perf(compaction): reuse message update parts for no-text tails

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/preemptive-compaction-degradation-monitor.ts

@@ -43,6 +43,7 @@ interface ClientLike {
 export interface AssistantCompactionMessageInfo {
   sessionID: string
   id?: string
+  parts?: unknown
 }
 
 async function withTimeout<TValue>(
@@ -185,6 +186,7 @@ export function createPostCompactionDegradationMonitor(args: {
       sessionID: info.sessionID,
       messageID: info.id,
       directory,
+      parts: info.parts,
     })
 
     if (!isNoTextTail) {

src/hooks/preemptive-compaction-no-text-tail.ts

@@ -46,8 +46,13 @@ export async function resolveNoTextTailFromSession(args: {
   sessionID: string
   messageID?: string
   directory: string
+  parts?: unknown
 }): Promise<boolean> {
-  const { client, sessionID, messageID, directory } = args
+  const { client, sessionID, messageID, directory, parts } = args
+
+  if (Array.isArray(parts)) {
+    return isStepOnlyNoTextParts(parts)
+  }
 
   try {
     const response = await client.session.messages({

src/hooks/preemptive-compaction.degradation-monitor.test.ts

@@ -192,4 +192,28 @@ describe("preemptive-compaction post-compaction degradation monitor", () => {
     // then
     expect(ctx.client.session.summarize).not.toHaveBeenCalled()
   })
+
+  it("uses message update parts without refetching session messages", async () => {
+    // given
+    const sessionHistory: AssistantHistoryMessage[] = []
+    const ctx = createMockCtx(sessionHistory)
+    const hook = createPreemptiveCompactionHook(ctx as never, {} as never)
+    const sessionID = "ses_tail_update_parts"
+    const stepOnlyParts = [{ type: "step-start" }, { type: "step-finish" }]
+
+    await hook.event({
+      event: {
+        type: "session.compacted",
+        properties: { sessionID },
+      },
+    })
+
+    // when
+    await hook.event(buildAssistantUpdate({ sessionID, id: "msg_1", parts: stepOnlyParts }))
+    await hook.event(buildAssistantUpdate({ sessionID, id: "msg_2", parts: stepOnlyParts }))
+
+    // then
+    expect(ctx.client.session.messages).not.toHaveBeenCalled()
+    expect(ctx.client.session.summarize).not.toHaveBeenCalled()
+  })
 })

src/hooks/preemptive-compaction.ts

@@ -76,6 +76,7 @@ export function createPreemptiveCompactionHook(
         modelID?: string
         finish?: boolean
         tokens?: TokenInfo
+        parts?: unknown
       } | undefined
 
       if (!info || info.role !== "assistant" || !info.finish || !info.sessionID) return
@@ -92,6 +93,7 @@ export function createPreemptiveCompactionHook(
       await postCompactionMonitor.onAssistantMessageUpdated({
         sessionID: info.sessionID,
         id: info.id,
+        parts: info.parts,
       })
     }
   }