fix: filter empty-parts messages to prevent follow-up conversation crash (#402)

* fix: filter out messages with empty parts to prevent follow-up crash

When an assistant response is interrupted or errors before producing content,
a UIMessage with empty parts remains in the chat state. On the next send, the
AI SDK validates all messages and rejects the empty-parts message with
"Message must contain at least one part". This filters them out when not
streaming and adds a safety guard in formatConversationHistory.

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

* fix: filter empty-parts messages before persisting to storage

Addresses race condition where the save effect could persist messages
with empty parts before the cleanup effect's state update applies.

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

apps/agent/entrypoints/sidepanel/index/useChatSession.ts

@@ -304,6 +304,15 @@ export const useChatSession = () => {
     }),
   })
 
+  // Remove messages with empty parts (e.g. interrupted assistant responses)
+  // to prevent AI SDK validation errors on subsequent sends
+  useEffect(() => {
+    if (status === 'streaming') return
+    if (messages.some((m) => !m.parts?.length)) {
+      setMessages(messages.filter((m) => m.parts?.length > 0))
+    }
+  }, [messages, status, setMessages])
+
   useNotifyActiveTab({
     messages,
     status,
@@ -370,15 +379,14 @@ export const useChatSession = () => {
   // biome-ignore lint/correctness/useExhaustiveDependencies: only need to run when messages change
   useEffect(() => {
     messagesRef.current = messages
-    if (messages.length > 0) {
-      // Local storage: save on every change (including during streaming)
-      // Remote: only save when not streaming to avoid partial message saves
+    const messagesToSave = messages.filter((m) => m.parts?.length > 0)
+    if (messagesToSave.length > 0) {
       if (isLoggedIn) {
         if (status !== 'streaming') {
-          saveRemoteConversation(conversationIdRef.current, messages)
+          saveRemoteConversation(conversationIdRef.current, messagesToSave)
         }
       } else {
-        saveLocalConversation(conversationIdRef.current, messages)
+        saveLocalConversation(conversationIdRef.current, messagesToSave)
       }
     }
   }, [messages, isLoggedIn, status])

apps/agent/lib/conversations/formatConversationHistory.ts

@@ -17,6 +17,7 @@ export function formatConversationHistory(
 
   return recentMessages
     .map((msg) => {
+      if (!msg.parts?.length) return null
       const role: 'user' | 'assistant' =
         msg.role === 'user' ? 'user' : 'assistant'
       const textContent = msg.parts