diff --git a/src/cluster/bucketed_service.rs b/src/cluster/bucketed_service.rs
new file mode 100644
index 0000000..633f851
--- /dev/null
+++ b/src/cluster/bucketed_service.rs
@@ -0,0 +1,216 @@
+use anyhow::Result;
+
+use super::cluster::{AdmissionStats, Cluster};
+use crate::config::{BucketConfig, Config, ModelConfig};
+use crate::instance::Instance;
+use crate::router::{self, BucketId, GlobalRouter};
+use crate::trace::RequestRecord;
+
+pub struct ServiceBucket {
+    pub id: BucketId,
+    pub cfg: BucketConfig,
+    pub cluster: Cluster,
+}
+
+impl ServiceBucket {
+    pub fn instances(&self) -> &[Instance] {
+        &self.cluster.instances
+    }
+}
+
+pub struct BucketedService {
+    pub buckets: Vec<ServiceBucket>,
+    pub global_router: Box<dyn GlobalRouter>,
+}
+
+impl BucketedService {
+    pub fn new(config: &Config, model: &ModelConfig) -> Self {
+        let buckets = config
+            .cluster
+            .effective_buckets()
+            .into_iter()
+            .enumerate()
+            .map(|(idx, cfg)| ServiceBucket {
+                id: idx as BucketId,
+                cluster: Cluster::new_for_bucket(config, model, idx as BucketId, cfg.num_instances)
+                    .expect("bucket-local cluster construction should succeed"),
+                cfg,
+            })
+            .collect();
+
+        Self {
+            buckets,
+            global_router: router::build_global(config),
+        }
+    }
+
+    pub fn bucket(&self, bucket_id: BucketId) -> &ServiceBucket {
+        &self.buckets[bucket_id as usize]
+    }
+
+    pub fn route_and_admit(&mut self, req: &RequestRecord, now: f64) -> Result<AdmissionStats> {
+        let bucket_views = self
+            .buckets
+            .iter()
+            .map(|bucket| bucket.cluster.bucket_view(bucket.id, &bucket.cfg, req, now))
+            .collect::<Vec<_>>();
+        let global = self.global_router.route(req, &bucket_views, now)?;
+        let bucket = &mut self.buckets[global.chosen_bucket as usize];
+        Ok(bucket
+            .cluster
+            .route_and_admit_with_global(req, now, &global))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::config::{
+        BucketConfig, CalibrationConfig, ClusterConfig, Config, GlobalRouterConfig,
+        GlobalRouterMode, HardwareConfig, MetaStoreConfig, ModelConfig, RouterConfig, RouterMode,
+        SimConfig,
+    };
+    use crate::trace::RequestRecord;
+
+    fn test_config() -> Config {
+        Config {
+            model: ModelConfig {
+                name: "test".into(),
+                num_layers: 4,
+                num_kv_heads: 2,
+                head_dim: 64,
+                dtype_bytes: 2,
+                block_size_tokens: 16,
+                flops_per_token_prefill: Some(1.0e9),
+                attn_quadratic_coeff: Some(64.0),
+                ..Default::default()
+            },
+            hardware: HardwareConfig {
+                gpu_flops: 1.0e14,
+                gpu_fp8_flops: 0.0,
+                gpu_fp4_flops: 0.0,
+                gpu_mem_bw: 1.0e12,
+                hbm_bytes: 1.0e9,
+                dram_bytes: 4.0e9,
+                host_dram_bw: 5.0e11,
+                pcie_bw: 32.0e9,
+                pcie_latency_us: 1.0,
+                rdma_bw: 12.0e9,
+                rdma_latency_us: 5.0,
+                intra_node_tp_bw: 9.0e11,
+                intra_node_tp_latency_us: 2.0,
+                tp_degree: 1,
+                max_batch_slots: 32,
+                prefill_chunk_tokens: 1024,
+            },
+            calibration: CalibrationConfig::default(),
+            cluster: ClusterConfig {
+                num_instances: None,
+                buckets: vec![
+                    BucketConfig {
+                        name: "short".into(),
+                        input_length_min: 0,
+                        input_length_max: 32,
