We are a young unfounded company and work with most of the major us investment banks and the answer is there is no sandbox or API that they will share with you.

We are a young unfounded company and work with most of the major us investment banks and the answer is there is no sandbox or API that they will share with you. Our experience is that the banks won't waste their time on anything that feels beta so you have to invest in having a solution built out and people using it (a catch 22). The exception to this rule however is that banks work for their clients (this pertains to investment banks not retail) and they will use a new tool in the context of an engagement when asked by the issuer, investor or firm they are representing. Not sure what you're planning to build but go to their customers first (issuers or large investors) who are much more agile. Once you're in the system you can navigate through the onboarding. Happy to share our experiences further if you want to discuss - reach out directly.