diff --git a/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/class/isValidNBJson..st b/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/class/isValidNBJson..st
new file mode 100644
index 000000000..a902079f7
--- /dev/null
+++ b/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/class/isValidNBJson..st
@@ -0,0 +1,17 @@
+nbformat
+isValidNBJson: aJsonString
+	validateNBJson ifNil: [
+		[ {[ validateNBJson := (Polyglot eval: #python string: 'import nbformat
+def is_valid_nb_json(nb_json):
+  nb = nbformat.reads(nb_json, 4)
+  try:
+    nbformat.validate(nb)
+    return True
+  except nbformat.ValidationError:
+    return False
+is_valid_nb_json') ]} do: [ :block | block value ]
+				displayingProgress: [ :block | 'Initializing nbformat in Python...' ]
+				every: 100 ] on: Error do: [ :e |
+					self error: 'nbformat does not seem to be installed (`pip install nbformat`).'.
+					^ false ] ].
+	^ validateNBJson value: aJsonString interopAsString "interopAsString hack: this might not be needed after 21.2"
\ No newline at end of file
diff --git a/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/class/offerToValidateOutput..st b/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/class/offerToValidateOutput..st
new file mode 100644
index 000000000..2db6710ca
--- /dev/null
+++ b/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/class/offerToValidateOutput..st
@@ -0,0 +1,6 @@
+nbformat
+offerToValidateOutput: aJsonString
+	(self confirm: 'Would you like to validate the output with nbformat?') ifTrue: [
+		(self isValidNBJson: aJsonString)
+			ifTrue: [ self inform: 'Validation with nbformat succeeded!' ]
+			ifFalse: [ self error: 'Validation with nbformat failed!' ] ]
\ No newline at end of file
diff --git a/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/instance/save.st b/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/instance/save.st
index dc7b39dc7..634f9ae18 100644
--- a/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/instance/save.st
+++ b/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/instance/save.st
@@ -1,6 +1,8 @@
 actions
 save
-	| stream fName |
+	| stream fName json |
+	json := self asJson.
 	fName := UIManager default saveFilenameRequest:'Please enter the name' translated initialAnswer: 'polyglot-notebook.ipynb'.
+	(Polyglot isPermitted: #python) ifTrue: [ self class offerToValidateOutput: json asJsonString ].
 	stream := FileDirectory default forceNewFileNamed: fName.
-	self asJson jsonWriteOn: stream
\ No newline at end of file
+	json jsonWriteOn: stream
\ No newline at end of file
diff --git a/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/methodProperties.json b/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/methodProperties.json
index d31c87eac..773cefd01 100644
--- a/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/methodProperties.json
+++ b/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/methodProperties.json
@@ -1,6 +1,8 @@
 {
 	"class" : {
-		"for:" : "fn 3/30/2019 00:31" },
+		"for:" : "fn 3/30/2019 00:31",
+		"isValidNBJson:" : "fn 6/30/2021 12:37",
+		"offerToValidateOutput:" : "fn 6/30/2021 12:38" },
 	"instance" : {
 		"addCodeCell" : "fn 4/13/2019 23:24",
 		"addCodeCellFor:" : "fn 4/13/2019 23:24",
@@ -20,6 +22,6 @@
 		"resetCells" : "fn 3/27/2019 11:15",
 		"resetExecutionCounter" : "fn 3/25/2019 18:28",
 		"runAll" : "fn 3/27/2019 11:19",
-		"save" : "fn 3/25/2019 18:18",
+		"save" : "fn 6/30/2021 12:37",
 		"step" : "fn 3/26/2019 11:51",
 		"stepTime" : "fn 3/26/2019 11:53" } }
diff --git a/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/properties.json b/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/properties.json
index da800810f..0b35f873e 100644
--- a/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/properties.json
+++ b/src/TruffleSqueak-Tools-Notebook-Core.package/PNBCodeCellContainer.class/properties.json
@@ -1,7 +1,7 @@
 {
 	"category" : "TruffleSqueak-Tools-Notebook-Core",
 	"classinstvars" : [
-		 ],
+		"validateNBJson" ],
 	"classvars" : [
 		 ],
 	"commentStamp" : "",