+                        num_instances: 2,
+                    },
+                    BucketConfig {
+                        name: "long".into(),
+                        input_length_min: 33,
+                        input_length_max: 96,
+                        num_instances: 1,
+                    },
+                ],
+                global_router: GlobalRouterConfig {
+                    mode: GlobalRouterMode::StrictInputLength,
+                    length_penalty_weight: 1.0,
+                    load_weight: 1.0,
+                    cache_weight: 1.0,
+                },
+                meta_store: MetaStoreConfig {
+                    ttl_seconds: 1000.0,
+                },
+                router: RouterConfig {
+                    mode: RouterMode::LeastLoaded,
+                    precise_probe_latency_us: 10.0,
+                    precise_probe_topk: 2,
+                    load_alpha: 0.0,
+                    score_alpha: 1.0,
+                    score_beta: 0.1,
+                    prefix_k: 8,
+                    affinity_fan_out: 2,
+                },
+            },
+            sim: SimConfig {
+                trace_path: String::new(),
+                max_requests: None,
+                output_dir: String::new(),
+                sample_interval_s: 0.0,
+                seed: 7,
+                input_length_min: None,
+                input_length_max: None,
+            },
+        }
+    }
+
+    fn req(req_id: u64, input_len: u32, hashes: &[u64]) -> RequestRecord {
+        RequestRecord {
+            req_id,
+            chat_id: req_id as i64,
+            parent_chat_id: -1,
+            turn: 0,
+            arrival: 0.0,
+            input_len,
+            output_len: 16,
+            hash_ids: hashes.to_vec(),
+        }
+    }
+
+    #[test]
+    fn strict_input_length_routes_into_matching_bucket() {
+        let cfg = test_config();
+        let mut service = BucketedService::new(&cfg, &cfg.model);
+        let stats = service
+            .route_and_admit(&req(1, 24, &[10, 11]), 0.0)
+            .unwrap();
+        assert_eq!(stats.bucket, 0);
+        assert_eq!(stats.decision.chosen_bucket, 0);
+        assert_eq!(
+            stats.decision.global_reason,
+            "unique bucket range contains input_length"
+        );
+        assert_eq!(
+            stats.decision.local_reason,
+            "argmin(kv_used + alpha * queue_len)"
+        );
+        assert_eq!(service.bucket(0).instances().len(), 2);
+    }
+
+    #[test]
+    fn bucket_meta_store_is_isolated() {
+        let cfg = test_config();
+        let mut service = BucketedService::new(&cfg, &cfg.model);
+        let _ = service
+            .route_and_admit(&req(1, 24, &[10, 11]), 0.0)
+            .unwrap();
+        let long_stats = service
+            .route_and_admit(&req(2, 64, &[10, 11, 12, 13]), 1.0)
+            .unwrap();
+        assert_eq!(long_stats.bucket, 1);
+        assert_eq!(long_stats.remote_hit_blocks, 0);
+        assert_eq!(long_stats.l1_hit_blocks, 0);
+    }
+
+    #[test]
+    fn unmatched_input_length_returns_recoverable_error() {
+        let mut cfg = test_config();
+        cfg.cluster.buckets[1].input_length_min = 40;
+        let mut service = BucketedService::new(&cfg, &cfg.model);
+
+        let err = service
+            .route_and_admit(&req(3, 36, &[20, 21, 22]), 0.0)
+            .unwrap_err();
+
+        assert!(err.to_string().contains("no bucket"));
+        assert!(err.to_string().contains("input_length=36"));
+    }
+}
diff --git a/src/cluster/cluster.rs b/src/cluster/cluster.rs
index 9927cdc..322b867 100644
--- a/src/cluster/cluster.rs
+++ b/src/cluster/cluster.rs
@@ -1,18 +1,21 @@
 //! Cluster: routes arrivals, performs the L0 / L1 / remote-RDMA fetch chain
 //! described in the design diagram, and bookkeeps the global meta store.
