diff --git a/HISTORY.md b/HISTORY.md
index bc9bf5b9..961361b3 100755
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -48,6 +48,7 @@
 
 ### 2021-10-26
 * objects: don't show codification and js-func blocks in search results unless enabled
+* gui, objects: new "showingExtensions" session setting for showing extension prims in the palette
 
 ### 2021-10-25
 * byob: fixed #2902
diff --git a/snap.html b/snap.html
index 663d0abc..94835487 100755
--- a/snap.html
+++ b/snap.html
@@ -20,7 +20,7 @@
         <script src="src/threads.js?version=2021-10-22"></script>
         <script src="src/objects.js?version=2021-10-26"></script>
         <script src="src/scenes.js?version=2021-10-12"></script>
-        <script src="src/gui.js?version=2021-10-20"></script>
+        <script src="src/gui.js?version=2021-10-26"></script>
         <script src="src/paint.js?version=2021-07-05"></script>
         <script src="src/lists.js?version=2021-07-19"></script>
         <script src="src/byob.js?version=2021-10-25"></script>
diff --git a/src/gui.js b/src/gui.js
index b71cc9a9..17b0d1a3 100644
--- a/src/gui.js
+++ b/src/gui.js
@@ -85,7 +85,7 @@ Animation, BoxMorph, BlockDialogMorph, RingMorph, Project, ZERO, BLACK*/
 
 // Global stuff ////////////////////////////////////////////////////////
 
-modules.gui = '2021-October-20';
+modules.gui = '2021-October-26';
 
 // Declarations
 
@@ -3808,6 +3808,18 @@ IDE_Morph.prototype.settingsMenu = function () {
             'NOTE: You will have to manually\n' +
             'sign in again to access your account.' */
     );
+    addPreference(
+        'Extension blocks',
+        () => {
+            SpriteMorph.prototype.showingExtensions =
+                !SpriteMorph.prototype.showingExtensions;
+            this.flushBlocksCache('variables');
+            this.refreshPalette();
+        },
+        SpriteMorph.prototype.showingExtensions,
+        'uncheck to hide extension\nprimitives in the palette',
+        'check to show extension\nprimitives in the palette'
+    );
     addPreference(
         'Add scenes',
         () => this.isAddingScenes = !this.isAddingScenes,
diff --git a/src/objects.js b/src/objects.js
index a1786228..e5908176 100644
--- a/src/objects.js
+++ b/src/objects.js
@@ -183,6 +183,7 @@ SpriteMorph.prototype.isCachingPrimitives = true;
 
 SpriteMorph.prototype.enableNesting = true;
 SpriteMorph.prototype.enableFirstClass = true;
+SpriteMorph.prototype.showingExtensions = false;
 SpriteMorph.prototype.useFlatLineEnds = false;
 SpriteMorph.prototype.highlightColor = new Color(250, 200, 130);
 SpriteMorph.prototype.highlightBorder = 8;
@@ -2743,13 +2744,8 @@ SpriteMorph.prototype.blockTemplates = function (
         blocks.push(block('doInsertInList'));
         blocks.push(block('doReplaceInList'));
 
-        // for debugging: ///////////////
-        if (devMode) {
-            blocks.push('-');
-            blocks.push(this.devModeText());
-            blocks.push('-');
-            blocks.push(block('doShowTable'));
-            blocks.push('-');
+        if (SpriteMorph.prototype.showingExtensions) {
+            blocks.push('=');
             blocks.push(block('doApplyExtension'));
             blocks.push(block('reportApplyExtension'));
         }
@@ -2762,6 +2758,14 @@ SpriteMorph.prototype.blockTemplates = function (
             blocks.push('-');
             blocks.push(block('reportMappedCode'));
         }
+
+        // for debugging: ///////////////
+        if (this.world().isDevMode) {
+            blocks.push('-');
+            blocks.push(this.devModeText());
+            blocks.push('-');
+            blocks.push(block('doShowTable'));
+        }
     }
 
     return blocks;
@@ -3194,6 +3198,12 @@ SpriteMorph.prototype.isDisablingBlock = function (aBlock) {
     if (sel === 'reportJSFunction') {
         return !Process.prototype.enableJS;
     }
+    if (
+        sel === 'doApplyExtension' ||
+        sel === 'reportApplyExtension'
+    ) {
+        return !SpriteMorph.prototype.showingExtensions;
+    }
     if (
         sel === 'doMapCodeOrHeader' ||
         sel === 'doMapValueCode' ||
@@ -8998,25 +9008,27 @@ StageMorph.prototype.blockTemplates = function (
         blocks.push(block('doInsertInList'));
         blocks.push(block('doReplaceInList'));
 
+        if (SpriteMorph.prototype.showingExtensions) {
+            blocks.push('=');
+            blocks.push(block('doApplyExtension'));
+            blocks.push(block('reportApplyExtension'));
+        }
+
+        if (StageMorph.prototype.enableCodeMapping) {
+            blocks.push('=');
+            blocks.push(block('doMapCodeOrHeader'));
+            blocks.push(block('doMapValueCode'));
+            blocks.push(block('doMapListCode'));
+            blocks.push('-');
+            blocks.push(block('reportMappedCode'));
+        }
+
         // for debugging: ///////////////
         if (this.world().isDevMode) {
             blocks.push('-');
             blocks.push(this.devModeText());
             blocks.push('-');
             blocks.push(block('doShowTable'));
-            blocks.push('-');
-            blocks.push(block('doApplyExtension'));
-            blocks.push(block('reportApplyExtension'));
-        }
-
-        blocks.push('=');
-
-        if (StageMorph.prototype.enableCodeMapping) {
-            blocks.push(block('doMapCodeOrHeader'));
-            blocks.push(block('doMapValueCode'));
-            blocks.push(block('doMapListCode'));
-            blocks.push('-');
-            blocks.push(block('reportMappedCode'));
         }
     }