Start with proven experience, not the length of the client list. Ask to see three or four case studies that resemble your stack, and then ask specifically who actually wrote that code. A solid partner will introduce you to the people who would work on your project. Answers that name nobody at this stage usually mean you are talking to a reseller.
The agreement deserves a slower read than the pitch. Three sections matter more than the rest: custom software development london assignment of intellectual property, confidentiality, and which is better laravel or node js termination and handover. Everything produced must transfer to you as it is paid for, including designs, scripts backend and frontend technologies we use infrastructure configuration. Look closely at any clause that keeps framework code outside the transfer, since it is usually exactly the piece that locks you in.
Find out how the estimate was built. A serious estimate arrives with a written set of assumptions, a task-level breakdown and a range rather than a single number. A fixed-price contract only makes sense when the requirements are stable and documented; in any other case the supplier adds a risk premium and you pay for seo agency for software factory uncertainty either way. A time-and-materials model moves the risk back to the client, so it needs a cap, regular demos and transparent reporting.
How the work is run matters more than team size. Establish what happens when the scope changes, who signs off on a feature and how quality assurance works. A team should be able to walk you through a working build every one or two weeks. Written acceptance criteria are your only real protection against an argument at delivery time.
Last, consider the day you no longer need this vendor before it becomes urgent. Ask that the repository lives on infrastructure you own from the beginning, and that documentation is updated as part of the work. A vendor with nothing to hide will agree quickly; hesitation here tells you a great deal.