Pin the Zarr export to format v3 explicitly

services/exporter/exporter/handlers/grid.py

@@ -177,7 +177,11 @@ def export_zarr(
     tmp_dir = tempfile.mkdtemp()
     try:
         zarr_dir = os.path.join(tmp_dir, "export.zarr")
-        ds.to_zarr(zarr_dir)
+        # The format version is part of the export contract: users open this
+        # store with their own tooling, and zarr-python 2.x cannot read v3.
+        # Pinned rather than left to the library default so a future default
+        # flip can't silently change what users receive.
+        ds.to_zarr(zarr_dir, zarr_format=3)
 
         progress("Zipping Zarr...", 85)
         zip_path = os.path.join(tmp_dir, "export")

services/exporter/tests/handlers/test_zarr.py

@@ -0,0 +1,77 @@
+"""
+Tests for the zipped-Zarr exporter handler.
+
+Unit tests mock storage to test handler logic.
+"""
+
+import shutil
+import zipfile
+from unittest.mock import patch
+
+import numpy as np
+import rioxarray  # noqa: F401
+import xarray as xr
+import zarr
+from exporter.handlers.grid import export_zarr
+from rasterio.transform import from_bounds
+
+
+def make_test_dataset(
+    crs: str = "EPSG:32611",
+    shape: tuple[int, int] = (10, 10),
+) -> xr.Dataset:
+    """2D Dataset analogous to test_netcdf.make_test_dataset."""
+    ny, nx = shape
+    transform = from_bounds(
+        500000, 5200000, 500000 + nx * 30, 5200000 + ny * 30, nx, ny
+    )
+    ds = xr.Dataset(
+        {"fuel_load.1hr": (("y", "x"), np.random.rand(ny, nx).astype(np.float32))},
+        coords={"y": np.arange(ny), "x": np.arange(nx)},
+    )
+    ds = ds.rio.write_crs(crs)
+    ds = ds.rio.write_transform(transform)
+    return ds
+
+
+def noop_progress(message: str, percent: int | None = None) -> None:
+    pass
+
+
+class TestExportZarrUnit:
+    """Unit tests for the Zarr handler — mock storage and the GCS upload."""
+
+    @patch("google.cloud.storage.Client")
+    @patch("exporter.handlers.grid.load_grid_zarr")
+    def test_writes_zarr_format_3(self, mock_load, mock_client, tmp_path):
+        """The exported store must be Zarr format v3.
+
+        Users open this store with their own tooling and zarr-python 2.x
+        cannot read v3, so the format version is part of the export contract
+        and must not drift with the library default.
+        """
+        mock_load.return_value = make_test_dataset()
+
+        captured = {}
+
+        def capture(local_path: str) -> None:
+            dest = tmp_path / "captured.zip"
+            shutil.copy(local_path, dest)
+            captured["path"] = str(dest)
+
+        blob = mock_client.return_value.bucket.return_value.blob.return_value
+        blob.upload_from_filename.side_effect = capture
+
+        export_zarr(
+            {"id": "test-export"},
+            {"grid_id": "grid-abc", "name": "zarr"},
+            noop_progress,
+        )
+
+        extract_dir = tmp_path / "extracted"
+        with zipfile.ZipFile(captured["path"]) as zf:
+            zf.extractall(extract_dir)
+
+        group = zarr.open_group(str(extract_dir / "export.zarr"), mode="r")
+        assert group.metadata.zarr_format == 3
+        assert group["fuel_load.1hr"].metadata.zarr_format == 3