diff --git a/.crow/build-all-versions.yaml b/.crow/build-all-versions.yaml
index fb1229d..cbbc70d 100644
--- a/.crow/build-all-versions.yaml
+++ b/.crow/build-all-versions.yaml
@@ -35,6 +35,14 @@ variables:
       - 'resolute'
     default: '3.24'
   R_VERSION:
+    # The slot's *primary* R minor: what `local/build-all.R` builds into the
+    # generic slot. The loop below already runs `--sensitive-only` for every
+    # other installed minor, so filling a non-primary minor's gap needs this
+    # left alone, not changed.
+    #
+    # 4.6.0 was briefly offered here (#183) and removed: selecting it for a
+    # slot whose generic binaries are 4.5-built would publish 4.6 binaries
+    # into the generic slot and break every 4.5 client.
     description: 'Primary R version under /opt/R.'
     options:
       - 4.5.3
diff --git a/.crow/reindex.yaml b/.crow/reindex.yaml
index b8f0d86..b13245a 100644
--- a/.crow/reindex.yaml
+++ b/.crow/reindex.yaml
@@ -172,7 +172,12 @@ steps:
         from_secret: BUNNYNET_API_KEY
       # cran.rpkgs.com and cran.allianceswisspass.devxy.io are on separate
       # Bunny pull zones, so both must be purged after the shared origin changes.
-      BUNNY_PULLZONES: '3857050 cran.allianceswisspass.devxy.io'
+      # The staging zone is listed too. It shares the B2 origin, so an index
+      # it still holds is a stale copy of the same object, and its
+      # cache_expiration_time is the same ~370 days: without a purge here it
+      # serves pre-reindex indexes indefinitely and any verification run
+      # against it measures the past.
+      BUNNY_PULLZONES: '3857050 cran.allianceswisspass.devxy.io cran-rpkgs-test.b-cdn.net'
     commands:
       - apk add --no-cache -q bash curl jq
       # Crow carries the checkout from the re-index step into this step.
diff --git a/.crow/weekly-rebuild-missing.yaml b/.crow/weekly-rebuild-missing.yaml
index d764ba5..9639792 100644
--- a/.crow/weekly-rebuild-missing.yaml
+++ b/.crow/weekly-rebuild-missing.yaml
@@ -20,19 +20,6 @@ variables:
   # Gates this pipeline. A manual pipeline creation instantiates every file in
   # .crow/, and a declared default is applied even when the run never passed
   # this variable, so the default must be a value that matches no matrix row.
-  # Which R minor to rebuild for. The matrix pins each slot's primary minor,
-  # which is the only one the weekly cron ever builds - that is why non-primary
-  # minors accumulate a backlog and why an ABI-risky package can exist for 4.4
-  # and 4.5 but not 4.6. Override this to work that backlog off.
-  rebuild_r_version:
-    description: "R version to rebuild with: 'matrix' for each slot's primary, or an explicit x.y.z."
-    options:
-      - matrix
-      - 4.6.0
-      - 4.5.3
-      - 4.4.3
-    default: matrix
-
   weekly_rebuild_missing:
     description: "Manual run target: a specific <os>-<arch>, 'all' to run every os/arch in parallel, or 'none' to run nothing."
     options:
@@ -423,7 +410,6 @@ steps:
       UVR_PACKAGES_DIR: /mnt/cache/uvr/packages
       R_LIBS_USER: /mnt/cache/R-pkgs
       R_VERSION: ${R_VERSION}
-      REBUILD_R_VERSION: ${rebuild_r_version}
       CCACHE_DIR: /mnt/cache/ccache
       PLATFORM: ${OS}
       ARCH: ${ARCH}
@@ -438,26 +424,12 @@ steps:
       - git clone -q https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
       - mkdir -p /mnt/cache/uvr/cache /mnt/cache/uvr/packages /mnt/cache/R-pkgs /mnt/cache/ccache /mnt/cache/packages
       - rm -rf /mnt/cache/R-pkgs/00LOCK-*
-      # `matrix` keeps the slot's primary minor; anything else overrides it.
-      # rebuild-missing.R derives its target slot from the R it runs under, so
-      # selecting the interpreter is all that is needed to fill a minor's gap.
-      - |
-        if [ "$REBUILD_R_VERSION" = "matrix" ] || [ -z "$REBUILD_R_VERSION" ]; then
-          RV="$R_VERSION"
-        else
-          RV="$REBUILD_R_VERSION"
-        fi
-        if [ ! -x "/opt/R/$RV/bin/R" ]; then
-          echo "R $RV is not installed in this image; available: $(ls /opt/R)" >&2
-          exit 1
-        fi
-        echo "Rebuilding with R $RV"
-      - /opt/R/$RV/bin/Rscript local/install-bincraft.R
-      - /opt/R/$RV/bin/R -q -e 'packageVersion("bincraft")'
+      - /opt/R/$R_VERSION/bin/Rscript local/install-bincraft.R
+      - /opt/R/$R_VERSION/bin/R -q -e 'packageVersion("bincraft")'
       - XVFB=$(command -v xwfb-run 2>/dev/null || command -v xvfb-run); XVFB_ARGS=""; if command -v xwfb-run >/dev/null 2>&1; then dnf install -y -q weston 2>/dev/null; XVFB_ARGS="-c weston"; fi
-      - UVR_R_BIN=/opt/R/$RV/bin/R local/uvr-install.sh httr2
-      - /opt/R/$RV/bin/R -q -e 'source("local/fetch-rebuild-packages-from-issue.R")'
-      - $XVFB $XVFB_ARGS -n $SPLIT_INDEX -- /opt/R/$RV/bin/Rscript local/rebuild-missing.R $SPLIT_INTO $SPLIT_INDEX $REBUILD_BUDGET_HOURS 2>&1
+      - UVR_R_BIN=/opt/R/$R_VERSION/bin/R local/uvr-install.sh httr2
+      - /opt/R/$R_VERSION/bin/R -q -e 'source("local/fetch-rebuild-packages-from-issue.R")'
+      - $XVFB $XVFB_ARGS -n $SPLIT_INDEX -- /opt/R/$R_VERSION/bin/Rscript local/rebuild-missing.R $SPLIT_INTO $SPLIT_INDEX $REBUILD_BUDGET_HOURS 2>&1
     backend_options:
       docker:
         resources:
diff --git a/.crow/weekly-rebuild-reindex.yaml b/.crow/weekly-rebuild-reindex.yaml
index 2e0f2a0..c9f9a8a 100644
--- a/.crow/weekly-rebuild-reindex.yaml
+++ b/.crow/weekly-rebuild-reindex.yaml
@@ -175,7 +175,12 @@ steps:
         from_secret: BUNNYNET_API_KEY
       # cran.rpkgs.com and cran.allianceswisspass.devxy.io are on separate
       # Bunny pull zones, so both must be purged after the shared origin changes.
-      BUNNY_PULLZONES: '3857050 cran.allianceswisspass.devxy.io'
+      # The staging zone is listed too. It shares the B2 origin, so an index
+      # it still holds is a stale copy of the same object, and its
+      # cache_expiration_time is the same ~370 days: without a purge here it
+      # serves pre-reindex indexes indefinitely and any verification run
+      # against it measures the past.
+      BUNNY_PULLZONES: '3857050 cran.allianceswisspass.devxy.io cran-rpkgs-test.b-cdn.net'
     commands:
       - apk add --no-cache -q bash curl jq
       # Crow carries the checkout from the re-index step into this step.
diff --git a/cdn.tf b/cdn.tf
index 349486e..df30273 100644
--- a/cdn.tf
+++ b/cdn.tf
@@ -89,9 +89,19 @@ resource "bunnynet_compute_script" "rpkgs_router" {
 # slot that is not listed here would hide every package the per-minor index does
 # not carry, so this stays empty until a slot has been backfilled.
 resource "bunnynet_compute_script_variable" "rpkgs_router_union_slots" {
-  script        = bunnynet_compute_script.rpkgs_router.id
-  name          = "UNION_SLOTS"
-  default_value = ""
+  script = bunnynet_compute_script.rpkgs_router.id
+  name   = "UNION_SLOTS"
+  # Enabled. Until this was set, every client resolved against the generic
+  # index and never reached a per-minor binary: an R 4.6.1 client on resolute
+  # downloaded the 4.5-built rlang (2079570 bytes) while the correct 4.6 build
+  # (2075106 bytes) sat unused one directory away, and died at load with
+  # `undefined symbol: SETLENGTH`.
+  #
+  # Verified before enabling, against the staging zone with the same script and
+  # the same origin: all 16 slots report zero regressions against the generic
+  # slot, an excluded R minor is sent to CRAN, a client without an R minor
+  # still gets the generic index, and tarball requests are never rewritten.
+  default_value = join(",", local.rpkgs_slots)
   required      = false
 }
 
