What is the difference between cy.fixture() and cy.readFile()?

0-2 years experience

1. 1You're writing a test that needs a list of test users. The data lives in cypress/fixtures/users.json and never changes between runs. Which command do you reach for and why?
2. 2A test needs to read a JSON config file that sits outside the project (e.g., /tmp/test-config.json) and might change between test runs. Would you use cy.fixture() or cy.readFile()? What's the practical difference you'd see?

2-5 years experience

1. 1You're debugging a flaky test where cy.fixture('data.json') returns stale data after a previous test mutated the parsed object in memory. Walk me through why this happens and how you'd fix it without switching to cy.readFile().
2. 2Your team stores test data in a shared fixtures folder. A new feature requires reading a CSV export that the app generates at runtime into the downloads folder. The file path includes a timestamp. How do you structure this test, and why doesn't cy.fixture() work here?

5-8 years experience

1. 1In a large suite, you notice cy.fixture() calls are slowing down test startup because hundreds of JSON files are being parsed upfront. The data is only needed in a subset of specs. How would you refactor the data loading strategy to improve performance while keeping tests maintainable?
2. 2You're migrating from Cypress 9 to 13 and discover that cy.fixture() now caches differently across spec files in parallel mode. A test that worked in serial now fails because it expects a fresh object each run. How do you diagnose this and what's the minimal change to restore isolation?

8+ years experience

1. 1Your org has 50+ repos with inconsistent test data patterns — some use fixtures, some use readFile, some inline data. You're tasked with defining a cross-team test data strategy that supports parallel execution, deterministic runs, and easy onboarding. What principles do you establish, and how do you enforce them without blocking teams?
2. 2A legacy codebase has tests that mutate fixture objects directly (e.g., fixtureData.user.id = 123) causing cascade failures. You can't rewrite all tests at once. Design a migration path that introduces safe defaults, detects mutations in CI, and lets teams opt in to strict mode incrementally.