What do you prioritize when hiring an app development company?

We're evaluating several app development companies for an upcoming web and mobile project. Most agencies advertise similar strengths AI integration, modern frameworks, cross-platform development, UX/UI design, security, and fast delivery so it's becoming difficult to tell which one is actually the best fit. For those who've worked with an external development team, what factors ended up being the most important once the project was underway?

Some questions I have:

How do you evaluate a company's technical expertise beyond its portfolio?
How important are client reviews and industry-specific case studies?
Do you value communication and project management more than pricing?
What questions do you always ask before signing a contract?
How do you assess code quality, scalability, and long-term maintenance?
What are the biggest red flags you've encountered?

I've also been browsing resources like for development-related utilities and productivity tools while planning the project. I'd love to hear from developers, founders, product managers, or anyone who's hired an app development company. If you were starting over today, what would you do differently?