 
+use anyhow::Result;
+
 use crate::cluster::meta_store::MetaStore;
-use crate::config::{Config, ModelConfig};
+use crate::config::{BucketConfig, Config, ModelConfig};
 use crate::instance::instance::AdmittedRequest;
 use crate::instance::kv_cache::L1Change;
 use crate::instance::Instance;
-use crate::router::{self, RouteDecision, Router};
+use crate::router::{self, BucketId, BucketView, GlobalRouteDecision, RouteDecision, Router};
 use crate::trace::RequestRecord;
 use crate::ttft::{classify_prefix_tiers, TtftModel};
 use crate::types::InstanceId;
 
 #[derive(Debug, Clone)]
 pub struct AdmissionStats {
+    pub bucket: BucketId,
     pub instance: InstanceId,
     pub l0_hit_blocks: u32,
     pub l1_hit_blocks: u32,
@@ -36,39 +39,41 @@ pub struct Cluster {
 }
 
 impl Cluster {
-    pub fn new(config: &Config, model: &ModelConfig) -> Self {
-        let mut instances = Vec::with_capacity(config.cluster.num_instances as usize);
-        for id in 0..config.cluster.num_instances {
-            instances.push(Instance::new(
-                id as InstanceId,
-                model,
-                &config.hardware,
-                &config.calibration,
-            ));
-        }
-        let meta_store = MetaStore::new(config.cluster.meta_store.ttl_seconds);
-        let router = router::build(config, config.sim.seed);
-        Self {
-            instances,
-            meta_store,
-            router,
-            block_size_tokens: model.block_size_tokens,
-            kv_block_bytes: model.kv_block_bytes(),
-            ttft_model: TtftModel::new(
-                &config.hardware,
-                &config.calibration,
-                model.kv_block_bytes(),
-            ),
-        }
+    pub fn new(config: &Config, model: &ModelConfig) -> Result<Self> {
+        let total_instances = config.cluster.require_legacy_single_pool("Cluster::new")?;
+        Self::build_local_cluster(config, model, total_instances)
+    }
+
+    pub fn new_for_bucket(
+        config: &Config,
+        model: &ModelConfig,
+        _bucket_id: BucketId,
+        num_instances: u32,
+    ) -> Result<Self> {
+        let mut local_config = config.clone();
+        local_config.cluster.num_instances = Some(num_instances);
+        local_config.cluster.buckets.clear();
+        Self::build_local_cluster(&local_config, model, num_instances)
     }
 
     /// Route + admit a request. Returns the chosen instance plus rich
     /// per-request stats for metrics. Does NOT schedule the BatchTick — the
     /// simulator driver does that based on the returned `ready_at`.
     pub fn route_and_admit(&mut self, req: &RequestRecord, now: f64) -> AdmissionStats {
+        let global = GlobalRouteDecision::single_bucket(req.req_id, 0);
+        self.route_and_admit_with_global(req, now, &global)
+    }
+
+    pub fn route_and_admit_with_global(
+        &mut self,
+        req: &RequestRecord,
+        now: f64,
+        global: &GlobalRouteDecision,
+    ) -> AdmissionStats {
         let decision = self
             .router
-            .route(req, &self.instances, &self.meta_store, now);
+            .route(req, &self.instances, &self.meta_store, now)
+            .with_global(global);
         let inst_id = decision.chosen;
         let probe_overhead_s = decision.probe_overhead_s;
         let scheduler_overhead_s = self
@@ -151,6 +156,7 @@ impl Cluster {
         let fetch_time_s = (t - effective_now).max(0.0);
 
         AdmissionStats {
+            bucket: decision.chosen_bucket,
             instance: inst_id,
             l0_hit_blocks: l0_hits,
             l1_hit_blocks: l1_hits,
