diff --git a/app/assets/api/schemas_in.py b/app/assets/api/schemas_in.py
index c5a8ae75a..636be5ced 100644
--- a/app/assets/api/schemas_in.py
+++ b/app/assets/api/schemas_in.py
@@ -95,10 +95,15 @@ class ListAssetsQuery(BaseModel):
         elif isinstance(v, list):
             raw = []
             for item in v:
-                if isinstance(item, str):
-                    raw.extend([t.strip() for t in item.split(",") if t.strip()])
+                if not isinstance(item, str):
+                    raise ValueError(
+                        f"job_ids entries must be strings, got {type(item).__name__}"
+                    )
+                raw.extend([t.strip() for t in item.split(",") if t.strip()])
         else:
-            return v
+            raise ValueError(
+                f"job_ids must be a string or list of strings, got {type(v).__name__}"
+            )
 
         out: list[str] = []
         seen: set[str] = set()
diff --git a/tests-unit/assets_test/queries/test_list_assets_query.py b/tests-unit/assets_test/queries/test_list_assets_query.py
new file mode 100644
index 000000000..e8d3430e2
--- /dev/null
+++ b/tests-unit/assets_test/queries/test_list_assets_query.py
@@ -0,0 +1,60 @@
+"""Schema-level unit tests for ListAssetsQuery (no DB required)."""
+import uuid
+
+import pytest
+from pydantic import ValidationError
+
+from app.assets.api.schemas_in import ListAssetsQuery
+
+
+class TestJobIdsValidator:
+    def test_csv_string_parses_and_canonicalizes(self):
+        a = "AAAAAAAA-BBBB-CCCC-DDDD-EEEEEEEEEEEE"
+        b = "11111111-2222-3333-4444-555555555555"
+        q = ListAssetsQuery.model_validate({"job_ids": f"{a},{b}"})
+        # Canonicalized to lowercase
+        assert q.job_ids == [a.lower(), b]
+
+    def test_repeated_query_params_as_list(self):
+        a = "11111111-1111-1111-1111-111111111111"
+        b = "22222222-2222-2222-2222-222222222222"
+        q = ListAssetsQuery.model_validate({"job_ids": [a, b]})
+        assert q.job_ids == [a, b]
+
+    def test_dedup_preserves_first_seen_order(self):
+        a = "11111111-1111-1111-1111-111111111111"
+        b = "22222222-2222-2222-2222-222222222222"
+        q = ListAssetsQuery.model_validate({"job_ids": [a, b, a]})
+        assert q.job_ids == [a, b]
+
+    def test_default_empty(self):
+        q = ListAssetsQuery.model_validate({})
+        assert q.job_ids == []
+
+    def test_invalid_uuid_rejected(self):
+        with pytest.raises(ValidationError) as exc:
+            ListAssetsQuery.model_validate({"job_ids": "not-a-uuid"})
+        assert "must be UUIDs" in str(exc.value)
+
+    def test_non_string_list_item_rejected(self):
+        with pytest.raises(ValidationError) as exc:
+            ListAssetsQuery.model_validate(
+                {"job_ids": ["11111111-1111-1111-1111-111111111111", 42]}
+            )
+        assert "must be strings" in str(exc.value)
+
+    def test_non_string_non_list_value_rejected(self):
+        with pytest.raises(ValidationError) as exc:
+            ListAssetsQuery.model_validate({"job_ids": {"bad": "shape"}})
+        assert "must be a string or list of strings" in str(exc.value)
+
+    def test_max_length_enforced(self):
+        too_many = [str(uuid.uuid4()) for _ in range(501)]
+        with pytest.raises(ValidationError) as exc:
+            ListAssetsQuery.model_validate({"job_ids": too_many})
+        assert exc.value.errors()[0]["type"] == "too_long"
+
+    def test_max_length_boundary_accepted(self):
+        at_cap = [str(uuid.uuid4()) for _ in range(500)]
+        q = ListAssetsQuery.model_validate({"job_ids": at_cap})
+        assert len(q.job_ids) == 500