fix: route new prompt sync to target directory

Keeps optimistic prompt state in the session directory
Routes live assistant part updates using upstream event payloads
Adds regressions for startup session switch races

packages/ui/src/sync/sync-context.tsx

@@ -571,11 +571,21 @@ const getSessionIdFromPayload = (event: Event): string | null => {
   }
 
   if (event.type === "message.part.updated") {
+    const sessionID = props.sessionID
+    if (typeof sessionID === "string" && sessionID.length > 0) {
+      return sessionID
+    }
+
     const part = props.part
     if (!part || typeof part !== "object") {
       return null
     }
-    const sessionID = (part as { sessionID?: unknown }).sessionID
+    const partSessionID = (part as { sessionID?: unknown }).sessionID
+    return typeof partSessionID === "string" && partSessionID.length > 0 ? partSessionID : null
+  }
+
+  if (event.type === "message.part.delta" || event.type === "message.part.removed") {
+    const sessionID = props.sessionID
     return typeof sessionID === "string" && sessionID.length > 0 ? sessionID : null
   }
 
@@ -658,8 +668,8 @@ const getMessageIdFromPayload = (event: Event): string | null => {
     if (!part || typeof part !== "object") {
       return null
     }
-    const messageID = (part as { messageID?: unknown }).messageID
-    return typeof messageID === "string" && messageID.length > 0 ? messageID : null
+    const partMessageID = (part as { messageID?: unknown }).messageID
+    return typeof partMessageID === "string" && partMessageID.length > 0 ? partMessageID : null
   }
 
   return null
@@ -957,9 +967,12 @@ const updateRoutingIndexFromEvent = (
     }
 
     case "message.part.updated": {
-      const part = (payload.properties as { part?: Part }).part as (Part & { sessionID?: string; messageID?: string }) | undefined
-      if (part?.messageID && part.sessionID) {
-        setIndexedMessage(routingIndex, part.sessionID, part.messageID, directory)
+      const props = payload.properties as { sessionID?: string; part?: Part }
+      const part = props.part as (Part & { sessionID?: string; messageID?: string }) | undefined
+      const sessionID = part?.sessionID ?? props.sessionID
+      const messageID = part?.messageID
+      if (messageID && sessionID) {
+        setIndexedMessage(routingIndex, sessionID, messageID, directory)
       }
       return
     }