--- a/server/build/packages/mist/src/mist/internal/http.gleam
+++ b/server/build/packages/mist/src/mist/internal/http.gleam
@@ -489,6 +489,9 @@ pub fn upgrade_socket(
     |> result.replace_error(req),
   )

+  // Check for WebSocket subprotocol (e.g., graphql-transport-ws)
+  let protocol = request.get_header(req, "sec-websocket-protocol")
+
   let permessage_deflate = websocket.has_deflate(extensions)

   let accept_key = parse_websocket_key(key)
@@ -500,6 +503,12 @@ pub fn upgrade_socket(
     |> response.prepend_header("connection", "Upgrade")
     |> response.prepend_header("sec-websocket-accept", accept_key)

+  // Add subprotocol header if requested
+  let resp = case protocol {
+    Ok(proto) -> response.prepend_header(resp, "sec-websocket-protocol", proto)
+    Error(_) -> resp
+  }
+
   case permessage_deflate {
     True ->
       Ok(response.prepend_header(