diff --git a/cdn.tf b/cdn.tf
index fe1b25f..5e8899d 100644
--- a/cdn.tf
+++ b/cdn.tf
@@ -105,6 +105,21 @@ resource "bunnynet_pullzone" "cran_rpkgs_com" {
   request_coalescing_enabled = true
   block_post_requests        = true
 
+  # Set on the zone since before this configuration existed; declared here so
+  # `tofu apply` stops silently removing it.
+  #
+  # The router makes it redundant on paper: the only UA-dependent responses it
+  # produces are redirects, and those carry `Cache-Control: no-store`, while
+  # their targets are concrete per-slot, per-minor URLs whose content depends
+  # only on the path. Dropping it would also be a real win, because otherwise
+  # every distinct R version string keys its own copy of every tarball.
+  #
+  # It stays for now anyway: it is the second line of defence against the one
+  # failure that would be quiet and confusing (an R 4.6 client served the 4.5
+  # index), and removing it is worth doing on its own once per-minor routing is
+  # confirmed live, not as a side effect of enabling that routing.
+  cache_vary_headers = ["User-Agent"]
+
   limit_requests    = 5000
   limit_connections = 1000