diff --git a/docling_core/types/experimental/document.py b/docling_core/types/experimental/document.py
index bd4fd55..d188435 100644
--- a/docling_core/types/experimental/document.py
+++ b/docling_core/types/experimental/document.py
@@ -211,6 +211,8 @@ class NodeItem(BaseModel):
     parent: Optional[RefItem] = None
     children: List[RefItem] = []
 
+    model_config = ConfigDict(extra="forbid")
+
     def get_ref(self):
         """get_ref."""
         return RefItem(cref=self.self_ref)
diff --git a/test/data/docling_document/unit/SectionItem.yaml b/test/data/docling_document/unit/SectionItem.yaml
index c31847e..7429499 100644
--- a/test/data/docling_document/unit/SectionItem.yaml
+++ b/test/data/docling_document/unit/SectionItem.yaml
@@ -1,6 +1,6 @@
 children: []
 label: text
-level: 1
+level: 2
 orig: whatever
 parent: null
 prov: []
diff --git a/test/data/experimental/dummy_doc.yaml b/test/data/experimental/dummy_doc.yaml
index 2bd50a0..d72f454 100644
--- a/test/data/experimental/dummy_doc.yaml
+++ b/test/data/experimental/dummy_doc.yaml
@@ -36,7 +36,6 @@ texts:
   - orig: "arXiv:2206.01062v1 [cs.CV] 2 Jun 2022"
     text: "arXiv:2206.01062v1 [cs.CV] 2 Jun 2022"
     self_ref: "#/texts/0"
-    hash: 132103230
     label: "page_header"
     parent:
       $ref: "#/furniture"
@@ -52,7 +51,6 @@ texts:
   - orig: "DocLayNet: A Large Human-Annotated Dataset for\nDocument-Layout Analysis"
     text: "DocLayNet: A Large Human-Annotated Dataset for Document-Layout Analysis"
     self_ref: "#/texts/1"
-    hash: 2349732 # uint64 hash of self_ref
     label: "title"
     parent:
       $ref: "#/body"
@@ -68,7 +66,6 @@ texts:
   - orig: "OPERATION (cont.)" # nested inside the figure
     text: "OPERATION (cont.)"
     self_ref: "#/texts/2"
-    hash: 6978483
     label: "section_header"
     parent:
       $ref: "/pictures/0"
@@ -85,7 +82,6 @@ texts:
   - orig: "Figure 1: Four examples of complex page layouts across dif-\nferent document categories" # nested inside the figure
     text: "Figure 1: Four examples of complex page layouts across different document categories"
     self_ref: "#/texts/3"
-    hash: 6978483
     label: "caption"
     parent:
       $ref: "#/body"
@@ -103,7 +99,6 @@ texts:
 
 tables: # All tables...
   - self_ref: "#/table/0"
-    hash: 98574
     label: "table"
     parent:
       $ref: "#/body"
@@ -133,7 +128,6 @@ tables: # All tables...
 
 pictures: # All pictures...
   - self_ref: "#/pictures/0"
-    hash: 7782482
     label: "picture"
     parent:
       $ref: "#/body"
@@ -168,7 +162,6 @@ key_value_items: [ ] # All KV-items
 # We should consider this for pages
 pages: # Optional, for layout documents
   1:
-    hash: 6203680922337857390
     size:
       width: 768.23
       height: 583.15
diff --git a/test/test_docling_doc.py b/test/test_docling_doc.py
index eea3f1f..73f644d 100644
--- a/test/test_docling_doc.py
+++ b/test/test_docling_doc.py
@@ -1,32 +1,32 @@
+from collections import deque
+
 import pytest
 import yaml
 
-from collections import deque
-
 from docling_core.types.experimental.document import (
     BasePictureData,
     BaseTableData,
     DescriptionItem,
-    DoclingDocument,
-    TableCell,
-    NodeItem,
     DocItem,
-    TextItem,
+    DoclingDocument,
     FloatingItem,
     KeyValueItem,
-    SectionItem,
     PictureItem,
+    SectionItem,
+    TableCell,
     TableItem,
-    BasePictureData,
-    BaseTableData
+    TextItem,
 )
 from docling_core.types.experimental.labels import DocItemLabel, GroupLabel
 
+
 def test_docitems():
 
