How to run fewer tests and speed up CI with CircleCI Smarter Testing

CircleCI’s Smarter Testing aims to shorten CI feedback cycles by using test impact analysis to run only tests affected by code changes, automatically retrying failed tests to reduce disruption from flakes, and dynamically balancing tests across parallel nodes. Configured through the `circleci testsuite` CLI extension, the system maps source-file coverage to independently executable test units and supports runners including Jest, Vitest, Mocha, Cypress, pytest, RSpec, and Go. The tutorial demonstrates setup for a TypeScript application with Jest unit and integration suites, requiring JUnit XML reports, suite definitions, coverage plugins, and configuration of files that should trigger full test runs. After a complete default-branch run establishes the impact map, feature branches can execute only relevant tests while still catching regressions; in the example, a changed error message triggered the integration test that detected it while unrelated tests were skipped. Reported results showed unit-test time falling from 16.4 to 6.4 seconds for one-file changes and overall executor time per pull request declining by about 30%, while auto rerun retried a flaky test file in seconds rather than requiring a full job rerun.

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