MSC1804: Advertising capable room versions to clients

proposals/1804-advertising-capable-room-versions.md

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