fix(compaction): ignore compaction agent updates

Fixes #3819

src/hooks/preemptive-compaction.test.ts

@@ -55,7 +55,9 @@ function setupImmediateTimeouts(): () => void {
 
   globalThis.setTimeout = ((callback: (...args: unknown[]) => void, _delay?: number, ...args: unknown[]) => {
     callback(...args)
-    return 1 as unknown as ReturnType<typeof setTimeout>
+    const timeoutID = originalSetTimeout(() => undefined, 0)
+    originalClearTimeout(timeoutID)
+    return timeoutID
   }) as typeof setTimeout
 
   globalThis.clearTimeout = (() => {}) as typeof clearTimeout
@@ -637,6 +639,78 @@ describe("preemptive-compaction", () => {
     Date.now = originalNow
   })
 
+  // #given compaction already succeeded for a session
+  // #when the compaction agent emits its summary message update
+  // #then it should not clear the compaction guard or trigger a duplicate summary
+  it("should ignore compaction-agent message updates after successful compaction", async () => {
+    const hook = createPreemptiveCompactionHook(ctx as never, {} as never)
+    const sessionID = "ses_compaction_agent_update"
+
+    await hook.event({
+      event: {
+        type: "message.updated",
+        properties: {
+          info: {
+            role: "assistant",
+            sessionID,
+            providerID: "anthropic",
+            modelID: "claude-sonnet-4-6",
+            finish: true,
+            tokens: {
+              input: 170000,
+              output: 0,
+              reasoning: 0,
+              cache: { read: 10000, write: 0 },
+            },
+          },
+        },
+      },
+    })
+
+    await hook["tool.execute.after"](
+      { tool: "bash", sessionID, callID: "call_1" },
+      { title: "", output: "test", metadata: null }
+    )
+
+    expect(ctx.client.session.summarize).toHaveBeenCalledTimes(1)
+
+    const originalNow = Date.now
+    try {
+      Date.now = () => originalNow() + 61_000
+
+      await hook.event({
+        event: {
+          type: "message.updated",
+          properties: {
+            info: {
+              agent: "compaction",
+              role: "assistant",
+              sessionID,
+              providerID: "anthropic",
+              modelID: "claude-sonnet-4-6",
+              finish: true,
+              tokens: {
+                input: 170000,
+                output: 0,
+                reasoning: 0,
+                cache: { read: 10000, write: 0 },
+              },
+            },
+          },
+        },
+      })
+
+      await hook["tool.execute.after"](
+        { tool: "bash", sessionID, callID: "call_2" },
+        { title: "", output: "test", metadata: null }
+      )
+
+      expect(ctx.client.session.summarize).toHaveBeenCalledTimes(1)
+    } finally {
+      Date.now = originalNow
+    }
+  })
+
   // #given modelContextLimitsCache has model-specific limit (256k)
   // #when tokens are above default 78% of 200k but below 78% of 256k
   // #then should NOT trigger compaction