diff --git a/src/engine/runtime.js b/src/engine/runtime.js
index 169c880f7d7..3ce8a69f12a 100644
--- a/src/engine/runtime.js
+++ b/src/engine/runtime.js
@@ -1429,6 +1429,12 @@ class Runtime extends EventEmitter {
             break;
         }
 
+        // Check if "blockShape" is specified
+        if (blockInfo.blockShape) {
+            blockJSON.outputShape = blockInfo.outputShape || ScratchBlocksConstants.OUTPUT_SHAPE_ROUND;
+        }
+
+
         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/block-shape.js b/src/extension-support/block-shape.js
new file mode 100644
index 00000000000..854975ada88
--- /dev/null
+++ b/src/extension-support/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..20503d56dcf 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('./block-shape');
 const TargetType = require('./target-type');
 const Cast = require('../util/cast');
 
 const Scratch = {
     ArgumentType,
     BlockType,
+    BlockShape,
     TargetType,
     Cast
 };