diff --git a/src/plugin/event.ts b/src/plugin/event.ts
index 3755a88c..fb88bcd6 100644
--- a/src/plugin/event.ts
+++ b/src/plugin/event.ts
@@ -13,6 +13,7 @@ import { lspManager } from "../tools"
 import type { CreatedHooks } from "../create-hooks"
 import type { Managers } from "../create-managers"
 import { normalizeSessionStatusToIdle } from "./session-status-normalizer"
+import { pruneRecentSyntheticIdles } from "./recent-synthetic-idles"
 
 type FirstMessageVariantGate = {
   markSessionCreated: (sessionInfo: { id?: string; title?: string; parentID?: string } | undefined) => void
@@ -54,6 +55,12 @@ export function createEventHandler(args: {
   const DEDUP_WINDOW_MS = 500
 
   return async (input): Promise<void> => {
+    pruneRecentSyntheticIdles({
+      recentSyntheticIdles,
+      now: Date.now(),
+      dedupWindowMs: DEDUP_WINDOW_MS,
+    })
+
     if (input.event.type === "session.idle") {
       const sessionID = (input.event.properties as Record<string, unknown> | undefined)?.sessionID as string | undefined
       if (sessionID) {
diff --git a/src/plugin/recent-synthetic-idles.test.ts b/src/plugin/recent-synthetic-idles.test.ts
new file mode 100644
index 00000000..468e6e30
--- /dev/null
+++ b/src/plugin/recent-synthetic-idles.test.ts
@@ -0,0 +1,24 @@
+import { describe, it, expect } from "bun:test"
+
+import { pruneRecentSyntheticIdles } from "./recent-synthetic-idles"
+
+describe("pruneRecentSyntheticIdles", () => {
+  it("removes entries older than dedup window", () => {
+    // given
+    const recentSyntheticIdles = new Map<string, number>([
+      ["ses_old", 1000],
+      ["ses_new", 1600],
+    ])
+
+    // when
+    pruneRecentSyntheticIdles({
+      recentSyntheticIdles,
+      now: 2000,
+      dedupWindowMs: 500,
+    })
+
+    // then
+    expect(recentSyntheticIdles.has("ses_old")).toBe(false)
+    expect(recentSyntheticIdles.has("ses_new")).toBe(true)
+  })
+})
diff --git a/src/plugin/recent-synthetic-idles.ts b/src/plugin/recent-synthetic-idles.ts
new file mode 100644
index 00000000..971ff5e0
--- /dev/null
+++ b/src/plugin/recent-synthetic-idles.ts
@@ -0,0 +1,13 @@
+export function pruneRecentSyntheticIdles(args: {
+  recentSyntheticIdles: Map<string, number>
+  now: number
+  dedupWindowMs: number
+}): void {
+  const { recentSyntheticIdles, now, dedupWindowMs } = args
+
+  for (const [sessionID, emittedAt] of recentSyntheticIdles) {
+    if (now - emittedAt >= dedupWindowMs) {
+      recentSyntheticIdles.delete(sessionID)
+    }
+  }
+}