diff --git a/apps/mobile-shell/scripts/check-config.mjs b/apps/mobile-shell/scripts/check-config.mjs
index 7a4d49b0b..1210a3472 100644
--- a/apps/mobile-shell/scripts/check-config.mjs
+++ b/apps/mobile-shell/scripts/check-config.mjs
@@ -914,6 +914,13 @@ for (const expectedErrorCode of [
     );
   }
 }
+if (
+  protocolSource.includes('error instanceof Error') ||
+  protocolSource.includes('? error.message') ||
+  protocolSource.includes('error.message\n        :')
+) {
+  throw new Error('mobile shell protocol must not expose unknown native Error.message values to H5');
+}
 for (const snippet of [
   'parseRequest',
   'isHostBridgeRequest',
diff --git a/apps/mobile-shell/src/host-bridge/protocol.test.ts b/apps/mobile-shell/src/host-bridge/protocol.test.ts
index c2cd0df97..68feed41c 100644
--- a/apps/mobile-shell/src/host-bridge/protocol.test.ts
+++ b/apps/mobile-shell/src/host-bridge/protocol.test.ts
@@ -106,7 +106,7 @@ describe('mobile HostBridge protocol helpers', () => {
     });
     expect(normalizeMobileHostBridgeError(new Error('disk failed'))).toEqual({
       code: 'host_error',
-      message: 'disk failed',
+      message: 'mobile host bridge request failed',
     });
     expect(normalizeMobileHostBridgeError('boom')).toEqual({
       code: 'host_error',
diff --git a/apps/mobile-shell/src/host-bridge/protocol.ts b/apps/mobile-shell/src/host-bridge/protocol.ts
index 0fb2938e7..d5b8b2a58 100644
--- a/apps/mobile-shell/src/host-bridge/protocol.ts
+++ b/apps/mobile-shell/src/host-bridge/protocol.ts
@@ -107,9 +107,6 @@ export function normalizeMobileHostBridgeError(error: unknown): HostBridgeError
 
   return {
     code: 'host_error',
-    message:
-      error instanceof Error
-        ? error.message
-        : 'mobile host bridge request failed',
+    message: 'mobile host bridge request failed',
   };
 }