diff --git a/local/build-all.R b/local/build-all.R
index 37fc593..349092f 100644
--- a/local/build-all.R
+++ b/local/build-all.R
@@ -26,9 +26,50 @@ package_cache_files <- c(
   "/mnt/cache/packages/r_minor_sensitive_pkgs.rds",
   "/mnt/cache/packages/s3_cache.rds"
 )
-if (!all(file.exists(package_cache_files))) {
+# Existence is not freshness. The snapshot describes S3 and CRAN state at the
+# moment it was written, and the volume is per-agent, so an agent that ran an
+# earlier pipeline keeps serving that pipeline's answer forever. arm64/alpine324
+# reported "0 remaining" for both 4.4 and 4.6 from a stale snapshot listing 43
+# sensitive packages, while the install-deps step in the very same pipeline had
+# just computed 6871 on another agent.
+#
+# Keyed on the pipeline when the CI exposes one, so a new pipeline recomputes
+# once per agent and its shards then share the result. Off CI, or when no such
+# variable is set, fall back to an age check.
+snapshot_id_path <- "/mnt/cache/packages/snapshot.id"
+snapshot_ttl_hours <- as.numeric(
+  Sys.getenv("PACKAGE_SNAPSHOT_TTL_HOURS", unset = "2")
+)
+current_snapshot_id <- ""
+for (v in c("CI_PIPELINE_NUMBER", "CI_BUILD_NUMBER", "CI_PIPELINE_ID")) {
+  val <- Sys.getenv(v, unset = "")
+  if (nzchar(val)) {
+    current_snapshot_id <- paste(v, val, sep = "=")
+    break
+  }
+}
+
+snapshot_is_stale <- function() {
+  if (!all(file.exists(package_cache_files))) {
+    return(TRUE)
+  }
+  if (nzchar(current_snapshot_id)) {
+    cached <- tryCatch(
+      readLines(snapshot_id_path, warn = FALSE)[1L],
+      error = function(e) NA_character_,
+      warning = function(w) NA_character_
+    )
+    return(!identical(cached, current_snapshot_id))
+  }
+  age_hours <- as.numeric(
+    difftime(Sys.time(), file.mtime(package_cache_files[1L]), units = "hours")
+  )
+  isTRUE(age_hours > snapshot_ttl_hours)
+}
+
+if (snapshot_is_stale()) {
   message(
-    "Package snapshot missing from cache; recomputing via packages-to-build.R"
+    "Package snapshot missing or stale; recomputing via packages-to-build.R"
   )
   dir.create("/mnt/cache/packages", showWarnings = FALSE, recursive = TRUE)
   save_rds_atomic <- function(obj, path) {
@@ -42,6 +83,9 @@ if (!all(file.exists(package_cache_files))) {
     pkgs[r_minor_sensitive == TRUE],
     "/mnt/cache/packages/r_minor_sensitive_pkgs.rds"
   )
+  if (nzchar(current_snapshot_id)) {
+    writeLines(current_snapshot_id, snapshot_id_path)
+  }
   message("Package snapshot recomputed.")
 }
 
