diff --git a/frontend/src/data/examples.ts b/frontend/src/data/examples.ts
index 2a9a7c73..8b0456ad 100644
--- a/frontend/src/data/examples.ts
+++ b/frontend/src/data/examples.ts
@@ -5026,7 +5026,7 @@ void loop() {
       {
         id: 'w-btn',
         start: { componentId: 'arduino-uno', pinName: '2' },
-        end: { componentId: 'btn1', pinName: '1a' },
+        end: { componentId: 'btn1', pinName: '1.l' },
         color: '#00aaff',
       },
       {
@@ -5043,7 +5043,7 @@ void loop() {
       },
       {
         id: 'w-btn-gnd',
-        start: { componentId: 'btn1', pinName: '1b' },
+        start: { componentId: 'btn1', pinName: '2.l' },
         end: { componentId: 'arduino-uno', pinName: 'GND' },
         color: '#000000',
       },
diff --git a/frontend/src/simulation/Esp32Bridge.ts b/frontend/src/simulation/Esp32Bridge.ts
index da9f34e1..944cb616 100644
--- a/frontend/src/simulation/Esp32Bridge.ts
+++ b/frontend/src/simulation/Esp32Bridge.ts
@@ -48,8 +48,20 @@ export function toQemuBoardType(kind: BoardKind): 'esp32' | 'esp32-s3' | 'esp32-
   return 'esp32'; // esp32, esp32-devkit-c-v4, esp32-cam, wemos-lolin32-lite
 }
 
-const API_BASE = (): string =>
-  (import.meta.env.VITE_API_BASE as string | undefined) ?? 'http://localhost:8001/api';
+const API_BASE = (): string => {
+  // The desktop shell injects the sidecar URL at runtime (random port) via
+  // window.__VELXIO_API_BASE__; honor it first so the QEMU-board WebSocket
+  // reaches the local Python sidecar instead of the build-time / dev
+  // default. Without this, ESP32 / Pi / STM32 simulations never start in
+  // the desktop app (the WS dialed localhost:8001, not the sidecar port).
+  if (typeof window !== 'undefined') {
+    const injected = (window as { __VELXIO_API_BASE__?: string }).__VELXIO_API_BASE__;
+    if (typeof injected === 'string' && injected) {
+      return injected.replace(/\/+$/, '');
+    }
+  }
+  return (import.meta.env.VITE_API_BASE as string | undefined) ?? 'http://localhost:8001/api';
+};
 
 /** Returns a stable UUID for this browser tab (persists across reloads, resets on new tab). */
 export function getTabSessionId(): string {
diff --git a/frontend/src/simulation/RaspberryPi3Bridge.ts b/frontend/src/simulation/RaspberryPi3Bridge.ts
index eb66a2c0..352c8dd3 100644
--- a/frontend/src/simulation/RaspberryPi3Bridge.ts
+++ b/frontend/src/simulation/RaspberryPi3Bridge.ts
@@ -27,8 +27,20 @@
  *     { type: 'error',         data: { message: string } }
  */
 
-const API_BASE = (): string =>
-  (import.meta.env.VITE_API_BASE as string | undefined) ?? 'http://localhost:8001/api';
+const API_BASE = (): string => {
+  // The desktop shell injects the sidecar URL at runtime (random port) via
+  // window.__VELXIO_API_BASE__; honor it first so the QEMU-board WebSocket
+  // reaches the local Python sidecar instead of the build-time / dev
+  // default. Without this, ESP32 / Pi / STM32 simulations never start in
+  // the desktop app (the WS dialed localhost:8001, not the sidecar port).
+  if (typeof window !== 'undefined') {
+    const injected = (window as { __VELXIO_API_BASE__?: string }).__VELXIO_API_BASE__;
+    if (typeof injected === 'string' && injected) {
+      return injected.replace(/\/+$/, '');
+    }
+  }
+  return (import.meta.env.VITE_API_BASE as string | undefined) ?? 'http://localhost:8001/api';
+};
 
 export class RaspberryPi3Bridge {
   readonly boardId: string;
diff --git a/frontend/src/simulation/Stm32Bridge.ts b/frontend/src/simulation/Stm32Bridge.ts
index 517d1508..f9a5bb11 100644
--- a/frontend/src/simulation/Stm32Bridge.ts
+++ b/frontend/src/simulation/Stm32Bridge.ts
@@ -27,8 +27,20 @@
 import type { BoardKind } from '../types/board';
 import { generateUUID } from '../utils/uuid';
 
-const API_BASE = (): string =>
-  (import.meta.env.VITE_API_BASE as string | undefined) ?? 'http://localhost:8001/api';
+const API_BASE = (): string => {
+  // The desktop shell injects the sidecar URL at runtime (random port) via
+  // window.__VELXIO_API_BASE__; honor it first so the QEMU-board WebSocket
+  // reaches the local Python sidecar instead of the build-time / dev
+  // default. Without this, ESP32 / Pi / STM32 simulations never start in
+  // the desktop app (the WS dialed localhost:8001, not the sidecar port).
+  if (typeof window !== 'undefined') {
+    const injected = (window as { __VELXIO_API_BASE__?: string }).__VELXIO_API_BASE__;
+    if (typeof injected === 'string' && injected) {
+      return injected.replace(/\/+$/, '');
+    }
+  }
+  return (import.meta.env.VITE_API_BASE as string | undefined) ?? 'http://localhost:8001/api';
+};
 
 export function getTabSessionId(): string {
   if (typeof sessionStorage === 'undefined') return generateUUID();
diff --git a/frontend/src/simulation/cyw43/Cyw43Bridge.ts b/frontend/src/simulation/cyw43/Cyw43Bridge.ts
index b7781d73..d29ebdc0 100644
--- a/frontend/src/simulation/cyw43/Cyw43Bridge.ts
+++ b/frontend/src/simulation/cyw43/Cyw43Bridge.ts
@@ -35,8 +35,20 @@ export interface WifiStatus {
 export interface PacketOutFrame { ether: Uint8Array; }
 export interface PacketInFrame { ether: Uint8Array; }
 
-const API_BASE = (): string =>
-  (import.meta.env.VITE_API_BASE as string | undefined) ?? 'http://localhost:8001/api';
+const API_BASE = (): string => {
+  // The desktop shell injects the sidecar URL at runtime (random port) via
+  // window.__VELXIO_API_BASE__; honor it first so the QEMU-board WebSocket
+  // reaches the local Python sidecar instead of the build-time / dev
+  // default. Without this, ESP32 / Pi / STM32 simulations never start in
+  // the desktop app (the WS dialed localhost:8001, not the sidecar port).
+  if (typeof window !== 'undefined') {
+    const injected = (window as { __VELXIO_API_BASE__?: string }).__VELXIO_API_BASE__;
+    if (typeof injected === 'string' && injected) {
+      return injected.replace(/\/+$/, '');
+    }
+  }
+  return (import.meta.env.VITE_API_BASE as string | undefined) ?? 'http://localhost:8001/api';
+};
 
 export class Cyw43Bridge {
   readonly boardId: string;