Open with the business problem, not your preferred technology. Who will use the system, how many times a day, and what does the process look like without it? An experienced team who understands the goal often proposes a cheaper route to it; one who only sees a list of screens can only price your assumptions along with the work.
Set out the scope as short scenarios: who does what, and what happens next. Just as important, list what is out of scope. An explicit list of exclusions removes more disagreement later than the rest of the brief combined. Indicate as well which items are decided and which are still under discussion — the difference between rest and graphql changes the price, and hire nodejs engineer pretending everything is fixed only hurts you.
List the constraints. These include existing systems the software has to talk to, the data you have and where it lives, security and compliance rules, expected load, which devices matter and infrastructure that is already decided. If there is a hard date, say why: an experienced team is usually able to resequence the work to protect it, but not if the date is a secret.
Say what done means feature by feature. Testable acceptance criteria do not need special syntax: a short list stating the expected behaviour is enough. This one section compresses acceptance testing by a surprising margin and closes off the usual argument at handover.
One last thing, state what you want in the response. Require a task-level breakdown, llm application development the assumptions used, the risks the team sees and a low number and a high number. Read a wide range as a signal about the brief: is langchain a rag framework it usually points to where your description is thin. From there tighten that section and ask for a new estimate — the next version is far closer to reality.