How Do You Actually Get Data to an LLM?
An LLM is only as useful as the context it has. The model itself is frozen — its weights were fixed at training time. Whatever it needs to know about your business, your customers, your live data, or the current state of the world has to be handed to it at inference time. Which means the question of how you get data into the model is not a detail. It is a core architectural decision.
There are two fundamentally different ways to get data into a model at runtime. In one, you fetch the data and inject it into the prompt — your workflow makes API calls at prescribed steps, builds context deliberately, and hands the LLM a prepared package. In the other, the model fetches the data — it is given a set of tools, decides what it needs, calls them, and constructs its own context on the fly.
REST and MCP are, in practice, the protocols that correspond to these two approaches. REST is the workhorse of the first. MCP is the infrastructure of the second. And that distinction maps almost exactly onto the autonomous vs. deterministic question from the previous post. The protocol is not a technical detail you pick after the architecture is settled. It is an architecture decision.