diff --git a/HISTORY.md b/HISTORY.md
index c7e7cb6a..20866d45 100755
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -45,6 +45,9 @@
 * **Translation Updates:**
     * German
 
+### 2022-05-01
+* byob: programmatically reduce the number of inputs in a custom block
+
 ### 2022-04-28
 * threads, byob: programmatically re-define custom blocks, experimental, under construction
 * threads: programmatically re-categorize custom blocks
diff --git a/snap.html b/snap.html
index 3cf5e862..7bfa5597 100755
--- a/snap.html
+++ b/snap.html
@@ -23,7 +23,7 @@
         <script src="src/gui.js?version=2022-04-26"></script>
         <script src="src/paint.js?version=2021-07-05"></script>
         <script src="src/lists.js?version=2022-02-07"></script>
-        <script src="src/byob.js?version=2022-04-28"></script>
+        <script src="src/byob.js?version=2022-05-01"></script>
         <script src="src/tables.js?version=2022-01-28"></script>
         <script src="src/sketch.js?version=2021-11-03"></script>
         <script src="src/video.js?version=2019-06-27"></script>
diff --git a/src/byob.js b/src/byob.js
index eaaf544e..908212a4 100644
--- a/src/byob.js
+++ b/src/byob.js
@@ -111,7 +111,7 @@ ArgLabelMorph*/
 
 // Global stuff ////////////////////////////////////////////////////////
 
-modules.byob = '2022-April-28';
+modules.byob = '2022-May-01';
 
 // Declarations
 
@@ -569,7 +569,10 @@ CustomBlockDefinition.prototype.setBlockDefinition = function (aContext) {
         declarations = this.declarations,
         parts = [];
 
-    if (oldInputs.length !== newInputs.length) {
+    if (oldInputs.length > newInputs.length) {
+        this.removeInputs(oldInputs.length - newInputs.length);
+        spec = this.abstractBlockSpec();
+    } else if (oldInputs.length !== newInputs.length) {
         throw new Error('expecting the number of inputs to match');
     }
 
@@ -601,6 +604,31 @@ CustomBlockDefinition.prototype.setBlockDefinition = function (aContext) {
     this.body = aContext;
 };
 
+CustomBlockDefinition.prototype.removeInputs = function (count) {
+    // private - only to be called from a Process that also does housekeeping
+    var surplus = this.inputNames().slice(-count);
+
+    // remove the surplus input names from the spec
+    this.spec = this.parseSpec(this.spec).filter(str =>
+        !(str.length > 1 && (str[0]) === '%' && surplus.includes(str.slice(1)))
+    ).join(' ').trim();
+
+    // remove the surplus input names from the slot declarations
+    surplus.forEach(name => this.declarations.delete(name));
+};
+
+CustomBlockDefinition.prototype.addInputs = function (count) {
+    // private
+};
+
+CustomBlockDefinition.prototype.gensym = function (existing) {
+    var count = 1;
+    while (contains(existing, '#' + count)) {
+        count += 1;
+    }
+    return '#' + count;
+};
+
 // CustomBlockDefinition picturing
 
 CustomBlockDefinition.prototype.scriptsPicture = function () {