How does LLM tool calling (function calling) actually work under the hood?
Tool calling powers every agent. The signal is grasping that the model executes nothing: it emits a structured request your code runs, then you hand the result back. Here is the loop and the details that determine reliability.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Tool calling powers every agent. The signal is grasping that the model executes nothing: it emits a structured request your code runs, then you hand the result back. Here is the loop and the details that determine reliability.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
