Deterministic MCP Tools Proposed to Fix LLMs' Unreliable Financial Arithmetic

A developer has built the Couples Shared Account Engine, a Model Context Protocol (MCP) server designed to handle proportional expense splitting for households rather than relying on a language model's error-prone internal arithmetic. The system uses four calculation primitives — covering income proportions, individual obligations, reconciliation transfers, and partner surplus — to ensure mathematically rigorous, equity-based cost sharing. The approach addresses a known weakness of LLM-driven automation, where probabilistic models often produce plausible-looking but mathematically flawed outputs for deterministic tasks like bill splitting. The engine is part of a broader project called Vinkius, which aims to simplify authentication complexity in agentic workflows through a unified gateway called MCPFusion, replacing per-tool credential management with a single connection token. The developer highlights that financial decision-support tools carry a higher risk profile than standard data queries, making controlled, deterministic logic especially critical when AI agents interact with real bank APIs or sensitive financial data.