     # Iterative function to find all subclasses
     def find_all_subclasses_iterative(base_class):
-        subclasses = deque([base_class])  # Use a deque for efficient popping from the front
+        subclasses = deque(
+            [base_class]
+        )  # Use a deque for efficient popping from the front
         all_subclasses = []
 
         while subclasses:
@@ -40,57 +40,91 @@ def find_all_subclasses_iterative(base_class):
     def serialise(obj):
         return yaml.safe_dump(obj.model_dump(mode="json", by_alias=True))
 
-    def write(name:str, serialisation:str):
+    def write(name: str, serialisation: str):
         with open(f"./test/data/docling_document/unit/{name}.yaml", "w") as fw:
             fw.write(serialisation)
 
-    def read(name:str):
+    def read(name: str):
         with open(f"./test/data/docling_document/unit/{name}.yaml", "r") as fr:
             gold = fr.read()
         return gold
 
-    def generate(dc, obj):
-        write(dc.__name__, pred)
-    
     def verify(dc, obj):
-        pred = serialise(obj)            
-        #print(f"\t{dc.__name__}:\n {pred}")
+        pred = serialise(obj)
+        # print(f"\t{dc.__name__}:\n {pred}")
         gold = read(dc.__name__)
 
-        assert pred==gold, f"pred!=gold for {dc.__name__}"        
-    
+        assert pred == gold, f"pred!=gold for {dc.__name__}"
+
     # Iterate over the derived classes of the BaseClass
     derived_classes = find_all_subclasses_iterative(DocItem)
     for dc in derived_classes:
 
         if dc is TextItem:
-            obj = dc(text="whatever", orig="whatever", dloc="sdvsd", label=DocItemLabel.TEXT, self_ref="#")
+            obj = dc(
+                text="whatever",
+                orig="whatever",
+                dloc="sdvsd",
+                label=DocItemLabel.TEXT,
+                self_ref="#",
+            )
             verify(dc, obj)
-            
+
         elif dc is FloatingItem:
-            obj = dc(text="whatever", orig="whatever", dloc="sdvsd", label=DocItemLabel.TEXT, self_ref="#")
+            obj = dc(
+                text="whatever",
+                orig="whatever",
+                dloc="sdvsd",
+                label=DocItemLabel.TEXT,
+                self_ref="#",
+            )
             verify(dc, obj)
-            
+
         elif dc is KeyValueItem:
-            obj = dc(text="whatever", orig="whatever", dloc="sdvsd", label=DocItemLabel.TEXT, self_ref="#")
+            obj = dc(
+                text="whatever",
+                orig="whatever",
+                dloc="sdvsd",
+                label=DocItemLabel.TEXT,
+                self_ref="#",
+            )
             verify(dc, obj)
-            
+
         elif dc is SectionItem:
-            obj = dc(text="whatever", orig="whatever", dloc="sdvsd", label=DocItemLabel.TEXT, self_ref="#")
+            obj = dc(
+                text="whatever",
+                orig="whatever",
+                dloc="sdvsd",
+                label=DocItemLabel.TEXT,
+                self_ref="#",
+                level=2,
+            )
             verify(dc, obj)
-            
+
         elif dc is PictureItem:
-            obj = dc(text="whatever", orig="whatever", dloc="sdvsd", label=DocItemLabel.TEXT, self_ref="#",
-                     data=BasePictureData())
+            obj = dc(
+                text="whatever",
+                orig="whatever",
+                dloc="sdvsd",
+                label=DocItemLabel.TEXT,
+                self_ref="#",
+                data=BasePictureData(),
+            )
             verify(dc, obj)
-            
+
         elif dc is TableItem:
-            obj = dc(text="whatever", orig="whatever", dloc="sdvsd", label=DocItemLabel.TEXT, self_ref="#",
-                     data=BaseTableData(num_rows=3, num_cols=5, cells=[]))
+            obj = dc(
+                text="whatever",
+                orig="whatever",
+                dloc="sdvsd",
+                label=DocItemLabel.TEXT,
+                self_ref="#",
+                data=BaseTableData(num_rows=3, num_cols=5, cells=[]),
+            )
             verify(dc, obj)
-            
+
         else:
-            print(f"{dc.__name__} is not known")            
+            print(f"{dc.__name__} is not known")
             assert False, "new derived class detected {dc.__name__}: {e}"
 
 
@@ -315,4 +349,3 @@ def _construct_doc() -> DoclingDocument:
     fig_item = doc.add_picture(data=BasePictureData(), caption=fig_caption)
 
     return doc
-