diff --git a/src/auto-reply/reply/session.ts b/src/auto-reply/reply/session.ts
index 35ee66b71..49e9d2208 100644
--- a/src/auto-reply/reply/session.ts
+++ b/src/auto-reply/reply/session.ts
@@ -278,10 +278,13 @@ export async function initSessionState(params: {
       ctx.MessageThreadId,
     );
   }
-  // Fresh start for new sessions - don't inherit old state like compactionCount
-  sessionStore[sessionKey] = isNewSession
-    ? sessionEntry
-    : { ...sessionStore[sessionKey], ...sessionEntry };
+  if (isNewSession) {
+    sessionEntry.compactionCount = 0;
+    sessionEntry.memoryFlushCompactionCount = undefined;
+    sessionEntry.memoryFlushAt = undefined;
+  }
+  // Preserve per-session overrides while resetting compaction state on /new.
+  sessionStore[sessionKey] = { ...sessionStore[sessionKey], ...sessionEntry };
   await updateSessionStore(storePath, (store) => {
     if (groupResolution?.legacyKey && groupResolution.legacyKey !== sessionKey) {
       if (store[groupResolution.legacyKey] && !store[sessionKey]) {
@@ -289,10 +292,8 @@ export async function initSessionState(params: {
       }
       delete store[groupResolution.legacyKey];
     }
-    // Fresh start for new sessions - don't inherit old state like compactionCount
-    store[sessionKey] = isNewSession
-      ? sessionEntry
-      : { ...store[sessionKey], ...sessionEntry };
+    // Preserve per-session overrides while resetting compaction state on /new.
+    store[sessionKey] = { ...store[sessionKey], ...sessionEntry };
   });
 
   const sessionCtx: TemplateContext = {