Start with proven experience, not the size of the portfolio. Ask for two or three engagements that sit close to your stack, and then find out whether those engineers are still with the llm development company. An honest provider is happy to connect you with the engineers. Evasive answers at this stage usually mean the demo work came from somewhere else.

The paperwork warrants more scrutiny than the proposal. A few clauses carry most of the weight: intellectual property assignment, confidentiality, and exit terms and handover. Everything produced should transfer to you on payment, including designs, scripts and infrastructure configuration. Look closely at language that keeps so-called reusable libraries with the vendor, since this is frequently the dependency that makes switching painful.

Ask how they estimate. An honest estimate arrives with a written set of assumptions, a breakdown by feature or module and a range rather than a single number. A fixed-bid deal is only reasonable when the scope is genuinely frozen; when the scope is still moving the supplier prices the risk in and you pay for software development partner it anyway. Hourly billing moves the risk back to the client, so it requires a sprint cadence, demos and a budget cap.

How the work is run beats headcount. Establish how change requests are handled, who signs off on a feature and how quality assurance works. A well-run team will be able to show you a working build every one or two weeks. Written acceptance criteria are the only reliable protection against endless rounds of rework.

Last, think about the end of the engagement at the start rather than at the end. Ask that the repository sits on infrastructure you own from day one, and that the documentation is refreshed in every sprint. A partner who is comfortable with this accepts it without argument; resistance at this point reveals a great deal.