@@ -165,6 +171,30 @@ impl Cluster {
         }
     }
 
+    pub fn bucket_view(
+        &self,
+        bucket_id: BucketId,
+        cfg: &BucketConfig,
+        req: &RequestRecord,
+        now: f64,
+    ) -> BucketView {
+        let predicted_prefix = self
+            .meta_store
+            .score_prefix(&req.hash_ids, now, self.instances.len())
+            .into_iter()
+            .max()
+            .unwrap_or(0);
+        BucketView {
+            id: bucket_id,
+            input_length_min: cfg.input_length_min,
+            input_length_max: cfg.input_length_max,
+            num_instances: self.instances.len() as u32,
+            total_queue_len: self.instances.iter().map(Instance::queue_len).sum(),
+            total_load_blocks: self.instances.iter().map(|inst| inst.kv_blocks_used).sum(),
+            predicted_prefix,
+        }
+    }
+
     fn apply_l1_changes(
         meta_store: &mut MetaStore,
         inst_id: InstanceId,
@@ -178,14 +208,44 @@ impl Cluster {
             }
         }
     }
+
+    fn build_local_cluster(
+        config: &Config,
+        model: &ModelConfig,
+        num_instances: u32,
+    ) -> Result<Self> {
+        let mut instances = Vec::with_capacity(num_instances as usize);
+        for id in 0..num_instances {
+            instances.push(Instance::new(
+                id as InstanceId,
+                model,
+                &config.hardware,
+                &config.calibration,
+            ));
+        }
+        let meta_store = MetaStore::new(config.cluster.meta_store.ttl_seconds);
+        let router = router::build(config, config.sim.seed);
+        Ok(Self {
+            instances,
+            meta_store,
+            router,
+            block_size_tokens: model.block_size_tokens,
+            kv_block_bytes: model.kv_block_bytes(),
+            ttft_model: TtftModel::new(
+                &config.hardware,
+                &config.calibration,
+                model.kv_block_bytes(),
+            ),
+        })
+    }
 }
 
 #[cfg(test)]
 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;
 
@@ -226,7 +286,9 @@ mod tests {
                 ..CalibrationConfig::default()
             },
             cluster: ClusterConfig {
-                num_instances: 1,
+                num_instances: Some(1),
+                buckets: Vec::new(),
+                global_router: Default::default(),
                 meta_store: MetaStoreConfig {
                     ttl_seconds: 1000.0,
                 },
@@ -256,7 +318,7 @@ mod tests {
     #[test]
     fn l1_ready_at_includes_dram_and_transform_overhead() {
         let cfg = test_config(RouterMode::EstimatedTtft);
-        let mut cluster = Cluster::new(&cfg, &cfg.model);
+        let mut cluster = Cluster::new(&cfg, &cfg.model).unwrap();
         let req = RequestRecord {
             req_id: 1,
             chat_id: 0,
@@ -282,4 +344,22 @@ 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 result = Cluster::new(&cfg, &cfg.model);
+        assert!(result.is_err(), "bucketed Cluster::new should fail");
+        let err = result.err().unwrap();
+        assert!(err.to_string().contains("Cluster::new"));
+        assert!(err.to_string().contains("cluster.buckets"));
+    }
 }
diff --git a/src/cluster/mod.rs b/src/cluster/mod.rs
index bc36667..01893e8 100644
--- a/src/cluster/mod.rs
+++ b/src/cluster/mod.rs
@@ -1,6 +1,8 @@
+pub mod bucketed_service;
 #[allow(clippy::module_inception)]
 pub mod cluster;
 pub mod meta_store;
 
+pub use bucketed_service::BucketedService;
 pub use cluster::Cluster;
 pub use meta_store::MetaStore;
diff --git a/src/config.rs b/src/config.rs
