Look first at proven experience, not the size of the portfolio. Ask to see two or three projects that resemble your technology stack, and then ask which engineers actually built it. A solid partner will introduce you to the tech lead. Answers that name nobody at this stage almost always mean the delivery team is not the team you were shown.
The agreement deserves more scrutiny than the proposal. Three sections matter more than the rest: intellectual property assignment, confidentiality, and exit terms and handover. Everything produced has to transfer to you as it is paid for, together with designs, scripts and infrastructure configuration. Be careful with language that leaves so-called reusable libraries outside the transfer, since that is often exactly the piece that locks you in.
Ask where their numbers come from. An honest estimate is accompanied by the assumptions behind it, a breakdown by feature laravel or symfony module and flutter developer hourly rate a range rather than a single number. A fixed price only makes sense when the specification is complete; otherwise the provider pads the number and you fund the buffer regardless. A time-and-materials model puts the risk on your side, so it requires visible weekly reporting and a spending cap.
Process beats team size. Ask what happens when the scope changes, who signs off on a feature and what the QA setup looks like. A mature team should be able to walk you through running software rather than status reports. Clear, written acceptance criteria stay the practical protection against endless rounds of rework.
Before signing, consider the day you no longer need this vendor before it becomes urgent. Require that the code repository sits under your account from the first commit, and that documentation is updated as part of the work. A partner who is comfortable with this says yes immediately; hesitation here reveals a great deal.