Error converting zarr v3 to zarr v2: zarr 2 does not support "Serializer"

[rileyhales]

What happened?

I'm trying to convert a Zarr dataset in the Zarr format/version 3 to 2. Xarray can successfully read zarr v2 or v3. I can make an xarray dataset in memory and write to disc in either zarr v2 or v3. I seem to find this problem only when reading an existing zarr v3 and and attempting to write to a zarr v2. However i can successfully convert the opposite direction by reading an existing zarr v2 and writing to zarr v3. Given that all the other read/write scenarios that i've tried work fine, i'm guessing there's some internal config set when reading a dataset from zarr v3 which maybe doesn't exist or has an incompatible default for in zarr v2.

What did you expect to happen?

I expected this to be possible

xr.open_zarr('v3path.zarr', zarr_format=3).to_zarr('v2path.zarr', zarr_format=2)

Minimal Complete Verifiable Example

import xarray as xr
import numpy as np

# generate a random array and write it to zarr v3
arr = np.random.rand(100, 100)
(
    xr
    .DataArray(arr, dims=['x', 'y'], name='data')
    .to_zarr(
        './test-v3.zarr',
        zarr_format=3
    )
)

# now try to convert it to a zarr 2
(
    xr
    .open_zarr('./test-v3.zarr')
    .to_zarr('test-v2.zarr', zarr_format=2)
)

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

ValueError: Zarr format 2 arrays do not support `serializer`.

Anything else we need to know?

No response

Environment

This is on a google colab machine but i've also replicated this on so Mac and Windows installations

INSTALLED VERSIONS ------------------ commit: None python: 3.11.12 (main, Apr 9 2025, 08:55:54) [GCC 11.4.0] python-bits: 64 OS: Linux OS-release: 6.1.123+ machine: x86_64 processor: x86_64 byteorder: little LC_ALL: en_US.UTF-8 LANG: en_US.UTF-8 LOCALE: ('en_US', 'UTF-8') libhdf5: 1.14.6 libnetcdf: None

xarray: 2025.3.1
pandas: 2.2.2
numpy: 2.0.2
scipy: 1.15.3
netCDF4: None
pydap: None
h5netcdf: 1.6.1
h5py: 3.13.0
zarr: None
cftime: None
nc_time_axis: None
iris: None
bottleneck: 1.4.2
dask: 2024.12.1
distributed: 2024.12.1
matplotlib: 3.10.0
cartopy: None
seaborn: 0.13.2
numbagg: None
fsspec: 2025.3.2
cupy: 13.3.0
pint: None
sparse: None
flox: None
numpy_groupies: None
setuptools: 75.2.0
pip: 24.1.2
conda: None
pytest: 8.3.5
mypy: None
IPython: 7.34.0
sphinx: 8.2.3

[rileyhales]

I'm happy to make a pull request but I need explanation of what serializer is/does. The "does not support" language in the error makes me unsure if this is intended behavior or not. Either way, i feel that it should be possible to make this conversion.

[rileyhales]

Possibly related issue is #9987