diff --git a/src/cluster/cluster.rs b/src/cluster/cluster.rs
index 31d42fb..122945e 100644
--- a/src/cluster/cluster.rs
+++ b/src/cluster/cluster.rs
@@ -37,7 +37,10 @@ pub struct Cluster {
 
 impl Cluster {
     pub fn new(config: &Config, model: &ModelConfig) -> Self {
-        let total_instances = config.cluster.total_instances();
+        let total_instances = config
+            .cluster
+            .require_legacy_single_pool("Cluster::new")
+            .unwrap_or_else(|err| panic!("{err}"));
         let mut instances = Vec::with_capacity(total_instances as usize);
         for id in 0..total_instances {
             instances.push(Instance::new(
@@ -185,8 +188,8 @@ impl Cluster {
 mod tests {
     use super::*;
     use crate::config::{
-        CalibrationConfig, ClusterConfig, Config, HardwareConfig, MetaStoreConfig, ModelConfig,
-        RouterConfig, RouterMode, SimConfig,
+        BucketConfig, CalibrationConfig, ClusterConfig, Config, HardwareConfig, MetaStoreConfig,
+        ModelConfig, RouterConfig, RouterMode, SimConfig,
     };
     use crate::trace::RequestRecord;
 
@@ -285,4 +288,29 @@ mod tests {
 
         assert!(stats.ready_at > pure_pcie);
     }
+
+    #[test]
+    fn cluster_new_rejects_bucketed_configs() {
+        let mut cfg = test_config(RouterMode::EstimatedTtft);
+        cfg.cluster.num_instances = None;
+        cfg.cluster.buckets = vec![BucketConfig {
+            name: "short".into(),
+            input_length_min: 0,
+            input_length_max: 64,
+            num_instances: 2,
+        }];
+
+        let panic = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
+            Cluster::new(&cfg, &cfg.model);
+        }))
+        .expect_err("bucketed Cluster::new should panic");
+
+        let msg = panic
+            .downcast_ref::<String>()
+            .cloned()
+            .or_else(|| panic.downcast_ref::<&str>().map(|s| (*s).to_string()))
+            .expect("panic payload should be a string");
+        assert!(msg.contains("Cluster::new"));
+        assert!(msg.contains("cluster.buckets"));
+    }
 }