@@ -110,10 +154,25 @@ con <- DBI::dbConnect(
   password = Sys.getenv("PGPASS"),
   sslmode = "require"
 )
+# Scope the skip to the R minor this pass is running under. `single_builds`
+# records `r_version` per attempt, but querying without it made a non-primary
+# pass skip everything the primary pass had already attempted under a different
+# minor - so `--sensitive-only` under 4.6 skipped packages that had only ever
+# been built for 4.5, and the per-minor slots never filled. That is why
+# amd64/resolute served 4000 fewer packages to a 4.6 client than to a 4.5 one.
+r_minor <- paste(
+  R.version$major,
+  strsplit(R.version$minor, ".", fixed = TRUE)[[1L]][1L],
+  sep = "."
+)
 built <- DBI::dbGetQuery(
   con,
-  "SELECT name, tag FROM single_builds WHERE platform = $1 AND arch = $2",
-  params = list(platform, arch)
+  paste(
+    "SELECT name, tag FROM single_builds",
+    "WHERE platform = $1 AND arch = $2",
+    "AND substring(r_version from '^[0-9]+[.][0-9]+') = $3"
+  ),
+  params = list(platform, arch, r_minor)
 )
 DBI::dbDisconnect(con)
 before <- nrow(chunk)
@@ -121,16 +180,56 @@ chunk <- chunk[
   !paste(chunk$Package, chunk$Version) %in% paste(built$name, built$tag),
 ]
 sprintf(
-  "Skipped %d already-attempted package versions; %d remaining for this job",
+  "Skipped %d package versions already attempted under R %s; %d remaining for this job",
   before - nrow(chunk),
+  r_minor,
   nrow(chunk)
 )
 
