diff --git a/server/app/interfaces/discovery.py b/server/app/interfaces/discovery.py
index 47e202053..c9a85d97e 100644
--- a/server/app/interfaces/discovery.py
+++ b/server/app/interfaces/discovery.py
@@ -194,7 +194,14 @@ def get_all_aas_ids_by_asset_link(
             if not isinstance(item, dict):
                 raise werkzeug.exceptions.BadRequest("Each asset link must be a JSON object")
 
-            asset_link = server_model.AssetLink(item["name"], item["value"])
+            try:
+                asset_link = server_model.AssetLink(item["name"], item["value"])
+            except KeyError as e:
+                # Match the shells filter path (#511): missing name/value is a
+                # client error, not an unhandled KeyError → 500 (#672).
+                raise werkzeug.exceptions.BadRequest(
+                    f"Invalid assetId format: missing field {e}"
+                ) from e
             aas_keys = self.persistent_store.search_aas_ids_by_asset_link(asset_link)
             matching_aas_keys.update(aas_keys)
 
diff --git a/server/app/interfaces/repository.py b/server/app/interfaces/repository.py
index fc7b4b0a0..ff964c705 100644
--- a/server/app/interfaces/repository.py
+++ b/server/app/interfaces/repository.py
@@ -792,11 +792,14 @@ def post_submodel_submodel_elements_id_short_path(
         try:
             parent.add_referable(new_submodel_element)
         except model.AASConstraintViolation as e:
-            if e.constraint_id != 22:
-                raise
-            raise Conflict(
-                f"SubmodelElement with idShort {new_submodel_element.id_short} already exists within {parent}!"
-            )
+            # Constraint 22 is "idShort already taken" → 409; every other
+            # constraint (e.g. AASd-109 type mismatch in a SubmodelElementList)
+            # is invalid client input → 400 (#672).
+            if e.constraint_id == 22:
+                raise Conflict(
+                    f"SubmodelElement with idShort {new_submodel_element.id_short} already exists within {parent}!"
+                )
+            raise BadRequest(str(e)) from e
         self.object_store.commit(self._get_submodel(url_args))
         submodel = self._get_submodel(url_args)
         id_short_path = url_args.get("id_shorts", [])
diff --git a/server/test/interfaces/test_discovery_asset_ids.py b/server/test/interfaces/test_discovery_asset_ids.py
new file mode 100644
index 000000000..26cbae664
--- /dev/null
+++ b/server/test/interfaces/test_discovery_asset_ids.py
@@ -0,0 +1,33 @@
+# Copyright (c) 2026 the Eclipse BaSyx Authors
+#
+# This program and the accompanying materials are made available under the terms of the MIT License, available in
+# the LICENSE file of this project.
+#
+# SPDX-License-Identifier: MIT
+
+"""Client-error mapping for discovery ``GET /lookup/shells?assetIds=...`` (#672)."""
+
+import base64
+import json
+import unittest
+
+from app.interfaces.discovery import DiscoveryAPI, DiscoveryStore
+from werkzeug.test import Client
+
+
+def _b64url_json(obj) -> str:
+    raw = json.dumps(obj, separators=(",", ":")).encode()
+    return base64.urlsafe_b64encode(raw).decode().rstrip("=")
+
+
+class DiscoveryAssetIdsClientErrorsTest(unittest.TestCase):
+    def setUp(self) -> None:
+        self.client = Client(DiscoveryAPI(DiscoveryStore(), base_path=""))
+
+    def test_asset_ids_missing_value_returns_400(self) -> None:
+        response = self.client.get(f"/lookup/shells?assetIds={_b64url_json({'name': 'serial'})}")
+        self.assertEqual(400, response.status_code)
+
+    def test_asset_ids_missing_name_returns_400(self) -> None:
+        response = self.client.get(f"/lookup/shells?assetIds={_b64url_json({'value': '123'})}")
+        self.assertEqual(400, response.status_code)
diff --git a/server/test/interfaces/test_submodel_list_constraint.py b/server/test/interfaces/test_submodel_list_constraint.py
new file mode 100644
index 000000000..5635da4c5
--- /dev/null
+++ b/server/test/interfaces/test_submodel_list_constraint.py
@@ -0,0 +1,49 @@
+# Copyright (c) 2026 the Eclipse BaSyx Authors
+#
+# This program and the accompanying materials are made available under the terms of the MIT License, available in
+# the LICENSE file of this project.
+#
+# SPDX-License-Identifier: MIT
+
+"""SubmodelElementList constraint violations must be 400, not 500 (#672)."""
+
+import json
+import unittest
+from unittest import mock
+
+from app.interfaces import repository
+from basyx.aas import model
+from basyx.aas.adapter import aasx
+from basyx.aas.adapter.json import json_serialization
+from werkzeug.test import Client
+
+
+class SubmodelListConstraintClientErrorTest(unittest.TestCase):
+    def setUp(self) -> None:
+        self.object_store = model.DictIdentifiableStore()
+        self.file_store = mock.Mock(spec=aasx.AbstractSupplementaryFileContainer)
+        app = repository.WSGIApp(self.object_store, self.file_store, base_path="")
+        self.client = Client(app)
+
+    def test_post_property_with_wrong_value_type_returns_400(self) -> None:
+        submodel = model.Submodel("https://acplt.org/Test_Submodel_ListConstraint")
+        sml = model.SubmodelElementList(
+            "L",
+            model.Property,
+            value_type_list_element=model.datatypes.String,
+        )
+        submodel.add_referable(sml)
+        self.object_store.add(submodel)
+
+        payload = json.dumps(
+            model.Property(None, model.datatypes.Int, 1),
+            cls=json_serialization.AASToJsonEncoder,
+        )
+        from app.util.converters import base64url_encode
+
+        response = self.client.post(
+            f"/submodels/{base64url_encode(submodel.id)}/submodel-elements/L",
+            data=payload,
+            content_type="application/json",
+        )
+        self.assertEqual(400, response.status_code)