diff --git a/pangeo_forge_recipes/transforms.py b/pangeo_forge_recipes/transforms.py
index 226025ec..50b9d409 100644
--- a/pangeo_forge_recipes/transforms.py
+++ b/pangeo_forge_recipes/transforms.py
@@ -4,7 +4,7 @@
 import random
 import sys
 from dataclasses import dataclass, field
-from typing import Callable, Dict, Iterable, Iterator, List, Optional, Tuple, TypeVar
+from typing import Callable, Dict, Iterable, Iterator, List, Optional, Tuple, TypeVar, Union
 
 # PEP612 Concatenate & ParamSpec are useful for annotating decorators, but their import
 # differs between Python versions 3.9 & 3.10. See: https://stackoverflow.com/a/71990006
@@ -69,6 +69,19 @@
 P = ParamSpec("P")
 
 
+class RequiredAtRuntimeDefault:
+    """Sentinel class to use as default for transform attributes which are required to run a
+    pipeline, but may not be available (or preferable) to define during recipe develoment; for
+    example, the ``target_root`` kwarg of a transform that writes data to a target location. By
+    using this sentinel as the default value for such an kwarg, a recipe module can define all
+    required arguments on the transform (and therefore be importable, satisfy type-checkers, be
+    unit-testable, etc.) before it is deployed, with the understanding that the attribute using
+    this sentinel as default will be re-assigned to the desired value at deploy time.
+    """
+
+    pass
+
+
 # TODO: replace with beam.MapTuple?
 def _add_keys(
     func: Callable[Concatenate[T, P], R],
@@ -434,9 +447,17 @@ def expand(self, references: beam.PCollection) -> beam.PCollection:
 class WriteCombinedReference(beam.PTransform, ZarrWriterMixin):
     """Store a singleton PCollection consisting of a ``kerchunk.combine.MultiZarrToZarr`` object.
 
+    :param store_name: Name for the Zarr store. It will be created with
+        this name under `target_root`.
+    :param target_root: Root path the Zarr store will be created inside;
+        `store_name` will be appended to this prefix to create a full path.
     :param output_json_fname: Name to give the output references file. Must end in ``.json``.
     """
 
+    store_name: str
+    target_root: Union[str, FSSpecTarget, RequiredAtRuntimeDefault] = field(
+        default_factory=RequiredAtRuntimeDefault
+    )
     output_json_fname: str = "reference.json"
 
     def expand(self, reference: beam.PCollection) -> beam.PCollection:
@@ -452,6 +473,10 @@ class StoreToZarr(beam.PTransform, ZarrWriterMixin):
     """Store a PCollection of Xarray datasets to Zarr.
 
     :param combine_dims: The dimensions to combine
+    :param store_name: Name for the Zarr store. It will be created with
+        this name under `target_root`.
+    :param target_root: Root path the Zarr store will be created inside;
+        `store_name` will be appended to this prefix to create a full path.
     :param target_chunks: Dictionary mapping dimension names to chunks sizes.
         If a dimension is a not named, the chunks will be inferred from the data.
     """
@@ -459,6 +484,10 @@ class StoreToZarr(beam.PTransform, ZarrWriterMixin):
     # TODO: make it so we don't have to explicitly specify combine_dims
     # Could be inferred from the pattern instead
     combine_dims: List[Dimension]
+    store_name: str
+    target_root: Union[str, FSSpecTarget, RequiredAtRuntimeDefault] = field(
+        default_factory=RequiredAtRuntimeDefault
+    )
     target_chunks: Dict[str, int] = field(default_factory=dict)
 
     def expand(
diff --git a/pangeo_forge_recipes/writers.py b/pangeo_forge_recipes/writers.py
index 71ae025f..4ce5b01e 100644
--- a/pangeo_forge_recipes/writers.py
+++ b/pangeo_forge_recipes/writers.py
@@ -1,6 +1,5 @@
 import os
-from dataclasses import dataclass
-from typing import Tuple, Union
+from typing import Protocol, Tuple, Union
 
 import numpy as np
 import xarray as xr
@@ -114,22 +113,27 @@ def write_combined_reference(
         raise NotImplementedError(f"{file_ext = } not supported.")
 
 
-@dataclass
-class ZarrWriterMixin:
-    """Defines common attributes and methods for storing zarr datasets, which can be either actual
-    zarr stores or virtual (i.e. kerchunked) stores. This class should not be directly instantiated.
-    Instead, PTransforms in the `.transforms` module which write consolidated zarr stores should
-    inherit from this mixin, so that they share a common interface for target store naming.
-
-    :param target_root: Location the Zarr store will be created inside.
-    :param store_name: Name for the Zarr store. It will be created with this name
-                       under `target_root`.
+class ZarrWriterProtocol(Protocol):
+    """Protocol for mixin typing, following best practices described in:
+    https://mypy.readthedocs.io/en/stable/more_types.html#mixin-classes.
+    When used as a type hint for the `self` argument on mixin classes, this protocol just tells type
+    checkers that the given method is expected to be called in the context of a class which defines
+    the attributes declared here. This satisfies type checkers without the need to define these
+    attributes more than once in an inheritance heirarchy.
     """
 
-    target_root: Union[str, FSSpecTarget]
     store_name: str
+    target_root: Union[str, FSSpecTarget]
+
+
+class ZarrWriterMixin:
+    """Defines common methods relevant to storing zarr datasets, which can be either actual zarr
+    stores or virtual (i.e. kerchunked) stores. This class should not be directly instantiated.
+    Instead, PTransforms in the `.transforms` module which write zarr stores should inherit from
+    this mixin, so that they share a common interface for target store naming.
+    """
 
-    def get_full_target(self) -> FSSpecTarget:
+    def get_full_target(self: ZarrWriterProtocol) -> FSSpecTarget:
         if isinstance(self.target_root, str):
             target_root = FSSpecTarget.from_url(self.target_root)
         else:
diff --git a/tests/test_transforms.py b/tests/test_transforms.py
index 296e5786..e40f77c1 100644
--- a/tests/test_transforms.py
+++ b/tests/test_transforms.py
@@ -264,3 +264,17 @@ def _is_xr_dataset(actual):
         )
         open_store = target_store | OpenZarrStore()
         assert_that(open_store, is_xrdataset())
+
+
+def test_StoreToZarr_target_root_default_unrunnable(
+    pipeline,
+    netcdf_local_file_pattern_sequential,
+):
+    pattern: FilePattern = netcdf_local_file_pattern_sequential
+    with pytest.raises(TypeError, match=r"unsupported operand"):
+        with pipeline as p:
+            datasets = p | beam.Create(pattern.items()) | OpenWithXarray()
+            _ = datasets | StoreToZarr(
+                store_name="test.zarr",
+                combine_dims=pattern.combine_dim_keys,
+            )