diff --git a/dataclass_compat.py b/dataclass_compat.py
new file mode 100644
index 0000000..ccee120
--- /dev/null
+++ b/dataclass_compat.py
@@ -0,0 +1,123 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from enum import IntEnum, Enum
+import sys
+from typing import Any, Optional, Union, Mapping, Sequence
+
+JSONLike = Union[None, bool, int, float, str, Mapping[str, Any], Sequence[Any]]
+
+
+# @todo decide whether the django string choices are in fact light weight enough. Maybe it's only the action models that need a dataclass equivalent.
+class OutcomeSeverity(IntEnum):
+    EXECUTED = 1
+    PASSED = 2
+    WARNING = 3
+    ERROR = 4
+    NOT_APPLICABLE = 0
+
+
+class ValidationOutcomeCode(str, Enum):
+    # passed / N/A
+    PASSED = "P00010"
+    NOT_APPLICABLE = "N00010"
+
+    # errors
+    SYNTAX_ERROR = "E00001"
+    SCHEMA_ERROR = "E00002"
+    TYPE_ERROR = "E00010"
+    VALUE_ERROR = "E00020"
+    GEOMETRY_ERROR = "E00030"
+    CARDINALITY_ERROR = "E00040"
+    DUPLICATE_ERROR = "E00050"
+    PLACEMENT_ERROR = "E00060"
+    UNITS_ERROR = "E00070"
+    QUANTITY_ERROR = "E00080"
+    ENUMERATED_VALUE_ERROR = "E00090"
+    RELATIONSHIP_ERROR = "E00100"
+    NAMING_ERROR = "E00110"
+    REFERENCE_ERROR = "E00120"
+    RESOURCE_ERROR = "E00130"
+    DEPRECATION_ERROR = "E00140"
+    SHAPE_REPRESENTATION_ERROR = "E00150"
+    INSTANCE_STRUCTURE_ERROR = "E00160"
+
+    # warnings / executed
+    ALIGNMENT_CONTAINS_BUSINESS_LOGIC_ONLY = "W00010"
+    ALIGNMENT_CONTAINS_GEOMETRY_ONLY = "W00020"
+    WARNING = "W00030"
+    EXECUTED = "X00040"
+
+
+SeverityLike = Union[int, OutcomeSeverity]
+
+
+class FrozenDict(frozenset):
+    def __repr__(self):
+        return repr(dict(self))
+
+
+def freeze(obj):
+    """
+    Recursively convert dict-like structures into FrozenDict (immutable).
+    Lists and tuples are converted to tuples.
+    Strings are interned (todo benchmark)
+    """
+    if isinstance(obj, Mapping):
+        return FrozenDict((k, freeze(v)) for k, v in obj.items())
+    elif isinstance(obj, (list, tuple)):
+        return tuple(freeze(v) for v in obj)
+    elif isinstance(obj, (set, frozenset)):
+        return frozenset(freeze(v) for v in obj)
+    elif isinstance(obj, str):
+        return sys.intern(obj)
+    else:
+        return obj
+
+
+def unfreeze(obj):
+    """
+    Recursively convert MappingProxyType (or mapping-like) objects back to dicts.
+    Don't care about frozenset/tuples, just dicts for json serializability,
+    we don't actually want to mutate
+    """
+    if isinstance(obj, FrozenDict):
+        return {k: unfreeze(v) for k, v in obj}
+    else:
+        return obj
+
+
+
+@dataclass(slots=True, kw_only=True, frozen=True, eq=True)
+class ValidationOutcome:
+    """A memory-lean DTO equivalent of ValidationOutcome."""
+
+    inst: Optional[int] = None
+    feature: Optional[str] = None
+    feature_version: Optional[int] = None
+    severity: SeverityLike
+    outcome_code: ValidationOutcomeCode = None
+    expected: Optional[JSONLike] = None
+    observed: Optional[JSONLike] = None
+
+    def to_dict(
+        self, validation_task_public_id: Optional[str] = None
+    ) -> dict[str, Any]:
+        return {
+            "inst": self.inst,
+            "validation_task_id": validation_task_public_id,
+            "feature": self.feature,
+            "feature_version": self.feature_version,
+            "severity": int(self.severity),
+            "outcome_code": str(self.outcome_code),
+            "expected": unfreeze(self.expected),
+            "observed": unfreeze(self.observed),
+        }
+    
+    def __post_init__(self):
+        # convert all dicts to MappingProxyType for immutability
+        object.__setattr__(self, 'expected', freeze(self.expected))
+        object.__setattr__(self, 'observed', freeze(self.observed))
+        # intern strings for mem reduction (todo benchmark)
+        if self.feature:
+            object.__setattr__(self, 'feature', sys.intern(self.feature))
diff --git a/models.py b/models.py
index ff02cc1..33dd517 100644
--- a/models.py
+++ b/models.py
@@ -1297,7 +1297,7 @@ def __str__(self):
             "Expected": self.expected,
             "Observed": self.observed,
         }
-        return f" ".join(f"{k}={v}" for k, v in members.items() if v)
+        return f' '.join(f'{k}={repr(v)}' for k, v in members.items() if v is not None)
 
     def to_dict(self):
         return {