Fixture Architecture Playbook

Build helpers as pure functions first, then expose them via Playwright extend or Cypress commands so logic stays testable in isolation.
Compose capabilities with mergeTests (Playwright) or layered Cypress commands instead of inheritance; each fixture should solve one concern (auth, api, logs, network).
Keep HTTP helpers framework agnostic—accept all required params explicitly and return results so unit tests and runtime fixtures can share them.
Export fixtures through package subpaths ("./api-request", "./api-request/fixtures") to make reuse trivial across suites and projects.
Treat fixture files as infrastructure: document dependencies, enforce deterministic timeouts, and ban hidden retries that mask flakiness.

Source: Murat Testing Philosophy, cy-vs-pw comparison, SEON production patterns.