diff --git a/cam/export.py b/cam/export.py
index eb7b2ec..acc96a2 100644
--- a/cam/export.py
+++ b/cam/export.py
@@ -228,6 +228,20 @@ def export_transcripts(
 
     manifest.sort(key=lambda item: item["modified_at"], reverse=True)
     filter_label = context or context_mask
+
+    manifest_path = output_root / "manifest.json"
+    if manifest_path.is_file():
+        try:
+            prev = json.loads(manifest_path.read_text(encoding="utf-8"))
+            for item in prev.get("sessions", []):
+                sid = item.get("session_id")
+                if sid and sid not in message_counts:
+                    message_counts[sid] = int(item.get("message_count", 0))
+        except (json.JSONDecodeError, OSError):
+            pass
+    for item in manifest:
+        message_counts[item["session_id"]] = item["message_count"]
+
     (output_root / "manifest.json").write_text(
         json.dumps(
             {