When would you use gRPC instead of a REST API?

Use gRPC for internal service to service calls where you control both ends and want a strict contract, binary encoding, streaming and lower latency. Use REST over JSON for public APIs, browser clients and anything that benefits from being readable, cacheable and debuggable with curl. The trade is a generated, versioned schema and better performance against universal tooling and easy interoperability.

I split it by who the consumer is. For anything a browser or a third party talks to, I stay on REST with JSON, because everyone can call it, you can cache at the edge, and when something breaks at two in the morning you can reproduce it with curl. For internal service to service traffic where I own both sides, gRPC earns its keep. You get a proto file as an enforced contract, generated clients in every language so nobody hand rolls a serializer, real bidirectional streaming, and far fewer bytes on the wire. The costs are real though. Browsers need a proxy layer, your load balancer has to handle HTTP/2 properly, and debugging a binary payload takes more setup. On a system I worked on we kept the public edge as REST and used gRPC between the gateway and the internal services, which gave us type safety internally without asking a single customer to change anything.