diff --git a/local/build-all.R b/local/build-all.R
index eda92e9..349092f 100644
--- a/local/build-all.R
+++ b/local/build-all.R
@@ -201,10 +201,15 @@ s3_cache_paths <- readRDS("/mnt/cache/packages/s3_cache.rds")
 # nothing; hand it nothing and it rebuilds what it already has. amd64/resolute
 # recompiled 8683 packages that way.
 select_cache_for_pass <- function(paths, sensitive_only, r_minor) {
-  # A cache written before this change holds bare basenames. Filtering those by
-  # path would select nothing and trigger a full rebuild, so use them as they
-  # are; #190's staleness check replaces it on the next pipeline anyway.
-  if (!any(grepl("/", paths, fixed = TRUE))) {
+  # A cache written before #191 holds bare basenames. Filtering those by path
+  # would select nothing and trigger a full rebuild, so use them as they are;
+  # #190's staleness check replaces it on the next pipeline anyway.
+  #
+  # The format is read from the marker `packages-to-build.R` sets, not guessed
+  # from the content: a slot-relative cache with no per-minor or Archive object
+  # holds bare names too, and treating that as legacy would hand a per-minor
+  # pass the flat slot's binaries and build nothing.
+  if (!isTRUE(attr(paths, "slot_relative"))) {
     message("S3 cache is in the legacy basename format; using it unfiltered.")
     return(paths)
   }
diff --git a/local/packages-to-build.R b/local/packages-to-build.R
index 64a2f5f..9e17f4d 100644
--- a/local/packages-to-build.R
+++ b/local/packages-to-build.R
@@ -190,14 +190,44 @@ binary_cache <- setdiff(file_names, source_served)
 # So the cache keeps the path relative to the slot, and `build-all.R` selects
 # the part that matches the pass it is running: the flat slot for the primary,
 # `<minor>/` for a per-minor pass.
+#
+# `s3_dir_ls()` returns keys with the `s3://` scheme attached, so stripping the
+# prefix as a fixed substring takes it out of the middle and leaves
+# `s3://4.4/curl_1.0.tar.gz`. That leading scheme defeats the `^<minor>/` test
+# downstream, so every per-minor object is read as a flat-slot object: the
+# sensitive passes see an empty cache and recompile everything they already
+# have. Anchor the pattern and swallow the scheme with it.
 contrib_prefix <- sprintf(
-  "devxy-rpkgs-binaries/%s/%s/latest/src/contrib/",
+  "^(s3://)?devxy-rpkgs-binaries/%s/%s/latest/src/contrib/",
   arch,
   codename
 )
-relative_paths <- sub(contrib_prefix, "", s3_pkgs, fixed = TRUE)
+relative_paths <- sub(contrib_prefix, "", s3_pkgs)
+
+# The strip is load-bearing and fails silently, so assert it. Both counts are
+# derived from the same listing and use the same shape of pattern, so they must
+# agree exactly; a mismatch means the prefix no longer describes the keys.
+stripped_per_minor <- grepl("^[0-9]+\\.[0-9]+/[^/]+$", relative_paths)
+if (sum(stripped_per_minor) != sum(per_minor_object)) {
+  stop(sprintf(
+    paste0(
+      "S3 prefix strip failed: %d per-minor objects in the listing, %d after ",
+      "stripping /%s/. Example key: %s"
+    ),
+    sum(per_minor_object),
+    sum(stripped_per_minor),
+    contrib_prefix,
+    if (any(per_minor_object)) {
+      s3_pkgs[which(per_minor_object)[1L]]
+    } else {
+      "<none>"
+    }
+  ))
+}
 source_basenames <- source_served
-existence_cache <- relative_paths[!basename(relative_paths) %in% source_basenames]
+existence_cache <- relative_paths[
+  !basename(relative_paths) %in% source_basenames
+]
 cat(sprintf(
   "S3 cache: %d objects, %d served as CRAN source, %d usable binaries\n",
   length(file_names),
@@ -208,6 +238,12 @@ cat(sprintf(
 # Save the S3 file listing for the build step to use as s3_package_cache.
 # This avoids loading s3fs/reticulate in the build container, saving memory for
 # the dependency-installer subprocesses
+# Mark the format explicitly. `build-all.R` has to tell a slot-relative cache
+# from a pre-#191 basename one, and sniffing for a "/" cannot: a new-format
+# cache for a slot with no per-minor or Archive objects holds bare names too,
+# and would be read as legacy and used unfiltered, which makes a per-minor pass
+# believe the flat slot's binaries are its own and build nothing.
+attr(existence_cache, "slot_relative") <- TRUE
 saveRDS(existence_cache, "/mnt/cache/packages/s3_cache.rds")
 # Built from the filtered listing, not the raw one: `s3_dt` is subtracted from
 # the build list below, so a source fallback left in here would exclude the very