with
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other identifiers in the Matrix protocol, the `m.` prefix is reserved for
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definition in the Matrix specification; other values can be used within an
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organisation following the Java package naming conventions.
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The values of the `capabilities` object will depend on the capability
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identifier, though in general the empty object will suffice.
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### Initial capability identifiers
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As a starting point, a single capability identifier is proposed:
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`m.change_password`, which should be considered supported if it is possible to
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change the user's password via the `POST /_matrix/client/r0/account/password`
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API.
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The value of the `capabilities` object in the response should be the empty
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object.
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### Fallback behaviour
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Clients will need to be aware of servers which do not support the new endpoint,
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and fall back to their current behaviour if they receive a 404 response.
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### Suitable applications
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In general, capabilities advertised via this endpoint should depend in some way
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on the state of the user or server - in other words, they will be inherently
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"optional" features in the API.
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This endpoint should *not* be used to advertise support for experimental or
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unstable features, which is better done via `/client/versions` (see
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[MSC1497](https://github.com/matrix-org/matrix-doc/issues/1497)).
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Examples of features which might reasonably be advertised here include:
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* Whether the server supports user presence.
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* Whether the server supports other optional features. The following could be
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made optional via this mechanism:
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* Room directory
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* URL previews
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* Policy restricitions, such as:
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* Whether certain types of content are permitted on this server.
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* The number of rooms you are allowed in.
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* Configured ratelimits.
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Features which might be better advertised elsewhere include:
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* Support for e2e key backups
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([MSC1219](https://github.com/matrix-org/matrix-doc/issues/1219)) - list in
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`/client/versions`.
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* Support for lazy-loading of room members - list in `/client/r0/versions`.
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* Media size limits - list in `/media/r0/config`, because the media server may
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be a separate process.
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* Optional transports/encodings for the CS API - probably better handled via
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HTTP headers etc.
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* Variations in room state resolution - this is implied via the room version
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(which is in the `m.room.create` event).
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## Tradeoffs
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One alternative would be to provide specific ways of establishing support for
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each operation: for example, a client might send an `GET
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/_matrix/client/r0/account/password` request to see if the user can change
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their password. The concern with this approach is that this could require a
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large number of requests to establish which entries should appear on a menu or
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dialog box.
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Another alternative is to provide a generic query mechanism where the client
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can query for specific capabilities it is interested in. However, this adds
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complication and makes it harder to discover capability identifiers.
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## Potential issues
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None yet identified.
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## Security considerations
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None yet identified.
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## Conclusion
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We propose adding a new endpoint to the Client-Server API, which will allow
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clients to query for supported operations so that they can decide whether to
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expose them in their user-interface.
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