fix: recover from invalid XML in localStorage on startup (#261)

When LLM generates invalid XML, the app previously saved corrupted messages
to localStorage, causing an unrecoverable crash loop on restart.

This fix validates messages when restoring from localStorage and filters out
any with invalid diagram XML. Users see a toast notification when corrupted
messages are removed.

Fixes #240

components/chat-panel.tsx

@@ -40,6 +40,7 @@ interface MessagePart {
     type: string
     state?: string
     toolName?: string
+    input?: { xml?: string; [key: string]: unknown }
     [key: string]: unknown
 }
 
@@ -88,6 +89,37 @@ function hasToolErrors(messages: ChatMessage[]): boolean {
     return lastToolPart?.state === TOOL_ERROR_STATE
 }
 
+/**
+ * Check if a message contains valid diagram XML.
+ * Used to filter out corrupted messages when restoring from localStorage.
+ * Validates both display_diagram and append_diagram tool calls.
+ */
+function hasValidDiagramXml(message: {
+    parts?: Array<{ type?: string; input?: unknown }>
+}): boolean {
+    if (!message.parts) return true // No parts = valid (user messages, text-only)
+
+    const parser = new DOMParser()
+    for (const part of message.parts) {
+        // Check both display_diagram and append_diagram tools
+        const isDiagramTool =
+            part.type === "tool-display_diagram" ||
+            part.type === "tool-append_diagram"
+        const input = part.input as { xml?: string } | undefined
+        if (isDiagramTool && input?.xml) {
+            try {
+                const doc = parser.parseFromString(input.xml, "text/xml")
+                if (doc.querySelector("parsererror")) {
+                    return false
+                }
+            } catch {
+                return false
+            }
+        }
+    }
+    return true
+}
+
 export default function ChatPanel({
     isVisible,
     onToggleVisibility,
@@ -598,6 +630,7 @@ Continue from EXACTLY where you stopped.`,
     const messagesEndRef = useRef<HTMLDivElement>(null)
 
     // Restore messages and XML snapshots from localStorage on mount
+    // Validates and filters out corrupted messages to prevent crash loops
     useEffect(() => {
         if (hasRestoredRef.current) return
         hasRestoredRef.current = true
@@ -608,7 +641,32 @@ Continue from EXACTLY where you stopped.`,
             if (savedMessages) {
                 const parsed = JSON.parse(savedMessages)
                 if (Array.isArray(parsed) && parsed.length > 0) {
-                    setMessages(parsed)
+                    // Filter out messages with invalid XML to prevent crash loops
+                    const validMessages = parsed.filter((msg: ChatMessage) => {
+                        try {
+                            return hasValidDiagramXml(msg)
+                        } catch {
+                            return false
+                        }
+                    })
+
+                    if (validMessages.length < parsed.length) {
+                        const removedCount =
+                            parsed.length - validMessages.length
+                        console.warn(
+                            `[ChatPanel] Filtered ${removedCount} corrupted message(s) from storage`,
+                        )
+                        toast.warning(
+                            `Removed ${removedCount} message(s) with invalid diagrams to recover session.`,
+                        )
+                        // Update storage with cleaned messages
+                        localStorage.setItem(
+                            STORAGE_MESSAGES_KEY,
+                            JSON.stringify(validMessages),
+                        )
+                    }
+
+                    setMessages(validMessages)
                 }
             }
 
@@ -622,6 +680,10 @@ Continue from EXACTLY where you stopped.`,
             }
         } catch (error) {
             console.error("Failed to restore from localStorage:", error)
+            // On complete failure, clear storage to allow recovery
+            localStorage.removeItem(STORAGE_MESSAGES_KEY)
+            localStorage.removeItem(STORAGE_XML_SNAPSHOTS_KEY)
+            toast.error("Session data was corrupted. Starting fresh.")
         }
     }, [setMessages])