diff --git a/scripts/verify-r-minor-routing.sh b/scripts/verify-r-minor-routing.sh
index 28acd92..cad6c38 100755
--- a/scripts/verify-r-minor-routing.sh
+++ b/scripts/verify-r-minor-routing.sh
@@ -48,6 +48,11 @@ SAMPLE=${SAMPLE:-5}
 # one R minor carries fewer per-minor binaries for the others; until that gap
 # closes, "full coverage for ABI-sensitive packages" is not a claim we can make.
 PARITY_TOLERANCE=${PARITY_TOLERANCE:-25}
+# Largest share (percent) of per-minor entries that may be source fallbacks
+# rather than binaries. A source fallback resolves with HTTP 200 and installs
+# correctly, but it compiles on the client: routing to it is not the binary
+# service we advertise. Entries without a `Built:` field are source fallbacks.
+SOURCE_FALLBACK_TOLERANCE=${SOURCE_FALLBACK_TOLERANCE:-10}
 LIVE=0
 
 for arg in "$@"; do
@@ -108,6 +113,19 @@ pkg_names() {
   gunzip -c "$1" 2>/dev/null | awk '/^Package:/ {print $2}' | sort -u
 }
 
+# "<steered> <source-fallbacks>" for a per-minor index: how many entries carry a
+# Path: field, and how many of those lack a Built: field (i.e. are sources).
+fallback_counts() {
+  gunzip -c "$1" 2>/dev/null | awk '
+    /^Package:/ { pkg = $2; path = ""; built = "" }
+    /^Path:/    { path = $2 }
+    /^Built:/   { built = $2 }
+    /^$/        { if (pkg != "" && path != "") { n++; if (built == "") s++ } pkg = "" }
+    END         { if (pkg != "" && path != "") { n++; if (built == "") s++ }
+                  printf "%d %d\n", n, s }
+  '
+}
+
 # "<Package> <Path>" pairs for entries that carry a Path: field.
 path_entries() {
   gunzip -c "$1" 2>/dev/null | awk '
@@ -176,6 +194,19 @@ for arch in $ARCHES; do
         ok "$slot R $minor index: $minor_count packages, union holds"
       fi
 
+      # A per-minor entry that is a source fallback resolves fine but makes the
+      # client compile. Routing to a slot that is mostly fallbacks does not
+      # deliver the binaries we advertise.
+      read -r steered fallbacks <<< "$(fallback_counts "$minor_file")"
+      if [ "${steered:-0}" -gt 0 ]; then
+        pct=$((fallbacks * 100 / steered))
+        if [ "$pct" -gt "$SOURCE_FALLBACK_TOLERANCE" ]; then
+          bad "$slot R $minor: $fallbacks/$steered per-minor entries are source fallbacks (${pct}%), not binaries"
+        else
+          ok "$slot R $minor: $((steered - fallbacks))/$steered per-minor entries are binaries"
+        fi
+      fi
+
       # Path: entries steer to per-minor binaries; they must resolve.
       if [ "$SAMPLE" -gt 0 ]; then
         path_entries "$minor_file" > "$minor_file.paths"