diff --git a/hooks/assemble-agents.sh b/hooks/assemble-agents.sh
index 78ad68f..ca42f7d 100755
--- a/hooks/assemble-agents.sh
+++ b/hooks/assemble-agents.sh
@@ -27,9 +27,15 @@ case "$FILE" in
         "$ASSEMBLER" --in-place "$FILE" 2>&1 | sed 's/^/[assemble-agents] /'
         ;;
     */agents/_blocks/*.md)
-        # Block changed → rebuild everything (block is shared)
+        # Block changed → rebuild everything (block is shared).
+        # Always surface FAIL/ERROR lines; truncate only the OK tail.
         echo "[assemble-agents] block changed, rebuilding all agents..."
-        "$ASSEMBLER" --in-place 2>&1 | sed 's/^/[assemble-agents] /' | head -40
+        OUTPUT=$("$ASSEMBLER" --in-place 2>&1 || true)
+        FAILS=$(printf '%s\n' "$OUTPUT" | grep -E '^(FAIL|ERROR)' || true)
+        if [ -n "$FAILS" ]; then
+            printf '%s\n' "$FAILS" | sed 's/^/[assemble-agents] /'
+        fi
+        printf '%s\n' "$OUTPUT" | sed 's/^/[assemble-agents] /' | head -40
         ;;
     *)
         exit 0