Import AGENTS.md from CLAUDE.md and use pixi in the agent instructions

AGENTS.md

@@ -2,22 +2,26 @@
 
 ## Setup
 
-```bash
-uv sync
-```
+Development environments are managed with [pixi](https://pixi.sh), the same ones
+CI uses (see `[environments]` in `pixi.toml`). They are installed on first use.
 
 ## Run tests
 
 ```bash
-uv run pytest xarray -n auto  # All tests in parallel
-uv run pytest xarray/tests/test_dataarray.py  # Specific file
+pixi run -e test-py313 -- python -m pytest xarray -n auto  # All tests in parallel
+pixi run -e test-py313 -- python -m pytest xarray/tests/test_dataarray.py  # Specific file
 ```
 
+Environments like `test-py314-no-dask` or `test-py312-bare-minimum` run the tests
+without some optional dependencies.
+
+See `xarray/tests/AGENTS.md` for guidelines on writing tests.
+
 ## Linting & type checking
 
 ```bash
-pre-commit run --all-files  # Includes ruff and other checks
-uv run dmypy run  # Type checking with mypy
+pixi exec pre-commit run --all-files  # Includes ruff and other checks
+pixi run -e test-py314-with-typing -- python -m mypy  # Type checking with mypy
 ```
 
 ## Code Style Guidelines
@@ -32,6 +36,11 @@ uv run dmypy run  # Type checking with mypy
   2. Related third-party imports
   3. Local application/library specific imports
 
+### Type hints
+
+- Always add type hints to every function and method you create or edit,
+  including tests (`-> None`), fixtures and helpers
+
 ## CI
 
 - The `test-py314 (flaky)` job runs tests that depend on external services

CLAUDE.md

@@ -0,0 +1 @@
+@AGENTS.md

xarray/tests/AGENTS.md

@@ -0,0 +1,104 @@
+# Testing Guidelines for xarray
+
+## Handling Optional Dependencies
+
+xarray has many optional dependencies that may not be available in all testing environments. Always use the standard decorators and patterns when writing tests that require specific dependencies.
+
+### Standard Decorators
+
+**ALWAYS use decorators** like `@requires_dask`, `@requires_cftime`, etc. instead of conditional `if` statements.
+
+All available decorators are defined in `xarray/tests/__init__.py` (look for `requires_*` decorators).
+
+Use the dask helpers from `xarray.tests` instead of importing `dask.array`
+directly. In dask-array expression test mode those helpers point at the
+registered chunk manager, so tests can run against either implementation.
+
+### Skip at collection time, not at runtime
+
+❌ **WRONG - Do not check for a dependency inside the test:**
+
+```python
+def test_mean() -> None:
+    if has_dask:  # WRONG!
+        ds = ds.chunk({})
+    result = ds.mean()
+```
+
+❌ **ALSO WRONG - Avoid pytest.mark.skipif in parametrize:**
+
+```python
+@pytest.mark.parametrize(
+    "chunk",
+    [
+        pytest.param(
+            True, marks=pytest.mark.skipif(not has_dask, reason="requires dask")
+        ),
+        False,
+    ],
+)
+def test_something(chunk: bool) -> None: ...
+```
+
+✅ **CORRECT - Parametrize, e.g. with the `use_dask` fixture:** it runs the test
+once with `False` (id `numpy`) and once with `True` (id `dask`), and the dask case
+is skipped at collection time if dask is not installed. Sharing the test body
+avoids duplicate code and checks that numpy and dask give the same result:
+
+```python
+def test_mean(use_dask: bool) -> None:
+    ds = create_test_data()
+    if use_dask:
+        ds = ds.chunk()
+    result = ds.mean()
+```
+
+For other optional dependencies, mark the parameters that need them:
+
+```python
+@pytest.mark.parametrize(
+    "calendar", ["standard", pytest.param("noleap", marks=requires_cftime)]
+)
+def test_something(calendar: str) -> None: ...
+```
+
+Only write a separate test, decorated with e.g. `@requires_dask`, if it checks
+something specific to that dependency, e.g. that the result stays lazy.
+
+### Fixtures
+
+Shared fixtures are defined in `xarray/tests/conftest.py`, e.g. `use_dask`,
+`compute_backend`, `use_cftime` and `time_unit`, and test data like `ds`, `da` and
+`simple_datatree`. Check there before writing your own fixture or parametrization.
+
+### Multiple dependencies
+
+When a test requires multiple optional dependencies:
+
+```python
+@requires_dask
+@requires_scipy
+def test_interpolation_with_dask() -> None: ...
+```
+
+### Importing optional dependencies in tests
+
+Skip tests at collection time with the `requires_*` decorators (or
+`pytest.param(..., marks=requires_*)` in a parametrization), not at runtime with
+`pytest.importorskip`. Import the optional dependency inside the decorated test:
+
+```python
+@requires_cftime
+def test_cftime_functionality() -> None:
+    import cftime
+
+    ...
+```
+
+### Key Points
+
+- CI environments intentionally exclude certain dependencies (e.g.,
+  `test-py314-no-dask`, `test-py312-bare-minimum`)
+- A test failing in `test-py314-no-dask` because it uses dask is a test bug, not a
+  CI issue
+- Look at similar existing tests for patterns to follow

