How do you draw module boundaries for AI coding agents? Small tasks duplicate code, big tasks break neighbors

A developer building a multi-product SaaS with AI coding agents is struggling with how to draw module boundaries. The core issue is a tradeoff between small tasks, which can lead to duplicated code and parallel execution, and large tasks, which require more context and can break neighboring modules. The developer is seeking advice on how to manage this challenge in agent-driven development, particularly regarding internal versus interface changes and past scaling issues.

This post moves the discussion of AI agent development from theoretical capabilities to practical architectural challenges, unlike most reports focusing on new models or benchmarks.