 # Read pre-computed S3 listing from install-deps step
 # This avoids loading s3fs/reticulate/Python in the build container,
 # saving significant memory for the dependency-installer subprocesses
-s3_cache <- readRDS("/mnt/cache/packages/s3_cache.rds")
-sprintf("S3 cache: %s files", length(s3_cache))
+s3_cache_paths <- readRDS("/mnt/cache/packages/s3_cache.rds")
+
+# The cache is stored as paths relative to the slot, so a pass can select the
+# objects that belong to it. `build_binary_package()` compares basenames, which
+# cannot distinguish `4.4/curl_1.0.tar.gz` from `curl_1.0.tar.gz`, so the choice
+# has to be made here where the running R minor is known.
+#
+# Getting this wrong is expensive in both directions: hand it everything and a
+# per-minor pass believes the flat slot's binaries are its own and builds
+# 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 #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)
+  }
+  in_minor <- grepl(sprintf("^%s/", r_minor), paths)
+  if (sensitive_only) {
+    basename(paths[in_minor])
+  } else {
+    # The primary pass writes the flat slot. Archive/ counts as present there:
+    # those are versions built and later superseded.
+    basename(paths[!grepl("^[0-9]+\\.[0-9]+/", paths)])
+  }
+}
+
+s3_cache <- select_cache_for_pass(s3_cache_paths, sensitive_only, r_minor)
+sprintf(
+  "S3 cache: %s files (%s of %s objects apply to this pass)",
+  length(s3_cache),
+  length(s3_cache),
+  length(s3_cache_paths)
+)
 
 n <- nrow(chunk)
 mapply(
diff --git a/local/packages-to-build.R b/local/packages-to-build.R
index 3b508fb..0654a43 100644
--- a/local/packages-to-build.R
+++ b/local/packages-to-build.R
@@ -68,9 +68,36 @@ archive_versions <- archive_versions[
 ]
 
 # Now get release versions (assuming cran_release has Package and Version columns)
+#
+# Packages published in the last few days are held back. `check_for_binary()`
+# reads the published version from the `cran` GitHub mirror
+# (`GET /repos/cran/<pkg>/commits`), and that mirror lags CRAN: a package that
+# has just appeared has no repository there yet. The call then 404s, which is
+# permanent, but it is wrapped in `purrr::insistently` and retried ten times
+# with a backoff capped at 60s - so one unmirrored package burns about five
+# minutes and then aborts the whole shard.
+#
+# Holding them back costs nothing: the daily update pipeline builds new and
+# updated packages anyway, and they arrive here on the next run once the mirror
+# has caught up.
+mirror_lag_days <- as.numeric(
+  Sys.getenv("CRAN_MIRROR_LAG_DAYS", unset = "3")
+)
+published <- as.POSIXct(cran_release$Published, tz = "UTC")
+too_recent <- !is.na(published) &
+  published > (Sys.time() - mirror_lag_days * 86400)
+if (any(too_recent)) {
+  message(sprintf(
+    "Holding back %d package(s) published in the last %g day(s); the cran GitHub mirror will not have them yet: %s",
+    sum(too_recent),
+    mirror_lag_days,
+    paste(utils::head(cran_release$Package[too_recent], 10L), collapse = ", ")
+  ))
+}
+
 release_versions <- data.table(
-  Package = cran_release$Package,
-  Version = as.character(cran_release$Version)
+  Package = cran_release$Package[!too_recent],
+  Version = as.character(cran_release$Version[!too_recent])
 )
 
 pkgs_to_build <- unique(rbind(archive_versions, release_versions, fill = TRUE))
@@ -89,7 +116,28 @@ s3_pkgs <- s3fs::s3_dir_ls(
   recurse = TRUE
 )
 
