diff --git a/converters/dbt/README.md b/converters/dbt/README.md
index c39090fb..4b3ebd3e 100644
--- a/converters/dbt/README.md
+++ b/converters/dbt/README.md
@@ -119,8 +119,9 @@ manifest_json = result.output.model_dump_json(by_alias=True, exclude_none=True,
 | `NATURAL_ENTITY_DROPPED` | Ossie has no natural-key entity type |
 | `CUMULATIVE_SEMANTICS_LOSS` | Window/grain semantics cannot be expressed in an Ossie expression string; the base aggregation is preserved |
 
-**Ossie → MSI** reconstructs a best-effort MSI manifest from Ossie's simpler schema. Nothing is dropped, but Ossie carries less structural information than MSI, so the converter makes the following choices:
+**Ossie → MSI** reconstructs a best-effort MSI manifest from Ossie's simpler schema. Nothing is dropped for supported inputs, but Ossie carries less structural information than MSI, so the converter makes the following choices:
 
+- Composite primary and unique keys are rejected because MSI entities cannot preserve grouped key semantics
 - Single aggregations (`SUM(col)`, `COUNT(DISTINCT col)`, etc.) → SIMPLE metric with `metric_aggregation_params`
 - `(expr_a) / (expr_b)` → RATIO metric with auto-generated sub-metrics
 - Anything else → SIMPLE metric with the raw expression stored verbatim
diff --git a/converters/dbt/src/ossie_dbt/osi_to_msi.py b/converters/dbt/src/ossie_dbt/osi_to_msi.py
index c8f155fc..b5132889 100644
--- a/converters/dbt/src/ossie_dbt/osi_to_msi.py
+++ b/converters/dbt/src/ossie_dbt/osi_to_msi.py
@@ -154,6 +154,17 @@ def _convert_dataset(
     @staticmethod
     def _build_key_sets(dataset: OSIDataset, osi_sm: OSISemanticModel) -> _KeySets:
         """Return a _KeySets with primary, unique, and foreign key column sets for a dataset."""
+        for key_type, keys in (
+            ("primary key", [dataset.primary_key] if dataset.primary_key else []),
+            ("unique key", dataset.unique_keys or []),
+        ):
+            for key in keys:
+                if len(key) > 1:
+                    raise ValueError(
+                        f"Dataset {dataset.name!r} has composite {key_type} {key!r}; "
+                        "MetricFlow entities cannot represent composite keys losslessly"
+                    )
+
         return _KeySets(
             primary=set(dataset.primary_key or []),
             unique={col for keys in (dataset.unique_keys or []) for col in keys},
diff --git a/converters/dbt/tests/test_osi_to_msi.py b/converters/dbt/tests/test_osi_to_msi.py
index dfed1e42..77761b5e 100644
--- a/converters/dbt/tests/test_osi_to_msi.py
+++ b/converters/dbt/tests/test_osi_to_msi.py
@@ -125,6 +125,45 @@ def test_unique_key_field_becomes_unique_entity(self) -> None:
         assert sm.entities[0].name == "email"
         assert sm.entities[0].type.value == "unique"
 
+    @pytest.mark.parametrize(
+        ("primary_key", "unique_keys", "expected_error"),
+        [
+            (
+                ["tenant_id", "order_id"],
+                None,
+                "Dataset 'orders' has composite primary key ['tenant_id', 'order_id']; "
+                "MetricFlow entities cannot represent composite keys losslessly",
+            ),
+            (
+                None,
+                [["tenant_id", "external_id"]],
+                "Dataset 'orders' has composite unique key ['tenant_id', 'external_id']; "
+                "MetricFlow entities cannot represent composite keys losslessly",
+            ),
+        ],
+    )
+    def test_composite_key_is_rejected(
+        self,
+        primary_key: list[str] | None,
+        unique_keys: list[list[str]] | None,
+        expected_error: str,
+    ) -> None:
+        doc = _osi_doc(
+            datasets=[
+                _osi_dataset(
+                    "orders",
+                    fields=[_osi_field("tenant_id"), _osi_field("order_id")],
+                    primary_key=primary_key,
+                    unique_keys=unique_keys,
+                )
+            ]
+        )
+
+        with pytest.raises(ValueError) as exc_info:
+            OSIToMSIConverter().convert(doc)
+
+        assert str(exc_info.value) == expected_error
+
     def test_relationship_from_column_becomes_foreign_entity(self) -> None:
         doc = _osi_doc(
             datasets=[