Start with proven experience, rust software development company not the number of logos on the website. Request a couple of engagements that sit close to your technology stack, and then ask who actually wrote that code. A solid partner is happy to connect you with the tech lead. Vague answers at this stage almost always mean you are talking to a reseller.
The agreement deserves more scrutiny than the proposal. A few clauses carry most of the weight: intellectual property assignment, confidentiality, and exit terms and handover. Every artifact must transfer to you once invoices are settled, including documentation, pipelines and deployment scripts. Look closely at any clause that keeps reusable components with the vendor, typescript web frameworks since this is frequently the part you cannot replace later.
Find out how the estimate was built. An honest estimate arrives with a list of assumptions, a breakdown by feature or module and a best case and a worst case. A fixed-price contract only makes sense when the specification is complete; when the scope is still moving the supplier adds a risk premium and you pay for it anyway. Hourly billing moves the risk back to the client, so it requires a sprint cadence, demos and a budget cap.
How the work is run beats headcount. Establish what happens when the scope changes, who signs off on a feature and how testing is organised. A team can demonstrate running custom software development usa rather than status reports. Written acceptance criteria are your only real protection against endless rounds of rework.
Last, think about the handover while the relationship is still good. Insist that the source repository stays in your organisation from the beginning, and that the documentation is refreshed in every sprint. A partner who is comfortable with this accepts it without argument; a long negotiation over it tells you most of what you need to know.