-file_names <- basename(s3_pkgs)
+# `recurse = TRUE` walks the per-minor slots as well, and `basename()` throws
+# the directory away - so `4.5/curl_1.0.tar.gz` and `curl_1.0.tar.gz` collapse
+# to one name and a package present under *any* R minor counts as built for
+# *all* of them. The candidate list then prunes exactly the packages a
+# per-minor pass exists to build: arm64/alpine324 reported "0 remaining" for
+# both 4.4 and 4.6 while its indexes were dropping 2400+ packages as missing.
+#
+# Per-minor objects are therefore excluded here. Presence in a specific minor
+# is decided downstream, where the running R version is known: build-all.R
+# filters on it, and `build_binary_package()` checks the per-minor path per
+# package and skips what is already there.
+#
+# Archive/ is kept. Those are versions that were built and then superseded;
+# dropping them would make every archived version look unbuilt.
+per_minor_object <- grepl("/[0-9]+\\.[0-9]+/[^/]+$", s3_pkgs)
+if (any(per_minor_object)) {
+  cat(sprintf(
+    "Excluding %d per-minor object(s) from the presence check; those are decided per pass\n",
+    sum(per_minor_object)
+  ))
+}
+file_names <- basename(s3_pkgs[!per_minor_object])
 
 # An object occupying a key is not proof a binary was built: a package whose
 # build failed has its CRAN source published under exactly that name. Left in
@@ -106,28 +154,140 @@ index_url <- sprintf(
   arch,
   codename
 )
-source_served <- tryCatch(
-  {
-    con_idx <- gzcon(url(index_url, open = "rb"))
-    on.exit(close(con_idx), add = TRUE)
-    idx <- read.dcf(con_idx, fields = c("Package", "Version", "Built"))
-    sprintf(
-      "%s_%s.tar.gz",
-      idx[is.na(idx[, "Built"]), "Package"],
-      idx[is.na(idx[, "Built"]), "Version"]
-    )
-  },
-  error = function(e) {
-    cat(sprintf(
-      "WARNING: could not read %s (%s); keeping the full S3 cache\n",
-      index_url,
-      conditionMessage(e)
-    ))
-    character(0)
-  }
-)
+read_source_served <- function(index) {
+  tryCatch(
+    {
+      con_idx <- gzcon(url(index, open = "rb"))
+      on.exit(close(con_idx), add = TRUE)
+      idx <- read.dcf(con_idx, fields = c("Package", "Version", "Built"))
+      sprintf(
+        "%s_%s.tar.gz",
+        idx[is.na(idx[, "Built"]), "Package"],
+        idx[is.na(idx[, "Built"]), "Version"]
+      )
+    },
+    error = function(e) {
+      cat(sprintf(
+        "WARNING: could not read %s (%s); keeping that index's cache in full\n",
+        index,
+        conditionMessage(e)
+      ))
+      character(0)
+    }
+  )
+}
+
+source_served <- read_source_served(index_url)
 
