diff --git a/base_operations.yaml b/base_operations.yaml
index a44ead9..fc12828 100644
--- a/base_operations.yaml
+++ b/base_operations.yaml
@@ -3348,6 +3348,12 @@ paths:
                 link_rows: {}
                 summaries: {}
                 colors: {}
+        "400":
+          description: A view with this name already exists in the table.
+          content:
+            application/json:
+              example:
+                error_msg: params invalid
   /api-gateway/api/v2/dtables/{base_uuid}/views/{view_name}/:
     get:
       tags:
@@ -3549,6 +3555,12 @@ paths:
                 link_rows: {}
                 summaries: {}
                 colors: {}
+        "400":
+          description: The target view name already exists in the table.
+          content:
+            application/json:
+              example:
+                error_msg: params invalid
     delete:
       tags:
         - Views
@@ -3702,6 +3714,11 @@ paths:
                   conditions: []
                 permission_type: ""
                 permitted_users: []
+        "400":
+          description: A column with this name already exists in the table.
+          content:
+            application/json:
+              example: Column ColA exists.
     put:
       tags:
         - Columns
@@ -3773,6 +3790,13 @@ paths:
                 data: null
                 permission_type: ""
                 permitted_users: []
+        "400":
+          description: A column with this name already exists in the table.
+          content:
+            application/json:
+              example:
+                error_type: column_already_exists
+                error_message: column name already exists
     delete:
       tags:
         - Columns
diff --git a/tests/test_columns.py b/tests/test_columns.py
index ef97c08..23ffb35 100644
--- a/tests/test_columns.py
+++ b/tests/test_columns.py
@@ -378,3 +378,67 @@ def test_deleteColumn(base: Base):
     column_names = [c['name'] for c in response.json()['columns']]
     assert 'keep' in column_names
     assert 'delete-me' not in column_names
+
+
+def test_insertColumn_duplicate_name_returns_400(base: Base):
+    table_name = 'test_insertColumn_duplicate'
+    create_table(base, table_name, [{'column_name': 'existing', 'column_type': 'text'}])
+
+    path_parameters = {'base_uuid': base.uuid}
+    headers = {'Authorization': f'Bearer {base.token}'}
+    body = {'table_name': table_name, 'column_name': 'existing', 'column_type': 'text'}
+
+    # Inserting a column whose name collides with an existing column is rejected
+    case: Case = base_operations_schema.find_operation_by_id('insertColumn') \
+        .Case(path_parameters=path_parameters, body=body, headers=headers)
+    response = case.call()
+
+    assert response.status_code == 400
+    # application/json is also enforced by the conformance hook against the documented 400
+    assert response.headers['content-type'][0].startswith('application/json')
+
+
+@pytest.mark.xfail(
+    reason="insertColumn returns the plain-text body 'Column ColA exists.' with an "
+           "application/json content-type on a duplicate-column 400 (not valid JSON)",
+)
+def test_insertColumn_duplicate_name_returns_json(base: Base):
+    table_name = 'test_insertColumn_duplicate_json'
+    create_table(base, table_name, [{'column_name': 'existing', 'column_type': 'text'}])
+
+    path_parameters = {'base_uuid': base.uuid}
+    headers = {'Authorization': f'Bearer {base.token}'}
+    body = {'table_name': table_name, 'column_name': 'existing', 'column_type': 'text'}
+
+    case: Case = base_operations_schema.find_operation_by_id('insertColumn') \
+        .Case(path_parameters=path_parameters, body=body, headers=headers)
+    response = case.call()
+
+    assert response.status_code == 400
+    # The body should be valid JSON; currently raises JSONDecodeError (plain text)
+    response.json()
+
+
+def test_updateColumn_rename_collision_returns_400(base: Base):
+    table_name = 'test_updateColumn_collision'
+    create_table(base, table_name, [
+        {'column_name': 'col-a', 'column_type': 'text'},
+        {'column_name': 'col-b', 'column_type': 'text'},
+    ])
+
+    path_parameters = {'base_uuid': base.uuid}
+    headers = {'Authorization': f'Bearer {base.token}'}
+    body = {
+        'op_type': 'rename_column',
+        'table_name': table_name,
+        'column': 'col-b',
+        'new_column_name': 'col-a',
+    }
+
+    # Renaming a column to collide with an existing column name is rejected
+    case: Case = base_operations_schema.find_operation_by_id('updateColumn') \
+        .Case(path_parameters=path_parameters, body=body, headers=headers)
+    response = case.call()
+
+    assert response.status_code == 400
+    assert response.headers['content-type'][0].startswith('application/json')
diff --git a/tests/test_views.py b/tests/test_views.py
index d02512c..f8f94f6 100644
--- a/tests/test_views.py
+++ b/tests/test_views.py
@@ -153,3 +153,54 @@ def test_deleteView(base: Base):
     assert response.status_code == 200
     view_names = [v['name'] for v in response.json()['views']]
     assert 'View To Delete' not in view_names
+
+
+def test_createView_duplicate_name_returns_400(base: Base):
+    table_name = 'test_createView_duplicate'
+    create_table(base, table_name, SIMPLE_COLUMNS)
+
+    path_parameters = {'base_uuid': base.uuid}
+    query = {'table_name': table_name}
+    headers = {'Authorization': f'Bearer {base.token}'}
+    body = {'name': 'Collision View', 'type': 'table'}
+
+    # Create the view once
+    case: Case = base_operations_schema.find_operation_by_id('createView') \
+        .Case(path_parameters=path_parameters, query=query, body=body, headers=headers)
+    response = case.call()
+    assert response.status_code == 200
+
+    # Creating a second view with the same name is rejected
+    case = base_operations_schema.find_operation_by_id('createView') \
+        .Case(path_parameters=path_parameters, query=query, body=body, headers=headers)
+    response = case.call()
+
+    assert response.status_code == 400
+    # application/json is also enforced by the conformance hook against the documented 400
+    assert response.headers['content-type'][0].startswith('application/json')
+
+
+def test_updateView_rename_collision_returns_400(base: Base):
+    table_name = 'test_updateView_collision'
+    create_table(base, table_name, SIMPLE_COLUMNS)
+
+    path_parameters = {'base_uuid': base.uuid}
+    query = {'table_name': table_name}
+    headers = {'Authorization': f'Bearer {base.token}'}
+
+    # Create a second view (the table already has a "Default View")
+    body = {'name': 'Second View', 'type': 'table'}
+    case: Case = base_operations_schema.find_operation_by_id('createView') \
+        .Case(path_parameters=path_parameters, query=query, body=body, headers=headers)
+    response = case.call()
+    assert response.status_code == 200
+
+    # Renaming it to collide with the existing "Default View" is rejected
+    rename_path = {'base_uuid': base.uuid, 'view_name': 'Second View'}
+    rename_body = {'name': 'Default View'}
+    case = base_operations_schema.find_operation_by_id('updateView') \
+        .Case(path_parameters=rename_path, query=query, body=rename_body, headers=headers)
+    response = case.call()
+
+    assert response.status_code == 400
+    assert response.headers['content-type'][0].startswith('application/json')