diff --git a/src/shared/session-cursor.test.ts b/src/shared/session-cursor.test.ts
index bb395a483..2edf1e16e 100644
--- a/src/shared/session-cursor.test.ts
+++ b/src/shared/session-cursor.test.ts
@@ -50,4 +50,17 @@ describe("getNewMessages", () => {
     // #then
     expect(next).toEqual(shorter)
   })
+
+  it("returns all messages when last key is missing", () => {
+    // #given
+    const messages = [buildMessage("m1", 1), buildMessage("m2", 2)]
+    getNewMessages(sessionID, messages)
+    const replaced = [buildMessage("n1", 1), buildMessage("n2", 2)]
+
+    // #when
+    const next = getNewMessages(sessionID, replaced)
+
+    // #then
+    expect(next).toEqual(replaced)
+  })
 })
diff --git a/src/shared/session-cursor.ts b/src/shared/session-cursor.ts
index 0fc942b8a..d9c38a28c 100644
--- a/src/shared/session-cursor.ts
+++ b/src/shared/session-cursor.ts
@@ -57,8 +57,9 @@ export function getNewMessages<T extends CursorMessage>(
       const lastIndex = keys.lastIndexOf(cursor.lastKey)
       if (lastIndex >= 0) {
         startIndex = lastIndex + 1
-      } else if (cursor.lastCount <= messages.length) {
-        startIndex = cursor.lastCount
+      } else {
+        // History changed without a shrink; reset to avoid skipping messages.
+        startIndex = 0
       }
     }
   }