'Build or buy?' is the wrong shape for the question. Almost every serious AI system has parts that should be bought, parts that should be built and parts where a partner shortens the path. The skill is in deciding at the component level.
Three questions per component
- Does it differentiate us? If the capability is part of what customers choose you for, own it. If every competitor can buy the same thing, don't spend effort rebuilding it.
- Does it depend on our proprietary data or process? The more it does, the more it should be built or heavily configured by you.
- How fast does it change? Fast-moving layers such as models and tooling are better rented, with an exit plan, than frozen into your own code.
Where partners fit
Use a partner when you need a capability once, or for a limited period, and the knowledge should end up with you: the first production system, an evaluation setup, a security review. Agree up front what you will own at handover: code, documentation, test sets and runbooks.
Whatever you buy, keep the data, the test set and the business logic in a form you can move. That is what keeps the option to change vendors real.