Different brokerages take different positions on these sorts of integrations - some go after all third-party integrations aggressively (including screen scrapers), while some will turn a blind eye as long as you're not abusing their platform. For a brokerage like Robinhood, we've only seen them take enforcement actions against individuals if they send a high volume of orders in a short timeframe.

It's worth noting that even with sanctioned integrations (such as the one we developed for TD Ameritrade), sending too many orders at once or not appropriately batching orders will still get you in trouble - we guide people that integrate with us through these nuances so that their users don't get angry calls from risk departments :).