diff --git a/eslint.config.js b/eslint.config.js
index 5cb8b7fcfa..d305f75f87 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -25,6 +25,18 @@ const __dirname = path.dirname(__filename);
 const projectRoot = __dirname;
 const currentYear = new Date().getFullYear();
 
+const commonRestrictedSyntaxRules = [
+  {
+    selector: 'CallExpression[callee.name="require"]',
+    message: 'Avoid using require(). Use ES6 imports instead.',
+  },
+  {
+    selector: 'ThrowStatement > Literal:not([value=/^\\w+Error:/])',
+    message:
+      'Do not throw string literals or non-Error objects. Throw new Error("...") instead.',
+  },
+];
+
 export default tseslint.config(
   {
     // Global ignores
@@ -120,18 +132,7 @@ export default tseslint.config(
       'no-cond-assign': 'error',
       'no-debugger': 'error',
       'no-duplicate-case': 'error',
-      'no-restricted-syntax': [
-        'error',
-        {
-          selector: 'CallExpression[callee.name="require"]',
-          message: 'Avoid using require(). Use ES6 imports instead.',
-        },
-        {
-          selector: 'ThrowStatement > Literal:not([value=/^\\w+Error:/])',
-          message:
-            'Do not throw string literals or non-Error objects. Throw new Error("...") instead.',
-        },
-      ],
+      'no-restricted-syntax': ['error', ...commonRestrictedSyntaxRules],
       'no-unsafe-finally': 'error',
       'no-unused-expressions': 'off', // Disable base rule
       '@typescript-eslint/no-unused-expressions': [
@@ -171,6 +172,28 @@ export default tseslint.config(
       ],
     },
   },
+  {
+    // API Response Optionality enforcement for Code Assist
+    files: ['packages/core/src/code_assist/**/*.{ts,tsx}'],
+    rules: {
+      'no-restricted-syntax': [
+        'error',
+        ...commonRestrictedSyntaxRules,
+        {
+          selector:
+            'TSInterfaceDeclaration[id.name=/.+Response$/] TSPropertySignature:not([optional=true])',
+          message:
+            'All fields in API response interfaces (*Response) must be marked as optional (?) to prevent developers from accidentally assuming a field will always be present based on current backend behavior.',
+        },
+        {
+          selector:
+            'TSTypeAliasDeclaration[id.name=/.+Response$/] TSPropertySignature:not([optional=true])',
+          message:
+            'All fields in API response types (*Response) must be marked as optional (?) to prevent developers from accidentally assuming a field will always be present based on current backend behavior.',
+        },
+      ],
+    },
+  },
   {
     // Rules that only apply to product code
     files: ['packages/*/src/**/*.{ts,tsx}'],
diff --git a/packages/core/src/code_assist/server.ts b/packages/core/src/code_assist/server.ts
index dbb749e45d..536bf0c31a 100644
--- a/packages/core/src/code_assist/server.ts
+++ b/packages/core/src/code_assist/server.ts
@@ -508,6 +508,16 @@ export class CodeAssistServer implements ContentGenerator {
 }
 
 interface VpcScErrorResponse {
+  response?: {
+    data?: {
+      error?: {
+        details?: unknown[];
+      };
+    };
+  };
+}
+
+function isVpcScErrorResponse(error: unknown): error is VpcScErrorResponse & {
   response: {
     data: {
       error: {
@@ -515,9 +525,7 @@ interface VpcScErrorResponse {
       };
     };
   };
-}
-
-function isVpcScErrorResponse(error: unknown): error is VpcScErrorResponse {
+} {
   return (
     !!error &&
     typeof error === 'object' &&