diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index c419f2e17..89030d22e 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -6,6 +6,19 @@ on:
   pull_request:
     branches:
     - '*'
+  # Nightly cron and a manual trigger so the full corpus (slow lane
+  # included) runs at least once a day. PR runs stay on the fast lane
+  # via `-m "not slow"`; this job has no such filter and exercises
+  # every fixture in the golden corpus, including the heavier
+  # compression cells. See issue #1930 for the fast / slow split.
+  #
+  # GitHub Actions only fires `schedule` triggers on the workflow file
+  # in the default branch. The cron will not run from a feature branch
+  # or PR head -- use `workflow_dispatch` below for an on-demand run.
+  schedule:
+    # 03:00 UTC daily. Off-peak to avoid contention with weekday PRs.
+    - cron: '0 3 * * *'
+  workflow_dispatch:
 
 jobs:
   run:
@@ -27,5 +40,13 @@ jobs:
       run: |
         python -m pip install --upgrade pip
         pip install -e .[tests]
-    - name: Run pytest
+    - name: Run pytest (fast lane)
+      # PR triggers run the fast lane: `-m "not slow"` deselects the
+      # heavier corpus cells tagged via `_marks.fast_slow_marks_for`
+      # (today: the six compression fixtures). push-to-main and the
+      # nightly schedule run the full set with no filter.
+      if: github.event_name == 'pull_request'
+      run: pytest -m "not slow"
+    - name: Run pytest (full)
+      if: github.event_name != 'pull_request'
       run: pytest
diff --git a/xrspatial/geotiff/tests/golden_corpus/test_corpus_determinism.py b/xrspatial/geotiff/tests/golden_corpus/test_corpus_determinism.py
new file mode 100644
index 000000000..a29f7c216
--- /dev/null
+++ b/xrspatial/geotiff/tests/golden_corpus/test_corpus_determinism.py
@@ -0,0 +1,172 @@
+"""Corpus determinism gate (issue #1930).
+
+The golden corpus generator is built to be byte-deterministic: fixed
+seeds, sorted iteration, and an explicit ``os.utime`` pass that pins
+file mtimes to a constant epoch. This test guards that property in CI
+so a regression in the generator (or a manually-edited fixture on
+disk) fails the build instead of silently drifting.
+
+What this catches:
+
+* a generator-side change that flips RNG ordering, drops the mtime
+  normalisation, or otherwise breaks reproducibility -- the
+  regenerated bytes diverge from the committed bytes;
+* a fixture-on-disk drift where the manifest still says X but the
+  committed ``.tif`` was edited (or stale) so it no longer matches
+  what the manifest would produce.
+
+The test is fast (regenerating the 30-fixture corpus measured ~0.3s
+locally) and carries the ``fast`` tag in spirit: it is parameterised
+over every manifest entry but each parameter is a single md5
+compare, so the whole module runs comfortably inside the PR fast
+lane. We do not attach the ``slow`` pytest marker so
+``pytest -m "not slow"`` keeps it in scope.
+
+Fixtures the manifest declares but that are not committed on disk
+(today: ``example_tiled_uint16_deflate_pred2``, kept as a
+schema-illustrating example) are skipped here rather than failing,
+mirroring how the per-backend tests handle the same case.
+"""
+from __future__ import annotations
+
+import hashlib
+import pathlib
+
+import pytest
+
+# rasterio / pyyaml are runtime deps of the generator. importorskip
+# keeps minimal environments green by skipping the whole module when
+# either is missing.
+pytest.importorskip("yaml")
+pytest.importorskip("rasterio")
+
+from xrspatial.geotiff.tests.golden_corpus import generate  # noqa: E402
+
+
+FIXTURES_DIR = (
+    pathlib.Path(__file__).resolve().parent / "fixtures"
+)
+
+
+def _md5(path: pathlib.Path) -> str:
+    # ``usedforsecurity=False`` (Python 3.9+) keeps this working on
+    # FIPS-strict runners where ``hashlib.md5()`` otherwise raises a
+    # ValueError. Byte-identity comparison only, no security claim.
+    h = hashlib.md5(usedforsecurity=False)
+    with path.open("rb") as f:
+        for chunk in iter(lambda: f.read(65536), b""):
+            h.update(chunk)
+    return h.hexdigest()
+
+
+def _load_entries() -> list[dict]:
+    """Return validated manifest entries (defaults merged), sorted by id."""
+    return sorted(generate.validate(generate.load_manifest()), key=lambda e: e["id"])
+
+
+# Cached at import so parametrize collection and the orphan-file test
+# share one manifest load. Each call to ``validate()`` re-walks every
+# entry, so collapsing the two callers cuts validation work in half.
