diff --git a/scanpipe/pipes/codebase.py b/scanpipe/pipes/codebase.py
index 8f1bc61b30..ea26f7e371 100644
--- a/scanpipe/pipes/codebase.py
+++ b/scanpipe/pipes/codebase.py
@@ -34,7 +34,7 @@ def get_resource_fields(resource, fields):
     return {field: getattr(resource, field) for field in fields}
 
 
-def get_resource_tree(resource, fields, codebase=None, seen_resources=set()):
+def get_resource_tree(resource, fields, codebase=None, seen_resources=None):
     """
     Return a tree as a dictionary structure starting from the provided `resource`.
 
@@ -51,6 +51,8 @@ def get_resource_tree(resource, fields, codebase=None, seen_resources=set()):
     get_codebase_tree(). We keep track of child Resources we visit in
     `seen_resources`, so we don't visit them again in get_codebase_tree().
     """
+    if seen_resources is None:
+        seen_resources = set()
     resource_dict = get_resource_fields(resource, fields)
 
     if resource.is_dir:
diff --git a/scanpipe/tests/pipes/test_codebase.py b/scanpipe/tests/pipes/test_codebase.py
index 9dea648a9b..001b6a8c67 100644
--- a/scanpipe/tests/pipes/test_codebase.py
+++ b/scanpipe/tests/pipes/test_codebase.py
@@ -207,3 +207,50 @@ def test_scanpipe_pipes_codebase_get_basic_virtual_codebase(self):
         self.assertEqual("c03f67211a311b13d1294ac8af7cb139ee34c4f9", resource.sha1)
         self.assertEqual(19948, resource.size)
         self.assertEqual(True, resource.is_file)
+
+    def test_get_resource_tree_does_not_share_seen_resources_across_calls(self):
+        """
+        Regression test: get_resource_tree must NOT share the seen_resources
+        set across multiple calls.
+
+        Bug: seen_resources=set() as a mutable default argument means Python
+        creates the set ONCE when the function is defined. All subsequent calls
+        share the SAME set object, causing resources from previous calls to be
+        silently skipped in later calls.
+
+        Fix: use seen_resources=None and create a fresh set() inside the
+        function when seen_resources is None.
+        """
+        from scanpipe.pipes.codebase import get_resource_tree
+        from scanpipe.models import CodebaseResource
+
+        project1 = Project.objects.create(name="Project1")
+        project2 = Project.objects.create(name="Project2")
+
+        dir1 = project1.codebaseresources.create(
+            path="dir1", type=CodebaseResource.Type.DIRECTORY
+        )
+        project1.codebaseresources.create(
+            path="dir1/file1.py", type=CodebaseResource.Type.FILE
+        )
+
+        dir2 = project2.codebaseresources.create(
+            path="dir1", type=CodebaseResource.Type.DIRECTORY
+        )
+        project2.codebaseresources.create(
+            path="dir1/file2.py", type=CodebaseResource.Type.FILE
+        )
+
+        fields = ["path"]
+
+        tree1 = get_resource_tree(dir1, fields)
+        self.assertIn("children", tree1)
+        self.assertEqual(len(tree1["children"]), 1)
+
+        tree2 = get_resource_tree(dir2, fields)
+        self.assertIn("children", tree2)
+        self.assertEqual(
+            len(tree2["children"]),
+            1,
+            "Second call returned empty children — mutable default argument bug!",
+        )
\ No newline at end of file