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 prompt 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": {
+            "m.development": ["state-res-v2-test", "event-ids-as-hashes-test", "3"],
+            "m.beta": ["2"],
+            "m.default": "1",
+            "m.recommended": "1",
+            "m.mandatory": ["1"]
+        }
+    }
+}
+```
+
+The labels picked (`m.development`, etc) are based upon the categories for different room versions.
+Note that `m.default` and `m.recommended` reinforce that there is exactly 1 version in each category.
+
+Clients are encouraged to make use of this capability to determine if the server supports a given
+version, and at what state.
+
+Clients should prompt people with sufficient permissions to perform an upgrade to upgrade their rooms
+to the `m.recommended` room version.
+
+Room versions might appear under multiple categories under some circumstances. In particular, it is
+expected that anything in `m.development` or `m.beta` appears exactly once in the whole capability
+whereas `m.default`, `m.recommended`, and `m.mandatory` may duplicate a room version. The duplication
+is possible due to the definitions of each category:
+
+* `m.default` - This is the room version that the server is going to apply to all new rooms by default.
+* `m.recommended` - The version clients should be prompting people to upgrade to.
+* `m.mandatory` - The version the server is going to enforce on all pre-existing rooms.
+
+With these definitions, it is possible that a room version fits multiple criteria (ie: "please upgrade
+your rooms to version X which is also the default for new rooms"). Clients will generally only be
+interested in the `m.recommended` room version, leaving the rest as informational for users.
+
+
+## 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.