Look first at domain experience, not the size of the portfolio. Ask for two or python software development company three projects that sit close to your stack, and then ask whether those engineers are still with the mvp development company. A serious vendor is happy to connect you with the engineers. Vague answers at this stage almost always mean the demo work came from somewhere else.
The agreement deserves more scrutiny than the proposal. Three clauses do most of the work: intellectual property assignment, the NDA, and termination and handover. Every artifact has to transfer to you once invoices are settled, including source code, designs and monolith vs microservices comparison infrastructure as code. Look closely at any clause that leaves so-called reusable libraries with the vendor, as that is often the dependency that makes switching painful.
Find out how the estimate was built. A serious estimate is accompanied by a list of assumptions, a breakdown per feature and a best case and a worst case. A fixed-price contract only makes sense when the scope is genuinely frozen; in any other case the provider pads the number and you pay for uncertainty either way. A time-and-materials model moves the risk back to the client, so it outsourcing usa needs a sprint cadence, demos and a budget cap.
How the work is run matters as much as the number of developers. Establish how a new requirement enters the plan, who writes the acceptance criteria and how quality assurance works. A well-run team should be able to walk you through running software rather than status reports. Written acceptance criteria stay the practical protection against an argument at delivery time.
Finally, think about the end of the engagement at the start rather than at the end. Ask that the repository lives on infrastructure you own from the first commit, and that documentation is updated as part of the work. A partner who is comfortable with this will agree quickly; resistance at this point says quite a lot.