A useful prototype proves one physical workflow
The best first scope is not a complete product. It is one device workflow: read a sensor, control an output, log an event, send telemetry, alert a human, or show a local operator screen. ESP32 is often enough for low-power sensor/control tasks. Raspberry Pi is useful when Linux, camera handling, local UI, storage, or gateway behavior matters.
The deliverable should include more than code
A client needs firmware, wiring notes, a BOM, setup steps, data format, dashboard or log output, test cases, known failure modes, and a list of what must change before a pilot. That is what separates a business prototype from a hobby demo.
Prototype, pilot, and production are different contracts
Manufacturing, certification, enclosure design, power reliability, weatherproofing, battery life, safety, and field support are separate phases. Clear phase boundaries keep expectations sane and make the prototype more useful.