diff --git a/src/engine/runtime.js b/src/engine/runtime.js
index f407dbdd5c8..503a1fbc449 100644
--- a/src/engine/runtime.js
+++ b/src/engine/runtime.js
@@ -1429,6 +1429,11 @@ class Runtime extends EventEmitter {
             break;
         }
 
+        // Allow extensiosn to override outputShape
+        if (blockInfo.blockShape) {
+            blockJSON.outputShape = blockInfo.blockShape;
+        }
+
         const blockText = Array.isArray(blockInfo.text) ? blockInfo.text : [blockInfo.text];
         let inTextNum = 0; // text for the next block "arm" is blockText[inTextNum]
         let inBranchNum = 0; // how many branches have we placed into the JSON so far?
diff --git a/src/extension-support/tw-block-shape.js b/src/extension-support/tw-block-shape.js
new file mode 100644
index 00000000000..854975ada88
--- /dev/null
+++ b/src/extension-support/tw-block-shape.js
@@ -0,0 +1,25 @@
+// Use the constants instead of manually redefining them again
+const ScratchBlocksConstants = require('../engine/scratch-blocks-constants');
+
+/**
+ * Types of block shapes
+ * @enum {number}
+ */
+const BlockShape = {
+    /**
+     * Output shape: hexagonal (booleans/predicates).
+     */
+    HEXAGONAL: ScratchBlocksConstants.OUTPUT_SHAPE_HEXAGONAL,
+
+    /**
+     * Output shape: rounded (numbers).
+     */
+    ROUND: ScratchBlocksConstants.OUTPUT_SHAPE_ROUND,
+
+    /**
+     * Output shape: squared (any/all values; strings).
+     */
+    SQUARE: ScratchBlocksConstants.OUTPUT_SHAPE_SQUARE
+};
+
+module.exports = BlockShape;
diff --git a/src/extension-support/tw-extension-api-common.js b/src/extension-support/tw-extension-api-common.js
index e2ac0b74d0f..1f5928fbc0f 100644
--- a/src/extension-support/tw-extension-api-common.js
+++ b/src/extension-support/tw-extension-api-common.js
@@ -1,11 +1,13 @@
 const ArgumentType = require('./argument-type');
 const BlockType = require('./block-type');
+const BlockShape = require('./tw-block-shape');
 const TargetType = require('./target-type');
 const Cast = require('../util/cast');
 
 const Scratch = {
     ArgumentType,
     BlockType,
+    BlockShape,
     TargetType,
     Cast
 };
diff --git a/test/unit/tw_block_shape.js b/test/unit/tw_block_shape.js
new file mode 100644
index 00000000000..a5ef50282a6
--- /dev/null
+++ b/test/unit/tw_block_shape.js
@@ -0,0 +1,38 @@
+const {test} = require('tap');
+const Runtime = require('../../src/engine/runtime');
+const Scratch = require('../../src/extension-support/tw-extension-api-common');
+
+test('blockShape', t => {
+    const rt = new Runtime();
+    rt._registerExtensionPrimitives({
+        id: 'shapetest',
+        name: 'shapetest',
+        blocks: [
+            {
+                blockType: Scratch.BlockType.REPORTER,
+                blockShape: Scratch.BlockShape.HEXAGONAL,
+                opcode: 'hexagonal',
+                text: 'hexagonal'
+            },
+            {
+                blockType: Scratch.BlockType.BOOLEAN,
+                blockShape: Scratch.BlockShape.ROUND,
+                opcode: 'round',
+                text: 'round'
+            },
+            {
+                blockType: Scratch.BlockType.REPORTER,
+                blockShape: Scratch.BlockShape.SQUARE,
+                opcode: 'square',
+                text: 'square'
+            }
+        ]
+    });
+
+    const json = rt.getBlocksJSON();
+    t.equal(json.length, 3);
+    t.equal(json[0].outputShape, 1);
+    t.equal(json[1].outputShape, 2);
+    t.equal(json[2].outputShape, 3);
+    t.end();
+});