diff --git a/web/assemble.sh b/web/assemble.sh
index 1fe1cd20..a1492d1e 100755
--- a/web/assemble.sh
+++ b/web/assemble.sh
@@ -73,18 +73,31 @@ sedi "s|'improvise_worker_server_bg\.wasm'|'improvise_worker_server_bg.wasm?v=${
 # three failed substitutions could still produce a "successful" assembly under
 # `set -e` (found by Copilot on PR #24). These checks are the only guard
 # against the generated glue changing shape, so each stamped reference is
-# verified on its own and names itself when it is the one missing.
+# verified on its own and names itself when it is missing.
+#
+# `stamped` RECORDS rather than exits, and the gate below is what fails the
+# build. Exiting on the first failure named one file and never reached the
+# rest, so a shape change that broke two stamps cost two wasm-pack rebuilds to
+# diagnose -- the slowest loop in this repo (improvise-x4wv). The gate is the
+# whole guard: without it every failure prints and the assembly succeeds, which
+# is the check-that-does-not-check this whole file is full of fixes for.
+missing=""
 stamped() {
-  grep -q "$2" "$1" || {
+  if grep -q "$2" "$1"; then
+    grep -n "$2" "$1"
+  else
     echo "assemble: $1 is missing the cache-bust stamp: $2" >&2
-    exit 1
-  }
-  grep -n "$2" "$1"
+    missing="${missing} $1"
+  fi
 }
 stamped "$app/app.js" "pkg/improvise_wasm_client.js?v=${token}"
 stamped "$app/worker.js" "worker-pkg/improvise_worker_server.js?v=${token}"
 stamped "$app/pkg/improvise_wasm_client.js" "improvise_wasm_client_bg.wasm?v=${token}"
 stamped "$app/worker-pkg/improvise_worker_server.js" "improvise_worker_server_bg.wasm?v=${token}"
+test -z "$missing" || {
+  echo "assemble: refusing to ship; unstamped:${missing}" >&2
+  exit 1
+}
 
 # Every relative import the two entry points make must have been copied. The
 # cp list above is a second answer to "what ships", and a missing sibling is a