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# Proposal for advertising capable room versions to clients
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Currently clients need to guess at which room versions the server supports, if any. This is particularly
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difficult to do as it generally requires knowing the state of the ecosystem and what versions are
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available and how keen users are to upgrade their servers and clients. The impossible judgement call
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for when to encourage people to upgrade shouldn't be impossible, or even a judgement call.
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## Proposal
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Building off of [MSC1753](https://github.com/matrix-org/matrix-doc/pull/1753) (capabilities API) and
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the [recommendations laid out for room versions](https://github.com/matrix-org/matrix-doc/pull/1773/files#diff-1436075794bb304492ca6953a6692cd0R463),
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this proposal suggests a `m.room_versions` capability be introduced like the following:
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```json
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{
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"capabilities": {
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"m.room_versions": {
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"default": "1",
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"available": {
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"1": "stable",
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"2": "stable",
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"state-res-v2-test": "unstable",
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"event-ids-as-hashes": "unstable",
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"3": "future-scifi-label"
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}
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}
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}
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}
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```
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Clients are encouraged to make use of this capability to determine if the server supports a given
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version, and at what level of stability. Anything not flagged explicitly as `stable` should be treated
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as `unstable` (ie: `future-scifi-label` is the same as `unstable`).
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The default version is the version that the server is using to create new rooms with. Clients can
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make the assumption that the default version is a stable version.
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Clients should encourage people with sufficient permissions to perform an upgrade to upgrade their
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rooms to the `default` room version when the room is using an `unstable` version.
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## Potential issues
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Changes aren't pushed to the client, which means clients may want to poll this endpoint on some
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heuristic instead. For example, clients may want to poll the endpoint weekly or when the user relaunches
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the client. Clients may also wish to provide users a way to upgrade without considering the capabilities
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of the server, expecting that the server may not support the user-provided version - the intention
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being such a feature would be used by eager room administrators which do not want to relaunch their
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client, for example.
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