---
type: convention
title: TDD and Test Isolation Methodology
summary: Rules for writing isolated, non-flaky tests — setup-based test data over fixtures, ENV cleanup, and avoiding config caching that breaks test overrides. Answers "why is this test flaky / coupled to other tests?"
tags:
  - type/convention
  - domain/testing
scope: global
last_updated: 2026-07-13
date: 2026-07-13
related:
  - phlex-component-design
  - ai-agent-rules
source: hyperthrive_dev
---

# TDD and Test Isolation Methodology

## Purpose
Governs how to set up test data and handle process-global state (ENV, constants) so tests stay isolated, parallel-safe, and non-flaky.

## Core Principles

**1. Build test data explicitly in `setup`, not via fixtures.**
Fixtures hide what data exists behind a separate file, creating coupling between unrelated tests that happen to share a fixture. Explicit `setup` blocks make dependencies visible in the test itself, isolate each test's data, and work cleanly under parallel test execution.

**2. Never cache dynamic/environment values in constants.**
A constant like `COMPANY_NAME = ENV.fetch(...).freeze` is captured once at class-load time — tests can't override it afterward because it's frozen before the test process even starts. Read ENV (or any config source that a test might need to vary) dynamically via a method call, not a frozen constant, so tests can verify fallback and override behavior.

**3. Any test that mutates process-global state must restore it, unconditionally.**
Tests run in parallel processes; a leaked ENV change or global mutation from one test can flake an unrelated test. Always restore in an `ensure` block, not just at the end of the happy path — restoration must run even when the test body raises.

## Patterns

**Setup-based fixtures.**
```ruby
setup do
  @company = Company.create!(name: "Test Company")
  @contact = Contact.create!(name: "John Doe", company: @company)
end
```

**ENV mutation with guaranteed restore.**
```ruby
test "with custom ENV" do
  original = ENV["MY_VAR"]
  ENV["MY_VAR"] = "test value"
  # ...
ensure
  original ? ENV["MY_VAR"] = original : ENV.delete("MY_VAR")
end
```

**Dynamic config reads.**
```ruby
# GOOD — tests can override via ENV at runtime
def self.company_name = ENV.fetch("COMPANY_NAME", "Default")
```

## Anti-Patterns

- **Fixture files for test data** → hidden coupling across tests; use `setup` blocks instead
- **`CONST = ENV.fetch(...).freeze` in a service/class body** → frozen at load time, untestable; wrap in a method
- **ENV mutation without `ensure`** → leaks into parallel test runs, causes flaky failures unrelated to the actual change
- **Testing an ActiveJob without `ActiveJob::TestHelper`** → can't assert `assert_enqueued_with`/perform behavior correctly

## Related

- [[phlex-component-design]] — the ≥90% coverage bar these test patterns support
- [[ai-agent-rules]] — the "test after every change" rule this methodology implements