diff --git a/tests/e2e/wdio.conf.js b/tests/e2e/wdio.conf.js
index 395d34a..7448ca4 100644
--- a/tests/e2e/wdio.conf.js
+++ b/tests/e2e/wdio.conf.js
@@ -112,6 +112,24 @@ export const config = {
       }
     } catch { /* no process on port — good */ }
 
+    // CRITICAL: The debug binary has devUrl (localhost:9700) baked in.
+    // If another app (e.g., BridgeCoach) is serving on that port, the Tauri
+    // WebView loads the WRONG frontend. Fail fast if port 9700 is in use.
+    const DEV_URL_PORT = 9700;
+    try {
+      const devPids = execSync(`lsof -ti:${DEV_URL_PORT} 2>/dev/null`, { encoding: 'utf8' }).trim();
+      if (devPids) {
+        throw new Error(
+          `Port ${DEV_URL_PORT} (Tauri devUrl) is in use by another process (PIDs: ${devPids}). ` +
+          `The debug binary will load that app instead of Agent Orchestrator frontend. ` +
+          `Either stop the process on port ${DEV_URL_PORT}, or use a release build.`
+        );
+      }
+    } catch (e) {
+      if (e.message.includes('Port 9700')) throw e;
+      // lsof returned non-zero = port is free, good
+    }
+
     // Verify binary exists
     if (!existsSync(tauriBinary)) {
       if (process.env.SKIP_BUILD) {