Validate a product end-to-end the way its real user uses it — a web app through the browser, a CLI through its binary, a library by writing consumer programs — black-box, deriving cases from the change's blast radius and spec acceptance criteria, and reporting every case as PASS, FAILURE, or BLOCKED without fixing anything. Use when the user says "/e2e-test", "e2e test this", "test it in the browser", "manual e2e testing", "test it as the end user", "re-test the changes comprehensively", or before a release when the next stop is prod.