index 562ac5e..087b960 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -379,11 +379,76 @@ fn default_first_token_ready_us() -> f64 {
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ClusterConfig {
-    pub num_instances: u32,
+    #[serde(default)]
+    pub num_instances: Option<u32>,
+    #[serde(default)]
+    pub buckets: Vec<BucketConfig>,
+    #[serde(default)]
+    pub global_router: GlobalRouterConfig,
     pub meta_store: MetaStoreConfig,
     pub router: RouterConfig,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq, Eq)]
+pub struct BucketConfig {
+    pub name: String,
+    pub input_length_min: u32,
+    pub input_length_max: u32,
+    pub num_instances: u32,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+pub struct GlobalRouterConfig {
+    #[serde(default)]
+    pub mode: GlobalRouterMode,
+    #[serde(default = "default_global_router_length_penalty_weight")]
+    pub length_penalty_weight: f64,
+    #[serde(default = "default_global_router_load_weight")]
+    pub load_weight: f64,
+    #[serde(default = "default_global_router_cache_weight")]
+    pub cache_weight: f64,
+}
+
+impl Default for GlobalRouterConfig {
+    fn default() -> Self {
+        Self {
+            mode: GlobalRouterMode::StrictInputLength,
+            length_penalty_weight: default_global_router_length_penalty_weight(),
+            load_weight: default_global_router_load_weight(),
+            cache_weight: default_global_router_cache_weight(),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
+#[serde(rename_all = "snake_case")]
+pub enum GlobalRouterMode {
+    #[default]
+    StrictInputLength,
+    BucketScore,
+}
+
+impl GlobalRouterMode {
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            Self::StrictInputLength => "strict_input_length",
+            Self::BucketScore => "bucket_score",
+        }
+    }
+}
+
+fn default_global_router_length_penalty_weight() -> f64 {
+    1.0
+}
+
+fn default_global_router_load_weight() -> f64 {
+    1.0
+}
+
+fn default_global_router_cache_weight() -> f64 {
+    1.0
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct MetaStoreConfig {
     pub ttl_seconds: f64,
@@ -432,6 +497,105 @@ fn default_prefix_k() -> usize {
     8
 }
 
+impl ClusterConfig {
+    pub fn require_legacy_single_pool(&self, context: &str) -> Result<u32> {
+        if !self.buckets.is_empty() {
+            anyhow::bail!("{context} does not support cluster.buckets until Task 2 lands");
+        }
+
+        self.legacy_num_instances()
+            .ok_or_else(|| anyhow::anyhow!("{context} requires cluster.num_instances"))
+    }
+
+    pub fn legacy_num_instances(&self) -> Option<u32> {
+        if self.buckets.is_empty() {
+            self.num_instances
+        } else {
+            None
+        }
+    }
+
+    pub fn total_instances(&self) -> u32 {
+        self.legacy_num_instances()
+            .unwrap_or_else(|| self.buckets.iter().map(|bucket| bucket.num_instances).sum())
+    }
+
+    pub fn bucket_index_for_input_len(&self, input_len: u32) -> Result<usize> {
+        if self.buckets.is_empty() {
+            return Ok(0);
+        }
+
+        self.buckets
+            .iter()
+            .position(|bucket| {
+                bucket.input_length_min <= input_len && input_len <= bucket.input_length_max
+            })
+            .ok_or_else(|| {
+                anyhow::anyhow!(
+                    "cluster.global_router.mode={} has no bucket for input_length={input_len}",
+                    self.global_router.mode.as_str()
+                )
+            })
+    }
+
+    pub fn effective_buckets(&self) -> Vec<BucketConfig> {
