Start with the reason this software should exist, not a feature list. Which people will use this, how many times a day, and how is the job done today? A vendor who knows what you are trying to achieve will suggest a simpler way to reach it; someone handed only a feature list can only price your assumptions along with the work.
Set out the scope as concrete flows: next.js development services who does what, and what happens next. Every bit as useful, state explicitly what is out of scope. An explicit exclusion list saves more disagreement later than almost anything else in the document. Mark too which decisions are settled langchain and rag difference which are still open — the difference changes the price, and pretending everything is fixed helps nobody.
Write down the hard constraints. The list covers systems you must integrate with, the data you already hold and its condition, regulatory obligations, expected load, target platforms and stacks you cannot change. If a deadline is real, say what depends on it: an experienced team is usually able to rearrange the plan to hit it, provided they hear about it early.
Define what the word done means for the important items. Acceptance criteria do not require formal language: a short paragraph setting out the expected behaviour is enough. This one section shortens the review at the end dramatically and removes the most common source of disputes.
Finally, say what you expect back. Ask for an itemised estimate, the assumptions used, whatever the team considers risky and a range rather than a single figure. Treat a wide range as a signal about the brief: it tells you the part of the brief that needs work. Then clarify that area and ask again — the revised figure tends to be much more reliable.