When should an agent write code instead of emitting JSON tool calls?
Tool calling costs one LLM round trip per action. Code costs one round trip for a loop over 500 records. The signal is knowing exactly what you trade away when you let the model write the program instead of the call.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Tool calling costs one LLM round trip per action. Code costs one round trip for a loop over 500 records. The signal is knowing exactly what you trade away when you let the model write the program instead of the call.
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.
