Begin with relevant experience, not the size of the portfolio. Ask to see three or four engagements that resemble your stack, and then ask specifically which engineers actually built it. A solid partner is happy to connect you with the engineers. Evasive answers at this stage generally mean the demo work came from somewhere else.
The paperwork deserves more scrutiny than the proposal. Three sections matter more than the rest: intellectual property assignment, the NDA, and ai development services exit terms and handover. Everything produced has to transfer to you once invoices are settled, together with source code, enterprise software development services designs and infrastructure as code. Be careful with wording that leaves reusable components with the vendor, since that is often the part you cannot replace later.
Find out how the estimate was built. A credible estimate comes with a list of assumptions, a breakdown by feature or module and a best case and a worst case. A fixed-bid deal is only reasonable when the specification is complete; when the scope is still moving the vendor pads the number and you pay software development company for startups it anyway. Time and materials shifts that risk to you, so it demands a cap, mvp development services regular demos and transparent reporting.
Process beats team size. Find out what happens when the scope changes, who defines done and what the QA setup looks like. A team can demonstrate a live build at the end of each sprint. Written acceptance criteria are your only real protection against endless rounds of rework.
Before signing, consider the day you no longer need this vendor before it becomes urgent. Insist that the source repository sits under your account from the beginning, and that documentation is written as you go rather than left to the end. A vendor with nothing to hide accepts it without argument; hesitation here reveals a great deal.