diff --git a/frontend/package.json b/frontend/package.json
index 84409185..bfd2c1be 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -33,7 +33,6 @@
     "@google/genai": "^0.6.0",
     "@monaco-editor/react": "^4.7.0",
     "@tanstack/react-table": "^8.20.5",
-    "monaco-editor": "0.55.1",
     "@types/react-syntax-highlighter": "^15.5.13",
     "@wokwi/elements": "1.9.2",
     "@xterm/addon-fit": "^0.11.0",
@@ -47,6 +46,7 @@
     "jszip": "^3.10.1",
     "littlefs": "^0.1.0",
     "lucide-react": "^0.460.0",
+    "monaco-editor": "0.55.1",
     "react": "^19.2.0",
     "react-dom": "^19.2.0",
     "react-i18next": "^15.1.0",
@@ -65,6 +65,7 @@
     "@types/node": "^24.11.0",
     "@types/react": "^19.2.7",
     "@types/react-dom": "^19.2.3",
+    "@types/w3c-web-serial": "^1.0.8",
     "@vitejs/plugin-react": "^5.1.1",
     "@vitest/coverage-v8": "^4.0.18",
     "@vitest/ui": "^4.0.18",
diff --git a/frontend/src/__tests__/pro-hardware-serial-seam.test.ts b/frontend/src/__tests__/pro-hardware-serial-seam.test.ts
new file mode 100644
index 00000000..c5b23de2
--- /dev/null
+++ b/frontend/src/__tests__/pro-hardware-serial-seam.test.ts
@@ -0,0 +1,28 @@
+/**
+ * The hardware-serial TX seam (lib/proHardwareSerial.ts): while a real
+ * board's monitor is attached, serial input routes to the interceptor;
+ * detaching restores simulator routing. Per-board, inert by default.
+ */
+import { describe, expect, it, vi } from 'vitest';
+import {
+  getSerialTxInterceptor,
+  installSerialTxInterceptor,
+} from '../lib/proHardwareSerial';
+
+describe('proHardwareSerial seam', () => {
+  it('has no interceptor by default (pure OSS build)', () => {
+    expect(getSerialTxInterceptor('board-1')).toBeNull();
+  });
+
+  it('routes per board and uninstalls with null', () => {
+    const toHardware = vi.fn();
+    installSerialTxInterceptor('board-1', toHardware);
+
+    expect(getSerialTxInterceptor('board-2')).toBeNull();
+    getSerialTxInterceptor('board-1')?.('hello\n');
+    expect(toHardware).toHaveBeenCalledWith('hello\n');
+
+    installSerialTxInterceptor('board-1', null);
+    expect(getSerialTxInterceptor('board-1')).toBeNull();
+  });
+});
diff --git a/frontend/src/lib/proHardwareSerial.ts b/frontend/src/lib/proHardwareSerial.ts
new file mode 100644
index 00000000..1fed8f4a
--- /dev/null
+++ b/frontend/src/lib/proHardwareSerial.ts
@@ -0,0 +1,37 @@
+/**
+ * Pro hardware-serial TX registry.
+ *
+ * The hardware serial monitor (pro overlay) mirrors a REAL board's UART
+ * into the existing SerialMonitor. Incoming bytes need no seam — the
+ * overlay appends them via `appendHardwareSerial` (useSimulatorStore).
+ * Outgoing text does: `serialWriteToBoard` normally targets the board's
+ * simulator, so while a hardware monitor is attached to a board, the
+ * overlay installs an interceptor here and the user's input goes to the
+ * physical port instead.
+ *
+ * Mirrors the other OSS->Pro seams (`proWebFlash.ts`, `proBoardGate.ts`).
+ * No imports on purpose: the store consults this module, never the other
+ * way around, so there is no cycle.
+ */
+
+type SerialTxInterceptor = (text: string) => void;
+
+const _interceptors = new Map<string, SerialTxInterceptor>();
+
+/**
+ * Route a board's serial input to the given sink instead of its simulator.
+ * Pass null to restore the default (simulator) routing. Installed by the
+ * pro overlay while its hardware serial monitor is connected.
+ */
+export function installSerialTxInterceptor(
+  boardId: string,
+  fn: SerialTxInterceptor | null,
+): void {
+  if (fn) _interceptors.set(boardId, fn);
+  else _interceptors.delete(boardId);
+}
+
+/** The interceptor for a board, or null when input should go to the sim. */
+export function getSerialTxInterceptor(boardId: string): SerialTxInterceptor | null {
+  return _interceptors.get(boardId) ?? null;
+}
diff --git a/frontend/src/store/useSimulatorStore.ts b/frontend/src/store/useSimulatorStore.ts
index 2cca724b..61fc752b 100644
--- a/frontend/src/store/useSimulatorStore.ts
+++ b/frontend/src/store/useSimulatorStore.ts
@@ -28,6 +28,7 @@ import type { Wire, WireInProgress, WireEndpoint } from '../types/wire';
 import type { BoardKind, BoardInstance, LanguageMode, WifiStatus } from '../types/board';
 import { BOARD_SUPPORTS_ESPIDF, BOARD_SUPPORTS_MICROPYTHON, isPiBoardKind, isStm32BoardKind } from '../types/board';
 import { boardGateDecision, proBoardFeatureName, triggerProUpgradePrompt } from '../lib/proBoardGate';
+import { getSerialTxInterceptor } from '../lib/proHardwareSerial';
 import { calculatePinPosition } from '../utils/pinPositionCalculator';
 import { useOscilloscopeStore } from './useOscilloscopeStore';
 import { RaspberryPi3Bridge } from '../simulation/RaspberryPi3Bridge';
@@ -1291,6 +1292,16 @@ const { append: appendSerial } = createSerialBatcher((perBoard) => {
   });
 });
 
+/**
+ * Pro overlay seam: append bytes coming from a REAL board's UART (Web
+ * Serial hardware monitor) into a board's serial output. Goes through the
+ * same per-frame batcher as simulator bytes, so the SerialMonitor needs no
+ * changes and hardware chatter cannot trigger the per-byte set() storm.
+ */
+export function appendHardwareSerial(boardId: string, chunk: string): void {
+  appendSerial(boardId, chunk);
+}
+
 // ── Store ─────────────────────────────────────────────────────────────────
 export const useSimulatorStore = create<SimulatorState>((set, get) => {
   // Initialise runtime objects for the default board
@@ -3756,6 +3767,13 @@ export const useSimulatorStore = create<SimulatorState>((set, get) => {
     serialWriteToBoard: (boardId: string, text: string) => {
       const board = get().boards.find((b) => b.id === boardId);
       if (!board) return;
+      // A connected hardware serial monitor (pro overlay) takes the input
+      // instead of the board's simulator.
+      const hardwareTx = getSerialTxInterceptor(boardId);
+      if (hardwareTx) {
+        hardwareTx(text);
+        return;
+      }
       if (isPiBoardKind(board.boardKind)) {
         const bridge = getBoardBridge(boardId);
         if (bridge) {