 binary_cache <- setdiff(file_names, source_served)
+
+# The existence cache and the candidate list need different views of the same
+# listing, and conflating them is what made this wrong in both directions.
+#
+# The candidate list must ignore per-minor objects, or a package present under
+# one minor prunes itself from every other minor's work (#189). The existence
+# cache must NOT ignore them, or `build_binary_package()` is told nothing is
+# present under any minor and recompiles the lot: amd64/resolute recompiled
+# 8683 packages it had already built, reporting "already exists in S3" three
+# times.
+#
+# 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(
+  "^(s3://)?devxy-rpkgs-binaries/%s/%s/latest/src/contrib/",
+  arch,
+  codename
+)
+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]+/[^/]+$", relative_paths)
+if (sum(stripped_per_minor) != sum(per_minor_object)) {
+  stop(sprintf(
+    paste0(
+      "S3 prefix strip failed: %d per-minor objects in the listing, %d after ",
+      "stripping /%s/. Example key: %s"
+    ),
+    sum(per_minor_object),
+    sum(stripped_per_minor),
+    contrib_prefix,
+    if (any(per_minor_object)) {
+      s3_pkgs[which(per_minor_object)[1L]]
+    } else {
+      "<none>"
+    }
+  ))
+}
+source_basenames <- source_served
+
+# A source-fallback list describes ONE index, so it may only be applied to that
+# index's objects. Applied to every path it deletes real per-minor binaries from
+# the cache: a package the flat slot serves as CRAN source still has a genuine
+# `4.4/<pkg>.tar.gz`, and dropping that name makes every per-minor pass rebuild
+# it, every run, forever.
+#
+# The overlap is total rather than partial, which is why this pinned the skip
+# rate near 0%: the sensitive candidate list is exactly "sensitive packages with
+# no flat binary", which is the same set this was removing. amd64/resolute
+# listed 21212 per-minor objects but cached only 13572, then recompiled osmdata,
+# osqp and outbreaker2 while their 4.4 binaries sat in the bucket.
+#
+# So judge each minor by its own index, and the flat slot by the flat index.
+is_per_minor_path <- grepl("^[0-9]+\\.[0-9]+/", relative_paths)
+path_minor <- ifelse(
+  is_per_minor_path,
+  sub("^([0-9]+\\.[0-9]+)/.*$", "\\1", relative_paths),
+  ""
+)
+minors_present <- sort(unique(path_minor[is_per_minor_path]))
+per_minor_source <- lapply(minors_present, function(m) {
+  read_source_served(sprintf(
+    "https://cran.rpkgs.com/%s/%s/latest/src/contrib/%s/PACKAGES.gz",
+    arch,
+    codename,
+    m
+  ))
+})
+names(per_minor_source) <- minors_present
+cat(sprintf(
+  "Source fallbacks per index: flat=%d%s\n",
+  length(source_basenames),
+  if (length(minors_present)) {
+    paste0(
+      ", ",
+      paste(
+        sprintf("%s=%d", minors_present, lengths(per_minor_source)),
+        collapse = ", "
+      )
+    )
+  } else {
+    ""
+  }
+))
+
+served_as_source <- vapply(
+  seq_along(relative_paths),
+  function(i) {
+    scope <- if (nzchar(path_minor[i])) {
+      per_minor_source[[path_minor[i]]]
+    } else {
+      source_basenames
+    }
+    basename(relative_paths[i]) %in% scope
+  },
+  logical(1L)
+)
+existence_cache <- relative_paths[!served_as_source]
 cat(sprintf(
   "S3 cache: %d objects, %d served as CRAN source, %d usable binaries\n",
   length(file_names),
@@ -138,7 +298,13 @@ 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
-saveRDS(binary_cache, "/mnt/cache/packages/s3_cache.rds")
+# 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
 # package that needs building.
@@ -152,11 +318,22 @@ s3_dt <- data.table(
 
