Before signing
Start with a written brief
A brief does not need to be long, but it needs to be specific. Describe the problem, the users, the core tasks the product must support, and any systems it must connect to. Note your constraints: budget range, timeline, and who on your side can make decisions. A partner who receives a clear brief can respond with a realistic plan. One who receives a vague idea will either ask a lot of questions or, worse, guess.
Two or three pages are usually enough for a first conversation. Write it in plain language your own team would use in a meeting, not in the vocabulary of a technical specification. If you cannot describe a requirement without a diagram, that is useful information too — it tells you the topic deserves its own working session with the partner before anyone estimates anything.