# MatrixRTC modes

Element Call is in the middle of a transition of how a call session is
represented and how participants pick an SFU:

- **Membership events**: from room _state_ events
  (`org.matrix.msc3401.call.member`) to _sticky_ events
  ([MSC4354](https://github.com/matrix-org/matrix-spec-proposals/pull/4354)),
  which are a much better fit for the short lived, per-device nature of call
  memberships.
- **SFU selection**: from "everyone connects to the SFU of the oldest member" to
  **multi SFU**, where each participant uses its own homeserver's SFU and the
  SFUs interconnect.

Not every homeserver supports sticky events yet. Multi SFU is supported on all current (August 2026)
element call clients. The three MatrixRTC modes are the steps of that transition,
so a deployment can pick the newest one its homeserver and its user base can
handle.

## The modes

| Mode            | Membership events | SFU selection | JWT endpoint                 |
| --------------- | ----------------- | ------------- | ---------------------------- |
| `legacy`        | state events      | oldest member | legacy                       |
| `compatibility` | state events      | multi SFU     | legacy                       |
| `matrix_2_0`    | sticky events     | multi SFU     | Matrix 2.0 (hashed identity) |

**`legacy`** — the lowest common denominator. Use it if calls need to work with
Element Call clients older than v0.17.0, which cannot handle multi SFU calls. (unused)

**`compatibility`** — multi SFU, but still state events. Use it when all Element
Call clients are v0.17.0 or later but the homeserver does not support sticky
events. This is the default. (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](https://github.com/element-hq/lk-jwt-service)
and identifies the room by a hashed identity instead of a `livekit_alias`.
(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:

```json
{
  "matrix_rtc_mode": "compatibility"
}
```

Valid values are `legacy`, `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.