An SDET is essentially a software developer who specializes in testing: they build frameworks, test tooling and CI integration, while a QA engineer usually owns the broader testing lifecycle, including test plans, exploratory testing and defect tracking. The titles overlap heavily, and companies use them inconsistently, so read the job description rather than the title. The skills that separate the roles are strong programming, framework and API design, CI/CD, and white-box thinking about the code under test. Google's own description of its roles set the pattern: the software engineer in test works on testability and the tooling that other engineers test with, while the test engineer owns test strategy and user-facing quality. Blind threads also show that at strong engineering companies, SDETs are treated as engineers on a separate track and are interviewed on coding.

A useful way to tell them apart is framework ownership versus framework consumption. An automation engineer mostly uses an existing framework to script user journeys, maintain locators and check acceptance criteria. An SDET designs and maintains the framework itself: lifecycle hooks, custom runners and retry policy, test data generators and teardown, the container images and parallel CI pipelines the suite runs on, and reviews of developers' pull requests for testability and accessibility attributes.

Sources: Google Testing Blog: How Google Tests Software, TestPro, Blind thread