+_ENTRIES = _load_entries()
+_MANIFEST_IDS = [e["id"] for e in _ENTRIES]
+_EXTERNAL_OVR_IDS = [e["id"] for e in _ENTRIES if e.get("external_overview")]
+
+
+@pytest.fixture(scope="module")
+def regenerated_dir(tmp_path_factory: pytest.TempPathFactory) -> pathlib.Path:
+    """Regenerate the entire corpus into a module-scoped tmp dir.
+
+    Module-scoped so the (few-second) write cost is paid once per
+    test session rather than per parametrised case.
+    """
+    out = tmp_path_factory.mktemp("regen_corpus_1930")
+    generate.generate(output_dir=out)
+    return out
+
+
+@pytest.mark.parametrize("fixture_id", _MANIFEST_IDS)
+def test_fixture_bytes_are_deterministic(
+    fixture_id: str, regenerated_dir: pathlib.Path
+) -> None:
+    """The committed ``.tif`` for each manifest id matches what the
+    generator would produce today, byte for byte.
+
+    Skip rather than fail when the committed file is missing -- that
+    means the fixture is declared but intentionally not shipped
+    (e.g. the schema-illustrating example fixture). The per-backend
+    tests handle this the same way.
+    """
+    committed = FIXTURES_DIR / f"{fixture_id}.tif"
+    if not committed.exists():
+        pytest.skip(
+            f"fixture {fixture_id!r} is in the manifest but not committed "
+            f"on disk; nothing to compare"
+        )
+    regenerated = regenerated_dir / f"{fixture_id}.tif"
+    assert regenerated.exists(), (
+        f"generator did not produce {fixture_id!r}; check the generator "
+        f"and manifest stayed in sync"
+    )
+    committed_md5 = _md5(committed)
+    regenerated_md5 = _md5(regenerated)
+    assert committed_md5 == regenerated_md5, (
+        f"fixture {fixture_id!r} drifted: committed md5 {committed_md5} "
+        f"does not match regenerated md5 {regenerated_md5}. Either the "
+        f"generator changed and the committed fixtures need re-running "
+        f"(`python -m xrspatial.geotiff.tests.golden_corpus.generate`), "
+        f"or the committed fixture was edited out of band."
+    )
+
+
+@pytest.mark.parametrize("fixture_id", _EXTERNAL_OVR_IDS or ["__none__"])
+def test_external_overview_sidecar_is_deterministic(
+    fixture_id: str, regenerated_dir: pathlib.Path
+) -> None:
+    """Fixtures with ``external_overview: true`` ship a sidecar
+    ``<id>.tif.ovr`` next to the main ``.tif``. The sidecar bytes are
+    part of the determinism contract too.
+
+    Iterates over every manifest entry with ``external_overview=True``
+    so a future fixture lands in this test automatically without a
+    code change. When no such entry exists today the placeholder id
+    short-circuits to a skip so pytest still reports a single
+    informative case.
+    """
+    if fixture_id == "__none__":
+        pytest.skip("manifest has no external_overview fixtures today")
+    sidecar_name = f"{fixture_id}.tif.ovr"
+    committed = FIXTURES_DIR / sidecar_name
+    if not committed.exists():
+        pytest.skip(
+            f"sidecar {sidecar_name!r} is not committed; nothing to compare"
+        )
+    regenerated = regenerated_dir / sidecar_name
+    assert regenerated.exists(), (
+        f"generator did not produce {sidecar_name!r}; external_overview "
+        f"path may be broken"
+    )
+    assert _md5(committed) == _md5(regenerated), (
+        f"{sidecar_name!r} drifted from the committed bytes; rerun the "
+        f"generator and recommit, or revert the on-disk edit"
+    )
+
+
+def test_no_orphan_fixtures_on_disk() -> None:
+    """Every committed ``.tif`` (and ``.tif.ovr`` sidecar) corresponds
+    to a manifest entry. Catches stale fixtures left behind after a
+    manifest delete.
+    """
+    manifest_ids = set(_MANIFEST_IDS)
+    orphans: list[str] = []
+    for path in sorted(FIXTURES_DIR.glob("*.tif")):
+        if path.stem not in manifest_ids:
+            orphans.append(path.name)
+    for path in sorted(FIXTURES_DIR.glob("*.tif.ovr")):
+        # sidecar stem: strip ``.tif`` to recover the fixture id
+        fid = path.name[: -len(".tif.ovr")]
+        if fid not in manifest_ids:
+            orphans.append(path.name)
+    assert not orphans, (
+        f"committed fixtures {orphans!r} have no matching manifest entry; "
+        f"either re-add them to the manifest or remove the orphan files"
+    )