diff --git a/frontend/src/__tests__/board-pin-mapping.test.ts b/frontend/src/__tests__/board-pin-mapping.test.ts
new file mode 100644
index 00000000..58a10268
--- /dev/null
+++ b/frontend/src/__tests__/board-pin-mapping.test.ts
@@ -0,0 +1,44 @@
+/**
+ * boardPinToNumber — the bridge between a board element's PAD NAMES and the
+ * GPIO numbers the simulators speak.
+ *
+ * A board whose kind has no branch here falls through to `return null`, and a
+ * null pin means every wire to that header silently connects to nothing: the
+ * part sits on the canvas, the sketch reads a dead pin, and nothing reports an
+ * error. That is what happened to the M5 Cardputer, whose EXT header and Grove
+ * Port A were unreachable even though the element drew them.
+ */
+import { describe, expect, it } from 'vitest';
+import { boardPinToNumber } from '../utils/boardPinMapping';
+
+describe('boardPinToNumber — M5 Cardputer ADV header', () => {
+  // The pad names CardputerElement.ts renders on its wireable bottom row.
+  const GPIO_NAMES = ['3', '4', '5', '6', '8', '9', '13', '14', '15', '39', '40', '2', '1'];
+
+  it('maps every GPIO pad to its own number', () => {
+    for (const name of GPIO_NAMES) {
+      expect(boardPinToNumber('cardputer-adv', name)).toBe(Number(name));
+    }
+  });
+
+  it('leaves no pad unmapped', () => {
+    const unmapped = GPIO_NAMES.filter((n) => boardPinToNumber('cardputer-adv', n) === null);
+    expect(unmapped).toEqual([]);
+  });
+
+  it('maps the power pads to -1, not to a GPIO', () => {
+    expect(boardPinToNumber('cardputer-adv', '5V')).toBe(-1);
+    expect(boardPinToNumber('cardputer-adv', 'GND')).toBe(-1);
+  });
+
+  it('accepts G40, which the classic ESP32 range would have rejected', () => {
+    // The shared esp32 branch clamps at 39; the S3 has 48 GPIOs and this board
+    // exposes 40 on its header.
+    expect(boardPinToNumber('cardputer-adv', '40')).toBe(40);
+  });
+
+  it('rejects a pad that is not a pin', () => {
+    expect(boardPinToNumber('cardputer-adv', 'NOPE')).toBeNull();
+    expect(boardPinToNumber('cardputer-adv', '99')).toBeNull();
+  });
+});
diff --git a/frontend/src/utils/boardPinMapping.ts b/frontend/src/utils/boardPinMapping.ts
index f6b0b027..d3497a16 100644
--- a/frontend/src/utils/boardPinMapping.ts
+++ b/frontend/src/utils/boardPinMapping.ts
@@ -350,6 +350,18 @@ export function boardPinToNumber(boardId: string, pinName: string): number | nul
     return null;
   }
 
+  // Overlay S3 boards whose pads are bare GPIO numbers but whose kind does not
+  // start with 'esp32' — the M5 Cardputer's EXT header and Grove Port A. Its
+  // pads go up to G40, above the classic ESP32's 39, so the shared branch below
+  // would reject the two highest ones even if the kind matched.
+  if (boardId === 'cardputer-adv') {
+    if (pinName.startsWith('GND') || pinName.startsWith('3V3') || pinName.startsWith('5V'))
+      return -1;
+    const num = parseInt(pinName, 10);
+    if (!isNaN(num) && num >= 0 && num <= 48) return num; // S3 has 48 GPIOs
+    return null;
+  }
+
   // ESP32 / ESP32-S3 / ESP32-C3 — GPIO numbers used directly
   if (boardId === 'esp32' || boardId.startsWith('esp32')) {
     // Power / GND pins (GND, GND.1, 3V3, 3V3.1, 5V, 5V.1, etc.)