Including the ones we would rather you did not ask us — and what a good answer sounds like.
Vendor selection tends to reward whoever demos best. These questions reward whoever will still be useful in year three.
1. Who owns the source code and the infrastructure accounts?
The answer should be you, in writing, from day one — including the cloud accounts, the domain registrar and the app store listings. A vendor holding these is holding leverage, not assets.
2. What happens if we want to leave in month six?
A confident vendor has a rehearsed answer. A nervous one changes the subject to how unlikely that is.
3. Which parts of this have you not done before?
Every project has some. A vendor who claims none is either not listening or not telling you the truth, and both are expensive.
4. Who exactly will work on this?
Ask for names and availability, and ask whether the people in the room are the people who will build it. Bait-and-switch staffing is the most common complaint about agencies of every size.
5. What is explicitly out of scope?
A proposal that lists only inclusions is a proposal designed to generate change requests. Integration, data migration, training and support are the usual silent omissions.
6. How will we hand this to our own team?
Documentation and training should appear in the plan with dates and effort attached, not as a line called "handover" in the final week.
7. Show me something that went wrong
Ask for a project that overran or was cancelled, and what changed afterwards. Anyone with a hundred projects has several. The willingness to discuss one is a better signal than any case study.
The vendor who answers question seven honestly is usually the one worth hiring.