xarray/tests/CLAUDE.md

@@ -1,136 +1 @@
-# Testing Guidelines for xarray
-
-## Handling Optional Dependencies
-
-xarray has many optional dependencies that may not be available in all testing environments. Always use the standard decorators and patterns when writing tests that require specific dependencies.
-
-### Standard Decorators
-
-**ALWAYS use decorators** like `@requires_dask`, `@requires_cftime`, etc. instead of conditional `if` statements.
-
-All available decorators are defined in `xarray/tests/__init__.py` (look for `requires_*` decorators).
-
-Use the dask helpers from `xarray.tests` instead of importing `dask.array`
-directly. In dask-array expression test mode those helpers point at the
-registered chunk manager, so tests can run against either implementation.
-
-### DO NOT use conditional imports or skipif
-
-❌ **WRONG - Do not do this:**
-
-```python
-def test_mean_with_cftime():
-    if has_dask:  # WRONG!
-        ds = ds.chunk({})
-        result = ds.mean()
-```
-
-❌ **ALSO WRONG - Avoid pytest.mark.skipif in parametrize:**
-
-```python
-@pytest.mark.parametrize(
-    "chunk",
-    [
-        pytest.param(
-            True, marks=pytest.mark.skipif(not has_dask, reason="requires dask")
-        ),
-        False,
-    ],
-)
-def test_something(chunk): ...
-```
-
-✅ **CORRECT - Do this instead:**
-
-```python
-def test_mean_with_cftime():
-    # Test without dask
-    result = ds.mean()
-
-
-@requires_dask
-def test_mean_with_cftime_dask():
-    # Separate test for dask functionality
-    ds = ds.chunk({})
-    result = ds.mean()
-```
-
-✅ **OR for parametrized tests, split them:**
-
-```python
-def test_something_without_dask():
-    # Test the False case
-    ...
-
-
-@requires_dask
-def test_something_with_dask():
-    # Test the True case with dask
-    ...
-```
-
-### Multiple dependencies
-
-When a test requires multiple optional dependencies:
-
-```python
-@requires_dask
-@requires_scipy
-def test_interpolation_with_dask(): ...
-```
-
-### Importing optional dependencies in tests
-
-For imports within test functions, use `pytest.importorskip`:
-
-```python
-def test_cftime_functionality():
-    cftime = pytest.importorskip("cftime")
-    # Now use cftime
-```
-
-### Common patterns
-
-1. **Split tests by dependency** - Don't mix optional dependency code with base functionality:
-
-   ```python
-   def test_base_functionality():
-       # Core test without optional deps
-       result = ds.mean()
-       assert result is not None
-
-
-   @requires_dask
-   def test_dask_functionality():
-       # Dask-specific test
-       ds_chunked = ds.chunk({})
-       result = ds_chunked.mean()
-       assert result is not None
-   ```
-
-2. **Use fixtures for dependency-specific setup**:
-
-   ```python
-   @pytest.fixture
-   def dask_array():
-       pytest.importorskip("dask.array")
-       import dask.array as da
-
-       return da.from_array([1, 2, 3], chunks=2)
-   ```
-
-3. **Check available implementations**:
-
-   ```python
-   from xarray.core.duck_array_ops import available_implementations
-
-
-   @pytest.mark.parametrize("implementation", available_implementations())
-   def test_with_available_backends(implementation): ...
-   ```
-
-### Key Points
-
-- CI environments intentionally exclude certain dependencies (e.g., `all-but-dask`, `bare-minimum`)
-- A test failing in "all-but-dask" because it uses dask is a test bug, not a CI issue
-- Look at similar existing tests for patterns to follow
+@AGENTS.md