diff --git a/Frontend/src/protocol.ts b/Frontend/src/protocol.ts
new file mode 100644
index 0000000..f38e050
--- /dev/null
+++ b/Frontend/src/protocol.ts
@@ -0,0 +1,33 @@
+// Wire messages exchanged with the backend over WebSocket.
+// Mirror of Backend/WireMessages.cs.
+
+export interface CursorMessage {
+  type: 'cursor';
+  x: number;
+  y: number;
+}
+
+export interface InitFrame {
+  type: 'init';
+  world: [number, number];
+}
+
+export interface StateFrame {
+  type: 'state';
+  x: number;
+  y: number;
+  angle: number;
+  engine: number;
+  target: [number, number];
+  step: number;
+}
+
+export type ServerFrame = InitFrame | StateFrame;
+
+export function isInit(frame: ServerFrame): frame is InitFrame {
+  return frame.type === 'init';
+}
+
+export function isState(frame: ServerFrame): frame is StateFrame {
+  return frame.type === 'state';
+}