+        if !self.buckets.is_empty() {
+            return self.buckets.clone();
+        }
+
+        vec![BucketConfig {
+            name: "default".to_string(),
+            input_length_min: 0,
+            input_length_max: u32::MAX,
+            num_instances: self
+                .num_instances
+                .expect("legacy single-pool cluster must have num_instances"),
+        }]
+    }
+
+    pub fn validate(&self) -> Result<()> {
+        if self.num_instances.is_some() && !self.buckets.is_empty() {
+            anyhow::bail!("cluster.num_instances and cluster.buckets are mutually exclusive");
+        }
+
+        if self.buckets.is_empty() {
+            let num_instances = self.num_instances.ok_or_else(|| {
+                anyhow::anyhow!("cluster must set either num_instances or buckets")
+            })?;
+            anyhow::ensure!(num_instances > 0, "cluster.num_instances must be positive");
+            return Ok(());
+        }
+
+        for bucket in &self.buckets {
+            anyhow::ensure!(
+                bucket.input_length_min <= bucket.input_length_max,
+                "cluster bucket '{}' has input_length_min > input_length_max",
+                bucket.name
+            );
+            anyhow::ensure!(
+                bucket.num_instances > 0,
+                "cluster bucket '{}' must have num_instances > 0",
+                bucket.name
+            );
+        }
+
+        let mut sorted = self.buckets.iter().collect::<Vec<_>>();
+        sorted.sort_by_key(|bucket| (bucket.input_length_min, bucket.input_length_max));
+        for pair in sorted.windows(2) {
+            let prev = pair[0];
+            let next = pair[1];
+            anyhow::ensure!(
+                prev.input_length_max < next.input_length_min,
+                "cluster buckets '{}' and '{}' overlap",
+                prev.name,
+                next.name
+            );
+        }
+
+        Ok(())
+    }
+}
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 #[serde(rename_all = "snake_case")]
 pub enum RouterMode {
@@ -544,7 +708,7 @@ impl Config {
             .with_context(|| format!("parsing config {}", path.display()))?;
         let yaml_dir = path.parent().unwrap_or(Path::new("."));
         raw.resolve(yaml_dir)
-            .with_context(|| format!("resolving config {}", path.display()))
+            .map_err(|err| anyhow::anyhow!("resolving config {}: {err}", path.display()))
     }
 }
 
@@ -678,7 +842,10 @@ impl RawConfig {
             model,
             hardware,
             calibration: self.calibration,
-            cluster: self.cluster,
+            cluster: {
+                self.cluster.validate()?;
+                self.cluster
+            },
             sim: self.sim,
         })
     }
@@ -1003,4 +1170,298 @@ sim:
         assert_eq!(cfg.model.weight_dtype.as_deref(), Some("fp4"));
         assert!((cfg.model.weight_dtype_bytes() - 0.5).abs() < 1e-12);
     }
+
+    #[test]
+    fn bucketed_config_loads_and_preserves_legacy_single_pool() {
+        let legacy = write_temp_config(
+            r#"
+model:
+  name: test
+  num_layers: 4
+  num_kv_heads: 2
+  head_dim: 64
+  dtype_bytes: 2
+  block_size_tokens: 16
+  flops_per_token_prefill: 1.0e9
+  attn_quadratic_coeff: 64.0
+hardware:
+  gpu_flops: 1.0e14
+  gpu_mem_bw: 1.0e12
+  hbm_bytes: 1.0e9
+cluster:
+  num_instances: 2
+  meta_store:
+    ttl_seconds: 10.0
+  router:
+    mode: cache_affinity
+  global_router:
+    mode: strict_input_length
+sim:
+  trace_path: trace.jsonl
+  output_dir: runs/test
+"#,
+        );
