fix(ui): prevent streaming text flicker and first-chunk loss

packages/ui/src/sync/event-pipeline.ts

@@ -314,9 +314,13 @@ export function createEventPipeline(input: EventPipelineInput) {
         } else {
           d.queue[i] = normalizedPayload
           if (normalizedPayload.type === "message.part.updated") {
-            const part = (normalizedPayload.properties as { part: { messageID: string; id: string } }).part
-            d.staleDeltas.add(deltaKey(part.messageID, part.id, "text"))
-            d.staleDeltas.add(deltaKey(part.messageID, part.id, "output"))
+            const part = (normalizedPayload.properties as { part: Record<string, unknown> & { messageID: string; id: string } }).part
+            for (const field of ["text", "output"] as const) {
+              const value = part[field]
+              if (typeof value === "string" && value.length > 0) {
+                d.staleDeltas.add(deltaKey(part.messageID, part.id, field))
+              }
+            }
           }
         }
         syncDebug.pipeline.coalesced(normalizedPayload.type, k)

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

@@ -1227,12 +1227,12 @@ function handleEvent(
     const messageID = getMessageIdFromPayload(payload) ?? undefined
     syncDebug.dispatch.eventNoChange(payload.type, sessionID, messageID)
 
-    // Parts-gap recovery: if a part event was dropped because the parts array
-    // was missing (message not yet inserted or parts lost), trigger a repair
-    // fetch for the session.
-    if (sessionID && messageID && (
-      payload.type === "message.part.delta" || payload.type === "message.part.updated"
-    )) {
+    // Parts-gap recovery: if a delta event was dropped because the parts array
+    // was missing or the partID was not found, trigger a repair fetch for the
+    // session. message.part.updated never needs repair — it only returns false
+    // for intentionally skipped types (step-start, step-finish, patch) or when
+    // preserving an existing finished tool part, neither of which indicates missing data.
+    if (sessionID && messageID && payload.type === "message.part.delta") {
       enqueuePartsRepair(resolvedDirectory, sessionID, childStores)
     }
   }