How it works

The Mux is a Companion Mode client and server at once. It holds CSP's single connection and offers a connection that looks like CSP's.

Clip Studio Paint
        │  one upstream connection
        ▼
   ┌─────────┐
   │ CSP Mux │  proxy QR, own credentials
   └─────────┘
     │   │   │  independent sessions
     ▼   ▼   ▼
  App  App  App

It scans CSP's real QR from the screen and authenticates as the smartphone-side client; CSP never learns anything else exists. If the upstream drops, so does every downstream session.

Each app that scans the proxy QR gets a full independent session on the Mux's own credentials and never sees CSP's. Authentication, password rotation and the serial number space stop at the Mux; commands and preview tails are forwarded. A dropped app rejoins with the same invitation, which is why Hide QR disconnects nobody. Defaults: 8 clients, 4 reads, 32 MB frames.

Serial remapping

The protocol tags each request with a serial, and two apps both start counting at the same place. The Mux remaps every downstream serial onto its own upstream space and maps the reply back, so each app sees only its own response. Broadcasts are fanned out the other way.

Command scheduling

Mutating commands — SetCurrentColor, SetColorSelectionModel, SetBrushSize, SetAlpha, DoGesture, DoNavigator, DoQuickAccess, SetServerSelectedTabKind, DoModeChange — go through an ordered queue, one at a time, so one app's colour change never lands inside another's brush change. Everything else is a read and runs concurrently, so a large canvas preview does not block everyone's state queries. DoQuickAccess is what CSP Palette Companion's Auto Action uses.

Broker and scheduler live in src/CspMultiplexer.Broker/, the wire format in src/CspMultiplexer.Protocol/, after the MIT-licensed chocolatkey/clipremote. Companion Mode is reverse-engineered: a CSP update that changes the private wire protocol can break it.