Originally published at tddbuddy.com.
Related reading: The Bar for TDD Just Moved names the floor; TDD Already Does BDD, Without the Gherkin names the craft underneath; Your Test Suite Is Your API for Agents names the audience. This post is the construction discipline those three assume.
Setup is world-building. Most teams treat it as plumbing.
That confusion is upstream of a dozen problems that get other names. Flaky tests. Brittle fixtures. Suites that resist refactoring. Setup blocks that have to be read line by line to figure out what scenario is even running. None of those are root causes. They are downstream of a single design failure: the arrange step of a test was never about getting the data into shape. It was always a claim about which world the behavior under test lives in. The team that hides that claim inside hand-assembled object construction has not removed the claim from the test. The team has only made the claim illegible.
The reason this matters now more than it ever did is the same reason every test-quality argument is louder now than it was five years ago. Agents recombine what they find. They do not bring a world model to the codebase. They sample one out of what is already there. A codebase with a coherent layered grammar for worlds gives an agent worlds to compose against. A codebase with hand-assembled setup gives an agent a pile of unrelated nouns and no way to form a sentence. The expressiveness of the world layer is the ceiling on what the agent can write.
