Why Spec-Driven Development Breaks Down in Microservices (Part 1): The Cross-Service Context Problem

The first article in a three-part series.

Part 1 - where an LLM loses cross-service context and why local specs are not enough.

Part 2 - archspec: a versioned architecture contract for services.

Part 3 - archspec: feature investigation, contract updates, and implementation.

1. Introduction

I work at a large product company with thousands of microservices. In a system like that, even a small feature often crosses several services, events, and internal contracts. Some teams already use spec-driven development with LLMs to plan and review features, so I wanted to understand where this approach helps and where it starts to fail.

As long as the task stays inside one service, things usually move quickly. The spec is short, and the description plus implementation fit into the model's context window. The trouble starts when a feature crosses several services. Each individual piece can look fine: clean layering, naming that follows the...

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