# 📦 WP-150 — `refine(tests): rescope seed-driven assertions as tier-D determinism`

> Engineering log · [Phase 13 — Contributor admission, augmentation guard and keypoint parity, v0.6.0](https://borda.github.io/lucid-YOLO/engineering/phase-13-contributor-admission/)
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> 🔗 Depends on [WP-149](https://borda.github.io/lucid-YOLO/work_packages/wp-149/) · Needed by [WP-153](https://borda.github.io/lucid-YOLO/work_packages/wp-153/)
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> ⬅️ [WP-149](https://borda.github.io/lucid-YOLO/work_packages/wp-149/) · [Roadmap](https://borda.github.io/lucid-YOLO/ROADMAP/) · [WP-151](https://borda.github.io/lucid-YOLO/work_packages/wp-151/) ➡️


## WP-150 — the tier that is allowed to move

<a id="wp-150"></a>

Six determinism assertions were scattered across five test files, each sitting beside the behaviour assertions for its own transform: `TestDeterminism` in `test_affine.py`, `test_fused_warp.py` and `test_mosaic.py`, `TestMixupDeterminism` and `TestCopyPasteDeterminism` in `test_mixup_copypaste.py`, and two seeded cases inside `test_photometric.py`'s jitter and flip classes. All six now live in `tests/data/test_aug_determinism.py`, labelled tier D.

**The move is not tidying. It is about what a failure will mean.** Every assertion in the file is keyed to a *seed*, and a seed pins whichever sampler happens to be installed. A replacement engine will not draw the same numbers from the same seed — different call order, different distributions, different consumption of the stream — so these values moving at an implementation swap is expected and says nothing at all about correctness. Tiers A, B and C moving at a swap is a defect. Those are opposite verdicts from the same red test run, and the only thing that tells them apart is which file the failure is in.

**Mixed in, the two verdicts share a keystroke.** A tier-D failure at swap time reads as a regression, and the natural response — re-freeze it, the values were always going to move — is exactly the response that would silently destroy a tier-B guard sitting three classes above it in the same file. Separated, the response follows from the filename. This is the same argument the plan makes for never re-freezing an expectation in the same commit as a call-site swap, applied one level up: keep the thing that is allowed to move away from the thing that is not.

**Two counterparts were added rather than only moved.** Tier D is a two-sided property — same seed gives the same output, *different* seeds give different output — and only the first half existed. The second half is what catches a transform that ignores its generator and reads the global RNG: such a transform passes the equal-seeds case whenever the global state happens to line up, which under a seeded test suite is most of the time. That is not hypothetical here; [WP-079](https://borda.github.io/lucid-YOLO/work_packages/wp-079/) was opened by an augmentation RNG collapse. The flip case gained a related assertion, that sixteen draws at `p=0.5` contain both outcomes, so a trigger stuck on one branch fails rather than agreeing with itself.

**`test_letterbox.py`'s `TestDeterminism` stayed where it is, and that is the useful distinction.** It asserts that two calls on the same input return the same output — purity, not reproducibility. `Letterbox` has no generator, nothing about it is keyed to a seed, and it will not be re-frozen at any swap. A rescope that swept every class named `TestDeterminism` into the tier-D file would have moved it, and the name is the only thing the two have in common.

No value in the tier-D file appears in `goldens/`, and none should: reproducibility is a property of a run rather than a number worth carrying across releases.