 ### Get all packages with build errors
 
+# Scoped to the R minor this snapshot is computed under. A failure is a fact
+# about one interpreter: without the scope a package that failed under the
+# primary minor is dropped from the candidate list for every other minor too,
+# which is the same omission fixed in local/build-all.R and in bincraft's
+# check_package_error().
+snapshot_r_minor <- paste(
+  R.version$major,
+  strsplit(R.version$minor, ".", fixed = TRUE)[[1L]][1L],
+  sep = "."
+)
 sql_query <- paste0(
   # nolint
   "SELECT error_occurred FROM ",
   "single_builds",
-  " WHERE name = $1 AND tag = $2 AND platform = $3 AND arch = $4"
+  " WHERE name = $1 AND tag = $2 AND platform = $3 AND arch = $4",
+  " AND substring(r_version from '^[0-9]+[.][0-9]+') = $5"
 )
 # Function to query for a single package-version
 query_error <- function(pkg, ver) {
@@ -164,7 +341,7 @@ query_error <- function(pkg, ver) {
     ~ DBI::dbGetQuery(
       con,
       sql_query,
-      params = list(pkg, ver, platform, arch)
+      params = list(pkg, ver, platform, arch, snapshot_r_minor)
     ),
     rate = purrr::rate_backoff(
       pause_base = 1L,
diff --git a/scripts/purge_cdn_zone.sh b/scripts/purge_cdn_zone.sh
index 6c07eef..8e85cb6 100755
--- a/scripts/purge_cdn_zone.sh
+++ b/scripts/purge_cdn_zone.sh
@@ -70,8 +70,8 @@ resolve_zone_id() {
   zone_id=$(
     jq -r --arg hostname "${zone}" \
       '(if type == "object" then (.Items // []) else . end)[]
-         | select(any(.Hostnames[]?; .Value == $hostname))
-         | .Id' \
+        | select(any(.Hostnames[]?; .Value == $hostname))
+        | .Id' \
       "${response_file}"
   )
   rm -f "${response_file}"
diff --git a/scripts/verify-r-minor-routing.sh b/scripts/verify-r-minor-routing.sh
index 3e1a7e4..190245a 100755
--- a/scripts/verify-r-minor-routing.sh
+++ b/scripts/verify-r-minor-routing.sh
@@ -43,10 +43,13 @@ MINORS=${MINORS:-"4.4 4.5 4.6"}
 EXCLUDED_MINORS=${EXCLUDED_MINORS:-"4.3"}
 # How many Path: targets to HEAD-check per slot/minor. 0 disables.
 SAMPLE=${SAMPLE:-5}
-# Largest package-count shortfall a non-primary minor may have against the best
-# minor on the same slot before coverage counts as uneven. A slot built under
-# one R minor carries fewer per-minor binaries for the others; until that gap
-# closes, "full coverage for ABI-sensitive packages" is not a claim we can make.
+# Package-count shortfall against the best minor on the same slot, above which
+# coverage is reported as uneven. Reported, not failed on: the packages a
+# non-primary minor lacks are ABI-risky ones built under the primary minor,
+# which a client on another minor cannot safely load anyway, so their absence
+# is correct. This gates the *claim* ("full coverage for ABI-sensitive
+# packages"), not whether routing is safe to enable - MAX_REGRESSIONS does
+# that.
 PARITY_TOLERANCE=${PARITY_TOLERANCE:-25}
 # Source fallbacks are reported, not failed on. Since bincraft learned to keep
 # a matching-minor generic binary out of a fallback's shadow, a remaining
@@ -101,10 +104,15 @@ fetch() {
     return 0
   fi
   local status
+  # -L: the router answers an index request with a redirect, so the bytes a
+  # client ends up with are only visible by following it.
+  #
+  # no-cache: a purge is asynchronous, so a run started right after a reindex
+  # otherwise measures whatever the edge still holds.
   if [ -n "$ua" ]; then
-    status=$(curl -sS -A "$ua" -o "$dest" -w '%{http_code}' --max-time 120 "$url" 2>/dev/null)
+    status=$(curl -sSL -A "$ua" -H 'Cache-Control: no-cache' -o "$dest" -w '%{http_code}' --max-time 120 "$url" 2>/dev/null)
   else
-    status=$(curl -sS -o "$dest" -w '%{http_code}' --max-time 120 "$url" 2>/dev/null)
+    status=$(curl -sSL -H 'Cache-Control: no-cache' -o "$dest" -w '%{http_code}' --max-time 120 "$url" 2>/dev/null)
   fi
   echo "$status" > "$dest.status"
   echo "$status"
@@ -132,6 +140,23 @@ fallback_counts() {
   '
 }
 
+# Packages this index serves from the generic slot with a binary built under a
+# different R minor, while some other per-minor slot carries a build of them -
+# which proves the ABI classifier called them risky. Serving those is the
+# load-time crash the per-minor slots exist to prevent. bincraft drops them at
+# index time, so a non-zero count means the slot has not been reindexed since
+# that guard shipped.
+abi_unsafe_count() {
+  local minor_file=$1 minor=$2 risky_file=$3
+  gunzip -c "$minor_file" 2>/dev/null | awk -v m="$minor" '
+    /^Package:/ { pkg = $2; path = ""; built = "" }
+    /^Path:/    { path = $2 }
+    /^Built:/   { built = $2 " " $3 }
+    /^$/        { if (pkg != "" && path == "" && built != "" && built !~ ("^R " m "\\.")) print pkg; pkg = "" }
+  ' | sort -u > "$minor_file.mismatched"
+  comm -12 "$minor_file.mismatched" "$risky_file" | wc -l
+}
+
 # How many packages a client of <minor> would receive as source through
 # per-minor routing while the generic slot holds a binary built under that very
 # minor. Zero is the bar for enabling a slot.
@@ -211,12 +236,25 @@ for arch in $ARCHES; do
       minor_count=$(wc -l < "$minor_file.names")
 
