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+# MSC 2379: Add /versions endpoint to Appservice API. 
+
+Bridges do not have a way to specify what version of the spec they support. This means that if the path
+of any of the appservice endpoints were to change in the spec, homeservers would not be able to
+intelligently discover the paths that a bridge supports.
+
+## Proposal
+
+A new endpoint is required, which is `/_matrix/app/versions`. This is nearly identical to the
+[C-S API](https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-versions) endpoint
+but lacks a `unstable_features` key, and is hosted by the appservice rather than the homeserver.
+
+All bridges MUST implement this endpoint and specify which version(s) of the `AS` API they support. 
+The homeserver MUST send requests to the endpoints specified by that version of the AS spec.
+
+## Potential issues
+
+None
+
+## Alternatives
+
+This proposal previously used the `registration` file as a way to specify the supported version, but
+this was dropped as it was hard for the bridge to be authoritive over what version it supports. Typically
+the registration format is generated once by the bridge and then handled by the homeserver. If a bridge were
+to update and require a new version of the AS API, the registration data would need to be updated/regenerated.
+
+## Security considerations
+
+None