diff --git a/packages/ui/src/sync/__tests__/event-reducer.test.ts b/packages/ui/src/sync/__tests__/event-reducer.test.ts
index 2dda4914..475727cd 100644
--- a/packages/ui/src/sync/__tests__/event-reducer.test.ts
+++ b/packages/ui/src/sync/__tests__/event-reducer.test.ts
@@ -128,6 +128,15 @@ describe("applyDirectoryEvent", () => {
       properties: { part: serverText },
     } as Event)).toBe(true)
     expect(draft.part.msg_1).toEqual([serverText, optimisticFile])
+
+    // The file echo follows the text echo; it must claim the optimistic file
+    // even though the first slot now holds a server part.
+    const serverFile = { id: "prt_server_file", messageID: "msg_1", sessionID: "ses_1", type: "file", filename: "a.png" } as Part
+    expect(applyDirectoryEvent(draft, {
+      type: "message.part.updated",
+      properties: { part: serverFile },
+    } as Event)).toBe(true)
+    expect(draft.part.msg_1).toEqual([serverText, serverFile])
   })
 
   test("returns typed materialization when delta arrives before parts", () => {
diff --git a/packages/ui/src/sync/event-reducer.ts b/packages/ui/src/sync/event-reducer.ts
index 13057258..2ac42246 100644
--- a/packages/ui/src/sync/event-reducer.ts
+++ b/packages/ui/src/sync/event-reducer.ts
@@ -433,11 +433,13 @@ export function applyDirectoryEvent(
           : part
       } else {
         // Replace optimistic part (no sessionID) with server part of same type.
-        // Gate: only scan if the first part lacks sessionID (optimistic parts are
-        // always inserted first). Assistant messages never have optimistic parts,
-        // so this check is effectively free during streaming.
-        const hasOptimistic = next.length > 0 && !(next[0] as { sessionID?: string }).sessionID
-        const optimisticIndex = hasOptimistic && (part.type === "text" || part.type === "file")
+        // Every optimistic part is a candidate, not only the first one: the
+        // server echoes a just-sent message part by part, and after the text
+        // echo replaced the first slot the file echo still has to find the
+        // optimistic file behind it, or the attachment shows twice until the
+        // next page fetch. The scan runs only when a part with a NEW id
+        // arrives, which during assistant streaming is once per part.
+        const optimisticIndex = part.type === "text" || part.type === "file"
           ? next.findIndex((p) => p.type === part.type && !(p as { sessionID?: string }).sessionID)
           : -1
         if (optimisticIndex >= 0) {