+        let cfg = Config::from_yaml_path(&legacy).unwrap();
+        assert_eq!(cfg.cluster.legacy_num_instances(), Some(2));
+        assert_eq!(cfg.cluster.buckets.len(), 0);
+
+        let bucketed = write_temp_config(
+            r#"
+model:
+  name: test
+  num_layers: 4
+  num_kv_heads: 2
+  head_dim: 64
+  dtype_bytes: 2
+  block_size_tokens: 16
+  flops_per_token_prefill: 1.0e9
+  attn_quadratic_coeff: 64.0
+hardware:
+  gpu_flops: 1.0e14
+  gpu_mem_bw: 1.0e12
+  hbm_bytes: 1.0e9
+cluster:
+  meta_store:
+    ttl_seconds: 10.0
+  router:
+    mode: cache_affinity
+  global_router:
+    mode: strict_input_length
+  buckets:
+    - name: short
+      input_length_min: 0
+      input_length_max: 32
+      num_instances: 2
+    - name: long
+      input_length_min: 33
+      input_length_max: 96
+      num_instances: 1
+sim:
+  trace_path: trace.jsonl
+  output_dir: runs/test
+"#,
+        );
+        let cfg = Config::from_yaml_path(&bucketed).unwrap();
+        assert_eq!(cfg.cluster.legacy_num_instances(), None);
+        assert_eq!(cfg.cluster.buckets.len(), 2);
+        assert_eq!(cfg.cluster.buckets[0].name, "short");
+        assert_eq!(cfg.cluster.buckets[1].num_instances, 1);
+        assert_eq!(cfg.cluster.total_instances(), 3);
+    }
+
+    #[test]
+    fn bucketed_config_deserializes_global_router_weights() {
+        let path = write_temp_config(
+            r#"
+model:
+  name: test
+  num_layers: 4
+  num_kv_heads: 2
+  head_dim: 64
+  dtype_bytes: 2
+  block_size_tokens: 16
+  flops_per_token_prefill: 1.0e9
+  attn_quadratic_coeff: 64.0
+hardware:
+  gpu_flops: 1.0e14
+  gpu_mem_bw: 1.0e12
+  hbm_bytes: 1.0e9
+cluster:
+  meta_store:
+    ttl_seconds: 10.0
+  router:
+    mode: cache_affinity
+  global_router:
+    mode: bucket_score
+    length_penalty_weight: 1.5
+    load_weight: 0.75
+    cache_weight: 2.25
+  buckets:
+    - name: short
+      input_length_min: 0
+      input_length_max: 32
+      num_instances: 2
+sim:
+  trace_path: trace.jsonl
+  output_dir: runs/test
+"#,
+        );
+
+        let cfg = Config::from_yaml_path(&path).unwrap();
+        assert_eq!(
+            cfg.cluster.global_router.mode,
+            GlobalRouterMode::BucketScore
+        );
+        assert!((cfg.cluster.global_router.length_penalty_weight - 1.5).abs() < 1e-12);
+        assert!((cfg.cluster.global_router.load_weight - 0.75).abs() < 1e-12);
+        assert!((cfg.cluster.global_router.cache_weight - 2.25).abs() < 1e-12);
+    }
+
+    #[test]
+    fn bucketed_config_rejects_overlapping_ranges_and_mixed_modes() {
+        let overlap = write_temp_config(
+            r#"
+model:
+  name: test
+  num_layers: 4
+  num_kv_heads: 2
+  head_dim: 64
+  dtype_bytes: 2
+  block_size_tokens: 16
+  flops_per_token_prefill: 1.0e9
+  attn_quadratic_coeff: 64.0
+hardware:
+  gpu_flops: 1.0e14
+  gpu_mem_bw: 1.0e12
+  hbm_bytes: 1.0e9
+cluster:
+  meta_store:
+    ttl_seconds: 10.0
+  router:
+    mode: cache_affinity
+  global_router:
+    mode: strict_input_length
+  buckets:
+    - name: short
+      input_length_min: 0
+      input_length_max: 32
+      num_instances: 2
+    - name: overlap
+      input_length_min: 32
+      input_length_max: 96
+      num_instances: 1
+sim:
+  trace_path: trace.jsonl
+  output_dir: runs/test
+"#,
+        );
+        let err = Config::from_yaml_path(&overlap).unwrap_err();
+        assert!(err.to_string().contains("overlap"));
+
+        let mixed = write_temp_config(
+            r#"
+model:
+  name: test
+  num_layers: 4
+  num_kv_heads: 2
+  head_dim: 64
+  dtype_bytes: 2
+  block_size_tokens: 16
+  flops_per_token_prefill: 1.0e9
