element-call/docs/matrix_rtc_modes.md

MatrixRTC modes

Element Call is in the middle of a transition of how a call session is represented: from room state events (org.matrix.msc3401.call.member) to sticky events (MSC4354), which are a much better fit for the short lived, per-device nature of call memberships.

Not every homeserver supports sticky events yet. The two MatrixRTC modes controls whether Element Call uses them.

The modes

Mode	Membership events	SFU selection	JWT endpoint
compatibility	state events	multi SFU	legacy
matrix_2_0	sticky events	multi SFU	Matrix 2.0 (hashed identity)

compatibility — multi SFU, but still state events. Use it when the homeserver does not support sticky events. This is the default.

matrix_2_0 — the target state. Requires a homeserver that advertises MSC4354 and all clients on v0.17.0 or later. The local membership requests its token from the Matrix 2.0 JWT endpoint of the MatrixRTC Authorization Service and identifies the room by a hashed identity instead of the Matrix room_id. (Remote memberships always try the new endpoint first and fall back to the legacy one, so remote participants can be on either.)

Selecting a mode

Users can choose a mode under Settings → Developer → MatrixRTC mode. The Matrix 2.0 option is disabled if the homeserver does not support sticky events.

A deployment can pin the mode for all its clients in config.json, which disables the Developer Settings choice:

{
  "matrix_rtc_mode": "compatibility"
}

Valid values are compatibility and matrix_2_0; an invalid value is ignored (with a warning) and the user's choice applies. Pinning matrix_2_0 on a homeserver without sticky event support makes joining fail with a "sticky events required" error.