In a Claude agentic loop, which signal should the loop treat as meaning the model has finished its work?

Answer: B — The response's stop_reason field is "end_turn"

stop_reason "end_turn" is the structured signal that the model has ended its turn; "tool_use" means it wants tools run and the loop should continue. A leading text block (a) can come before a tool_use block in the same response. Parsing prose (c) is a natural-language heuristic. An iteration limit (d) is a safety guard against runaway loops, not the completion signal.