The MCP Apps spec defines changing the size via ui/size-change. Though the SDK's app.sendSizeChange sends a ui/notification/size-change message. This discrepancy causes confusion.
From SEP:
{
jsonrpc: "2.0",
method: "ui/size-change",
params: {
width: number, // Viewport width in pixels
height: number // Viewport height in pixels
}
}
SDK Behavior:
{
"jsonrpc":"2.0",
"method":"ui/notifications/size-change",
"params": {
"width": 753,
"height": 318
}
}