diff --git a/frontend/src/__tests__/board-pin-mapping.test.ts b/frontend/src/__tests__/board-pin-mapping.test.ts
index 58a10268..d3ad2595 100644
--- a/frontend/src/__tests__/board-pin-mapping.test.ts
+++ b/frontend/src/__tests__/board-pin-mapping.test.ts
@@ -42,3 +42,28 @@ describe('boardPinToNumber — M5 Cardputer ADV header', () => {
     expect(boardPinToNumber('cardputer-adv', '99')).toBeNull();
   });
 });
+
+describe('m5stack-core header pins', () => {
+  // The Core's pads sat unmapped for months: the kind does not start with
+  // 'esp32', so it fell through every branch and each wire from the M-Bus
+  // header resolved to null — a part wired to the header never saw a signal,
+  // silently. These pin the dedicated branch.
+  const CORE_GPIO = ['21', '22', '23', '19', '18', '3', '1', '16', '17', '2', '5', '25', '26', '35', '36', '12', '13', '15', '0', '34'];
+
+  it('maps every M-Bus GPIO pad to its number', () => {
+    for (const name of CORE_GPIO) {
+      expect(boardPinToNumber('m5stack-core', name)).toBe(Number(name));
+    }
+  });
+
+  it('maps the power pads (including BAT) to -1', () => {
+    for (const name of ['5V', 'GND', '3V3', 'BAT']) {
+      expect(boardPinToNumber('m5stack-core', name)).toBe(-1);
+    }
+  });
+
+  it('rejects out-of-range and non-pins', () => {
+    expect(boardPinToNumber('m5stack-core', '40')).toBeNull(); // classic tops out at 39
+    expect(boardPinToNumber('m5stack-core', 'NOPE')).toBeNull();
+  });
+});
diff --git a/frontend/src/utils/boardPinMapping.ts b/frontend/src/utils/boardPinMapping.ts
index 12a0d06c..b3d836da 100644
--- a/frontend/src/utils/boardPinMapping.ts
+++ b/frontend/src/utils/boardPinMapping.ts
@@ -361,6 +361,24 @@ export function boardPinToNumber(boardId: string, pinName: string): number | nul
     return null;
   }
 
+  // M5Stack Core: same bare-GPIO-number pads, classic ESP32 range. Without
+  // this branch the kind fell through every one below — the shared ESP32
+  // branch requires boardId.startsWith('esp32') — and every wire from the
+  // M-Bus header resolved to null: a part wired to the header never saw a
+  // signal, with nothing anywhere saying why. BAT joins the power names.
+  if (boardId === 'm5stack-core') {
+    if (
+      pinName.startsWith('GND') ||
+      pinName.startsWith('3V3') ||
+      pinName.startsWith('5V') ||
+      pinName === 'BAT'
+    )
+      return -1;
+    const num = parseInt(pinName, 10);
+    if (!isNaN(num) && num >= 0 && num <= 39) return num;
+    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.)