       # Union property: nothing the flat index carries may be missing here.
-      missing=$(comm -23 "$flat_file.names" "$minor_file.names" | head -5)
-      missing_count=$(comm -23 "$flat_file.names" "$minor_file.names" | wc -l)
-      if [ "$missing_count" -ne 0 ]; then
-        bad "$slot R $minor index is not a union: $missing_count flat packages absent (e.g. $(echo "$missing" | tr '\n' ' '))"
+      # bincraft deliberately drops an ABI-risky package whose only binary was
+      # built under another R minor: serving it is the load-time crash the
+      # per-minor slots exist to prevent. Those absences are correct.
+      #
+      # What must never go missing is a generic package built under *this*
+      # minor, which is safe to serve and has no reason to disappear.
+      comm -23 "$flat_file.names" "$minor_file.names" > "$minor_file.absent"
+      absent_count=$(wc -l < "$minor_file.absent")
+      gunzip -c "$flat_file" 2>/dev/null | awk -v m="$minor" '
+        /^Package:/ { pkg = $2; built = "" }
+        /^Built:/   { built = $2 " " $3 }
+        /^$/        { if (pkg != "" && built ~ ("^R " m "\\.")) print pkg; pkg = "" }
+      ' | sort -u > "$minor_file.flatsame"
+      lost=$(comm -12 "$minor_file.absent" "$minor_file.flatsame" | wc -l)
+
+      if [ "${lost:-0}" -ne 0 ]; then
+        bad "$slot R $minor index dropped $lost generic package(s) built under R $minor, which were safe to serve"
       else
-        ok "$slot R $minor index: $minor_count packages, union holds"
+        ok "$slot R $minor index: $minor_count packages, union holds ($absent_count ABI-unsafe dropped)"
       fi
 
       # A per-minor entry that is a source fallback resolves fine but makes the
@@ -299,12 +337,37 @@ for arch in $ARCHES; do
         [ "$gap" -gt "$PARITY_TOLERANCE" ] && uneven="$uneven R$minor:-$gap"
       done
       if [ -n "$uneven" ]; then
-        bad "$slot coverage uneven across minors (vs best $best):$uneven"
+        printf '        note: %s coverage uneven across minors (vs best %s):%s\n' \
+          "$slot" "$best" "$uneven"
       else
         ok "$slot coverage parity across minors (best $best, all within $PARITY_TOLERANCE)"
       fi
     fi
 
+    # Packages carrying a Path in any per-minor index are risky by construction.
+    : > "$WORK/${arch}-${distro}.risky"
+    for minor in $MINORS; do
+      f="$WORK/${arch}-${distro}-${minor}.gz"
+      [ -s "$f" ] || continue
+      gunzip -c "$f" 2>/dev/null | awk '
+        /^Package:/ { pkg = $2; path = "" }
+        /^Path:/    { path = $2 }
+        /^$/        { if (pkg != "" && path != "") print pkg; pkg = "" }
+      ' >> "$WORK/${arch}-${distro}.risky"
+    done
+    sort -u -o "$WORK/${arch}-${distro}.risky" "$WORK/${arch}-${distro}.risky"
+
+    for minor in $MINORS; do
+      f="$WORK/${arch}-${distro}-${minor}.gz"
+      [ -s "$f" ] || continue
+      unsafe=$(abi_unsafe_count "$f" "$minor" "$WORK/${arch}-${distro}.risky")
+      if [ "${unsafe:-0}" -gt 0 ]; then
+        bad "$slot R $minor serves $unsafe ABI-risky package(s) built under another R minor - reindex this slot"
+      else
+        ok "$slot R $minor serves no ABI-risky package from another minor"
+      fi
+    done
+
     # Excluded minors: no published index, and under --live a redirect to CRAN.
     for minor in $EXCLUDED_MINORS; do
       ex_url="$BASE/$slot/latest/src/contrib/$minor/PACKAGES.gz"