An estimate that arrives instantly is a red flag rather than good service. An experienced provider will come back with a list of questions: about who owns the data and what happens on failure. A vendor that commits to a figure before understanding the scope is simply guessing, and that guess will be corrected later — at your expense.
Watch for a mismatch between the engineers on the sales call and the developers actually assigned. Request specific people rather than roles in the statement of work, hire react native developer with a provision covering replacement. A team that talks only about abstract roles and never names individuals is keeping its own flexibility at your cost.
Insist on the source repository from the start. A provider that shows code only at milestones expects you to take delivery on faith. Visible commits tell you how many people are really working far better than any status report. The same applies to the build and deployment setup: if it does not exist, quality claims are just talk.
Vague wording in the contract around code ownership is not an oversight. The agreement needs to state in plain terms that all outputs produced under it belong to the client on payment. Check also the governing law and outsource kotlin development the milestone terms: a large upfront payment with no milestone tied to it removes your only leverage.
Lastly, examine how they communicate. Establish how many hours you will share each day, which named person is expected to answer day-to-day questions and within what time. Some genuine overlap generally works; zero overlap stretches every clarification into a day of delay. Careless writing in the early emails rarely improves later.