diff --git a/src/hooks/useAutoCompactContinue.ts b/src/hooks/useAutoCompactContinue.ts
index 6a857d130..bb23f0406 100644
--- a/src/hooks/useAutoCompactContinue.ts
+++ b/src/hooks/useAutoCompactContinue.ts
@@ -1,5 +1,5 @@
 import { useRef, useEffect } from "react";
-import { useWorkspaceStoreRaw, type WorkspaceState } from "@/stores/WorkspaceStore";
+import { useWorkspaceStoreRaw } from "@/stores/WorkspaceStore";
 import { buildSendMessageOptions } from "@/hooks/useSendMessageOptions";
 
 /**
@@ -28,8 +28,6 @@ export function useAutoCompactContinue() {
   // re-rendering AppInner on every workspace state change. This hook only needs
   // to react when messages change to a single compacted message state.
   const store = useWorkspaceStoreRaw();
-  const workspaceStatesRef = useRef<Map<string, WorkspaceState>>(new Map());
-
   // Track which specific compaction summary messages we've already processed.
   // Key insight: Each compaction creates a unique message. Track by message ID,
   // not workspace ID, to prevent processing the same compaction result multiple times.
@@ -39,7 +37,6 @@ export function useAutoCompactContinue() {
   // Update ref and check for auto-continue condition
   const checkAutoCompact = () => {
     const newStates = store.getAllStates();
-    workspaceStatesRef.current = newStates;
 
     // Check all workspaces for completed compaction
     for (const [workspaceId, state] of newStates) {
@@ -59,22 +56,23 @@ export function useAutoCompactContinue() {
       // After compaction, history is replaced with a single summary message
       // The summary message has compaction-result metadata with the continueMessage
       const summaryMessage = state.cmuxMessages[0]; // Single compacted message
-      const messageId = summaryMessage.id;
-
-      // Have we already processed this specific compaction message?
-      // This check is race-safe because message IDs are unique and immutable.
-      if (processedMessageIds.current.has(messageId)) continue;
-
       const cmuxMeta = summaryMessage?.metadata?.cmuxMetadata;
       const continueMessage =
         cmuxMeta?.type === "compaction-result" ? cmuxMeta.continueMessage : undefined;
 
       if (!continueMessage) continue;
 
-      // Mark THIS MESSAGE as processed before sending
-      // Multiple concurrent checkAutoCompact() calls will all see the same message ID,
-      // so only the first call that reaches this point will proceed
-      processedMessageIds.current.add(messageId);
+      // Prefer compaction-request ID for idempotency; fall back to summary message ID
+      const idForGuard =
+        cmuxMeta?.type === "compaction-result" && cmuxMeta.requestId
+          ? `req:${cmuxMeta.requestId}`
+          : `msg:${summaryMessage.id}`;
+
+      // Have we already processed this specific compaction result?
+      if (processedMessageIds.current.has(idForGuard)) continue;
+
+      // Mark THIS RESULT as processed before sending to prevent duplicates
+      processedMessageIds.current.add(idForGuard);
 
       console.log(
         `[useAutoCompactContinue] Sending continue message for ${workspaceId}:`,
@@ -86,7 +84,7 @@ export function useAutoCompactContinue() {
       window.api.workspace.sendMessage(workspaceId, continueMessage, options).catch((error) => {
         console.error("Failed to send continue message:", error);
         // If sending failed, remove from processed set to allow retry
-        processedMessageIds.current.delete(messageId);
+        processedMessageIds.current.delete(idForGuard);
       });
     }
   };
diff --git a/src/stores/WorkspaceStore.ts b/src/stores/WorkspaceStore.ts
index be1f0218f..afc265210 100644
--- a/src/stores/WorkspaceStore.ts
+++ b/src/stores/WorkspaceStore.ts
@@ -417,6 +417,9 @@ export class WorkspaceStore {
    * Handle compact_summary tool completion.
    * Returns true if compaction was handled (caller should early return).
    */
+  // Track processed compaction-request IDs to dedupe performCompaction across duplicated events
+  private processedCompactionRequestIds = new Set<string>();
+
   private handleCompactionCompletion(
     workspaceId: string,
     aggregator: StreamingMessageAggregator,
@@ -430,6 +433,17 @@ export class WorkspaceStore {
       return false;
     }
 
+    // Extract the compaction-request message to identify this compaction run
+    const compactionRequestMsg = findCompactionRequestMessage(aggregator);
+    if (!compactionRequestMsg) {
+      return false;
+    }
+
+    // Dedupe: If we've already processed this compaction-request, skip re-running
+    if (this.processedCompactionRequestIds.has(compactionRequestMsg.id)) {
+      return true; // Already handled compaction for this request
+    }
+
     // Extract the summary text from the assistant's response
     const summary = aggregator.getCompactionSummary(data.messageId);
     if (!summary) {
@@ -437,6 +451,9 @@ export class WorkspaceStore {
       return false;
     }
 
+    // Mark this compaction-request as processed before performing compaction
+    this.processedCompactionRequestIds.add(compactionRequestMsg.id);
+
     this.performCompaction(workspaceId, aggregator, data, summary);
     return true;
   }
@@ -544,7 +561,7 @@ export class WorkspaceStore {
             : undefined,
         // Store continueMessage in summary so it survives history replacement
         cmuxMetadata: continueMessage
-          ? { type: "compaction-result", continueMessage }
+          ? { type: "compaction-result", continueMessage, requestId: compactRequestMsg?.id }
           : { type: "normal" },
       }
     );
diff --git a/src/types/message.ts b/src/types/message.ts
index a307766a4..4da548409 100644
--- a/src/types/message.ts
+++ b/src/types/message.ts
@@ -21,6 +21,7 @@ export type CmuxFrontendMetadata =
   | {
       type: "compaction-result";
       continueMessage: string; // Message to send after compaction completes
+      requestId?: string; // ID of the compaction-request user message that produced this summary (for idempotency)
     }
   | {
       type: "normal"; // Regular messages