fix(ui): unify tool attachment handling

packages/ui/src/sync/__tests__/materialization.test.ts

@@ -313,6 +313,80 @@ describe("materializeSessionSnapshots", () => {
     expect(mergedPart.state?.attachments).toHaveLength(1)
     expect((mergedPart.state?.attachments?.[0] as { id?: string })?.id).toBe("att-new")
   })
+
+  test("treats empty state.attachments in completed snapshot as authoritative", () => {
+    const livePart = {
+      id: "prt_1",
+      messageID: "msg_1",
+      sessionID: "ses_1",
+      type: "tool",
+      state: {
+        status: "completed",
+        output: "done",
+        time: { start: 100, end: 200 },
+        attachments: [{ id: "att-old", type: "file", mime: "image/png", url: "data:image/png,..." }],
+      },
+    } as unknown as Part
+    const snapshotPart = {
+      id: "prt_1",
+      messageID: "msg_1",
+      sessionID: "ses_1",
+      type: "tool",
+      state: {
+        status: "completed",
+        output: "done",
+        time: { start: 100, end: 200 },
+        attachments: [],
+      },
+    } as unknown as Part
+    const state = {
+      message: { ses_1: [message("msg_1")] },
+      part: { msg_1: [livePart] },
+    }
+
+    const result = materializeSessionSnapshots(
+      state,
+      "ses_1",
+      [{ info: message("msg_1"), parts: [snapshotPart] }],
+    )
+
+    const mergedPart = result.part.msg_1[0] as { state?: { attachments?: Array<unknown> } }
+    expect(mergedPart.state?.attachments).toEqual([])
+  })
+
+  test("treats empty state.attachments in streaming snapshot as authoritative", () => {
+    const livePart = {
+      id: "prt_1",
+      messageID: "msg_1",
+      sessionID: "ses_1",
+      type: "tool",
+      state: {
+        status: "running",
+        time: { start: 100 },
+        attachments: [{ id: "att-old", type: "file", mime: "image/png", url: "data:image/png,..." }],
+      },
+    } as unknown as Part
+    const snapshotPart = {
+      id: "prt_1",
+      messageID: "msg_1",
+      sessionID: "ses_1",
+      type: "tool",
+      state: { status: "running", time: { start: 100 }, attachments: [] },
+    } as unknown as Part
+    const state = {
+      message: { ses_1: [message("msg_1")] },
+      part: { msg_1: [livePart] },
+    }
+
+    const result = materializeSessionSnapshots(
+      state,
+      "ses_1",
+      [{ info: message("msg_1"), parts: [snapshotPart] }],
+    )
+
+    const mergedPart = result.part.msg_1[0] as { state?: { attachments?: Array<unknown> } }
+    expect(mergedPart.state?.attachments).toEqual([])
+  })
 })
 
 describe("getSessionMaterializationStatus", () => {

packages/ui/src/sync/materialization.ts

@@ -112,7 +112,7 @@ function getPartStateAttachments(part: Part): Array<unknown> | undefined {
   const state = (part as Record<string, unknown>).state as Record<string, unknown> | undefined
   if (!state) return undefined
   const attachments = state.attachments
-  return Array.isArray(attachments) && attachments.length > 0 ? attachments : undefined
+  return Array.isArray(attachments) ? attachments : undefined
 }
 
 function hasLiveStreamingField(part: Part): boolean {
@@ -137,7 +137,7 @@ function mergeMaterializedPart(existing: Part | undefined, next: Part): Part {
 
   if (getPartEndTime(next) !== undefined) {
     const existingAttachments = getPartStateAttachments(existing)
-    if (existingAttachments && !getPartStateAttachments(next)) {
+    if (existingAttachments?.length && getPartStateAttachments(next) === undefined) {
       const nextRecord = { ...next }
       const nextState = { ...((next as Record<string, unknown>).state as Record<string, unknown> ?? {}), attachments: existingAttachments }
       ;(nextRecord as Record<string, unknown>).state = nextState
@@ -161,7 +161,7 @@ function mergeMaterializedPart(existing: Part | undefined, next: Part): Part {
   }
 
   const existingAttachments = getPartStateAttachments(existing)
-  if (existingAttachments && !getPartStateAttachments(next)) {
+  if (existingAttachments?.length && getPartStateAttachments(next) === undefined) {
     if (merged === next) merged = { ...next }
     const mergedRecord = merged as Record<string, unknown>
     const nextState = (next as Record<string, unknown>).state as Record<string, unknown> | undefined