Traditional software engineering assumes deterministic components. A function called with the same arguments returns the same result. A module's behavior is specified, tested, and frozen. The engineering discipline — version control, type systems, unit tests, code review — exists to ensure that the system does what its specification says.
Multi-agent LLM systems violate every assumption in that paragraph. An agent prompted with the same input may produce different outputs. Agents communicate in natural language, which is ambiguous. The system's behavior is an emergent property of agent interactions that cannot be predicted from individual agent specifications. When something goes wrong, there is no stack trace pointing to the bug — there is a conversation history pointing to a misunderstanding.
Loosely-Structured Software formalizes this as an engineering problem (arXiv:2603.15690). The key move is reframing: instead of treating non-determinism as a defect to be eliminated, treat it as a managed resource — runtime entropy. The engineering task shifts from constructing deterministic logic to managing the rate, type, and consequences of unpredictability.
Three layers of engineering emerge. Context engineering manages what each agent knows and how its knowledge is structured — the equivalent of type systems for natural language. Structure engineering manages how agents connect and communicate — the equivalent of module interfaces. Evolution engineering manages how the system changes over time as agents learn, adapt, and reconfigure — the equivalent of version control for behavior that was never frozen.
The design patterns that result look nothing like traditional software patterns. Instead of singletons and factories, there are “entropy budgets” (limits on how much unpredictability a subsystem is allowed to inject) and “structure checkpoints” (periodic snapshots of the system's communication topology that enable rollback to known-good configurations).
The code is not the system. The conversation is the system. The engineering is of the conversation.