Start with relevant experience, not the number of logos on the website. Request a couple of case studies that match your technology stack, and then find out whether those engineers are still with the flutter app development company. A solid partner is happy to connect you with the tech lead. Vague answers at this stage almost always mean the delivery team is not the team you were shown.

The contract needs a slower read than the pitch. A few clauses carry most of the weight: ownership of the code, confidentiality, and exit terms and handover. All the work product should transfer to you on payment, symfony vs laravel performance along with designs, scripts and infrastructure configuration. Look closely at language that leaves so-called reusable libraries with the vendor, since that is often exactly the piece that locks you in.

Ask how they estimate. An honest estimate is accompanied by a written set of assumptions, a task-level breakdown and a range rather than a single number. A fixed price works only when the requirements are stable and documented; when the scope is still moving the provider adds a risk premium and you pay for it anyway. Time and materials moves the risk back to the client, so it needs a sprint cadence, demos and a budget cap.

How the work is run matters as much as team size. Ask what happens when the scope changes, who signs off on a feature and how testing is organised. A mature team will be able to show you a working build every one or two weeks. Acceptance criteria in writing stay the only reliable protection against an argument at delivery time.

Last, consider the day you no longer need this vendor at the start rather than at the end. Ask that the repository lives in your organisation from day one, and that documentation is written as you go rather than left to the end. A provider confident in its own work accepts it without argument; resistance at this point says a great deal.