Is XPC reply guaranteed to be dispatched in a sole single thread?

I have this protocol method in the XPC server:

- (void)ping:(void (^)(NSString* reply))resultHandler;

Then I connect to the XPC server:

xpc.remoteObjectInterface = [NSXPCInterface interfaceWithProtocol:@protocol(I_LoginHelperProtocol)]
_agent = [xpc remoteObjectProxyWithErrorHandler:^(NSError *error) {
  /
        MyLog(@"XPC error: %@", error);
        handler(error);
  }];

Then I call the ping method (or other methods) on the agent:

_agent ping:^(NSString *reply) {
        [_appLog log:@"LoginHelper Agent ping reply: %@ in thread %@", reply, [NSThread currentThread]];
    }];

My questions is - is it guaranteed that the reply handler is always dispatched in one single thread, or is it possible for XPC service to spawn a new thread to service the request?