fix(kernel-ui): validate KRef target at state-entry and improve cast safety

Validate target with isKRef() in the select onChange handler instead of
at send time, making invalid state unrepresentable. Add test for invalid
target rejection and clarifying comments on safe regex-guarded as-casts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rekmarks

packages/kernel-ui/src/components/SendMessageForm.test.tsx

@@ -17,7 +17,7 @@ import { useRegistry } from '../hooks/useRegistry.ts';
 import type { ObjectRegistry } from '../types.ts';
 import { SendMessageForm } from './SendMessageForm.tsx';
 
-setupOcapKernelMock();
+const { setMockBehavior } = setupOcapKernelMock();
 
 vi.mock('../context/PanelContext.tsx', () => ({
   usePanelContext: vi.fn(),
@@ -193,6 +193,20 @@ describe('SendMessageForm Component', () => {
     expect(sendButton).toBeDisabled();
   });
 
+  it('does not set target when value fails KRef validation', async () => {
+    setMockBehavior({ isKRef: false });
+    const { getByTestId } = render(<SendMessageForm />);
+
+    const targetSelect = getByTestId('message-target');
+    fireEvent.change(targetSelect, { target: { value: 'not-a-kref' } });
+
+    // Target should remain unset, button stays disabled
+    expect(targetSelect).toHaveValue('');
+    expect(getByTestId('message-send-button')).toBeDisabled();
+
+    setMockBehavior({ isKRef: true });
+  });
+
   it('calls callKernelMethod with correct parameters when Send button is clicked', async () => {
     const { getByTestId } = render(<SendMessageForm />);
 

packages/kernel-ui/src/components/SendMessageForm.tsx

@@ -10,6 +10,7 @@ import {
 } from '@metamask/design-system-react';
 import { stringify } from '@metamask/kernel-utils';
 import { isKRef } from '@metamask/ocap-kernel';
+import type { KRef } from '@metamask/ocap-kernel';
 import type { Json } from '@metamask/utils';
 import { useState, useMemo } from 'react';
 
@@ -27,7 +28,7 @@ const inputStyle =
 export const SendMessageForm: React.FC = () => {
   const { callKernelMethod, logMessage, objectRegistry } = usePanelContext();
   const { fetchObjectRegistry } = useRegistry();
-  const [target, setTarget] = useState('');
+  const [target, setTarget] = useState<KRef | null>(null);
   const [method, setMethod] = useState('__getMethodNames__');
   const [paramsText, setParamsText] = useState('[]');
   const [result, setResult] = useState<Json | null>(null);
@@ -64,8 +65,7 @@ export const SendMessageForm: React.FC = () => {
   }, [objectRegistry]);
 
   const handleSend = (): void => {
-    if (!isKRef(target)) {
-      logMessage('Invalid target', 'error');
+    if (!target) {
       return;
     }
     Promise.resolve()
@@ -121,8 +121,11 @@ export const SendMessageForm: React.FC = () => {
           </label>
           <select
             id="message-target"
-            value={target}
-            onChange={(event) => setTarget(event.target.value)}
+            value={target ?? ''}
+            onChange={(event) => {
+              const { value } = event.target;
+              setTarget(isKRef(value) ? value : null);
+            }}
             data-testid="message-target"
             className={`${inputStyle} cursor-pointer`}
           >
@@ -176,7 +179,7 @@ export const SendMessageForm: React.FC = () => {
             variant={ButtonVariant.Primary}
             size={ButtonSize.Sm}
             onClick={handleSend}
-            isDisabled={!(target.trim() && method.trim())}
+            isDisabled={!(target && method.trim())}
             className="h-9 rounded-md"
             data-testid="message-send-button"
           >

packages/kernel-ui/src/services/db-parser.ts

@@ -81,6 +81,7 @@ export function parseObjectRegistry(
       matches[1] &&
         objCList.push({
           vat: matches[1],
+          // Safe cast: regex capture group guarantees KRef format
           kref: matches[2] as KRef,
           eref: value.replace(/^R\s*/u, ''),
         });
@@ -90,6 +91,7 @@ export function parseObjectRegistry(
       matches[1] &&
         prmCList.push({
           vat: matches[1],
+          // Safe cast: regex capture group guarantees KRef format
           kref: matches[2] as KRef,
           eref: value.replace(/^R\s*/u, ''),
         });