Architecting LLM Agents: The SDB Primitive

The integration of stochastic Large Language Models (LLMs) with deterministic software systems in production LLM agents has created a critical, yet often overlooked, architectural element: the boundary between these two paradigms. This paper introduces the Stochastic-Deterministic Boundary (SDB) as a foundational, four-part contract (proposer, verifier, commit, reject) that governs how LLM outputs translate into system actions. As detailed by Vasundra Srinivasan, the SDB is posited as the load-bearing primitive for production agent runtimes, a concept elaborated upon with a catalog of six production LLM agent runtime patterns.

The SDB: A New Architectural Primitive

The SDB is presented not merely as an interface, but as the central architectural object defining the interaction between probabilistic AI outputs and structured software execution. This framework organizes agent runtime design into three key concerns: Coordination, State, and Control. By treating the SDB as a first-class primitive, Srinivasan proposes a structured approach to building more reliable and predictable LLM-powered systems. The paper traces the lineage of these runtime patterns to established distributed-systems concepts, highlighting the unique challenges and adaptations required when the 'worker' is a stochastic LLM.

Mitigating Failure in Stochastic Systems

Beyond defining the SDB, the research offers practical tools for managing failures inherent in LLM agents. A five-step methodology guides the selection of appropriate runtime patterns, while a diagnostic procedure maps production failures to specific pattern weaknesses. A novel failure mode, 'replay divergence,' is identified, describing how LLM consumers of event logs can produce inconsistent downstream outputs due to model or prompt changes. This highlights a critical shift in reliability engineering: as per-call model variance decreases, the choice of production LLM agent runtime patterns and the strength of the SDB become paramount for achieving long-term system robustness. A stylized reliability decomposition further separates model-specific variance from the 'architectural momentum' of the runtime, underscoring the strategic importance of pattern selection and SDB design.