+  attn_quadratic_coeff: 64.0
+hardware:
+  gpu_flops: 1.0e14
+  gpu_mem_bw: 1.0e12
+  hbm_bytes: 1.0e9
+cluster:
+  num_instances: 2
+  meta_store:
+    ttl_seconds: 10.0
+  router:
+    mode: cache_affinity
+  global_router:
+    mode: strict_input_length
+  buckets:
+    - name: short
+      input_length_min: 0
+      input_length_max: 32
+      num_instances: 2
+sim:
+  trace_path: trace.jsonl
+  output_dir: runs/test
+"#,
+        );
+        let err = Config::from_yaml_path(&mixed).unwrap_err();
+        assert!(err.to_string().contains("num_instances"));
+        assert!(err.to_string().contains("buckets"));
+    }
+
+    #[test]
+    fn bucketed_config_reports_unmatched_input_length_gaps_clearly() {
+        let gapful = write_temp_config(
+            r#"
+model:
+  name: test
+  num_layers: 4
+  num_kv_heads: 2
+  head_dim: 64
+  dtype_bytes: 2
+  block_size_tokens: 16
+  flops_per_token_prefill: 1.0e9
+  attn_quadratic_coeff: 64.0
+hardware:
+  gpu_flops: 1.0e14
+  gpu_mem_bw: 1.0e12
+  hbm_bytes: 1.0e9
+cluster:
+  meta_store:
+    ttl_seconds: 10.0
+  router:
+    mode: cache_affinity
+  global_router:
+    mode: strict_input_length
+  buckets:
+    - name: short
+      input_length_min: 0
+      input_length_max: 32
+      num_instances: 2
+    - name: long
+      input_length_min: 64
+      input_length_max: 96
+      num_instances: 1
+sim:
+  trace_path: trace.jsonl
+  output_dir: runs/test
+"#,
+        );
+        let cfg = Config::from_yaml_path(&gapful).unwrap();
+
+        let err = cfg.cluster.bucket_index_for_input_len(40).unwrap_err();
+        assert!(err.to_string().contains("40"));
+        assert!(err.to_string().contains("no bucket"));
+    }
+
+    #[test]
+    fn bucketed_config_requires_legacy_single_pool_for_legacy_runtime_paths() {
+        let bucketed = write_temp_config(
+            r#"
+model:
+  name: test
+  num_layers: 4
+  num_kv_heads: 2
+  head_dim: 64
+  dtype_bytes: 2
+  block_size_tokens: 16
+  flops_per_token_prefill: 1.0e9
+  attn_quadratic_coeff: 64.0
+hardware:
+  gpu_flops: 1.0e14
+  gpu_mem_bw: 1.0e12
+  hbm_bytes: 1.0e9
+cluster:
+  meta_store:
+    ttl_seconds: 10.0
+  router:
+    mode: cache_affinity
+  global_router:
+    mode: strict_input_length
+  buckets:
+    - name: short
+      input_length_min: 0
+      input_length_max: 32
+      num_instances: 2
+sim:
+  trace_path: trace.jsonl
+  output_dir: runs/test
+"#,
+        );
+        let cfg = Config::from_yaml_path(&bucketed).unwrap();
+
+        let err = cfg
+            .cluster
+            .require_legacy_single_pool("driver run")
+            .unwrap_err();
+        assert!(err.to_string().contains("driver run"));
+        assert!(err.to_string().contains("cluster.buckets"));
+    }
 }
diff --git a/src/driver.rs b/src/driver.rs
index 6c1c064..0494942 100644
--- a/src/driver.rs
+++ b/src/driver.rs
@@ -6,7 +6,7 @@ use std::collections::{HashMap, VecDeque};
 use std::path::Path;
 use std::sync::{Arc, Mutex};
 
-use crate::cluster::Cluster;
+use crate::cluster::BucketedService;
 use crate::config::{Config, RouterMode};
 use crate::metrics::ablation::AblationRow;
 use crate::metrics::per_request::{PerRequestRow, PerRequestWriter};
@@ -37,7 +37,9 @@ pub struct RunOutputs {
 #[derive(Debug, Clone)]
 struct InflightInfo {
     arrival: f64,
+    bucket: u32,
     instance: u32,
+    length_bucket_match: bool,
     total_blocks: u32,
     l0_hit_blocks: u32,
     l1_hit_blocks: u32,
@@ -49,7 +51,7 @@ struct InflightInfo {
 }
 
 pub fn run(config: &Config, output_subdir: Option<&str>) -> Result<RunOutputs> {
-    let mut cluster = Cluster::new(config, &config.model);
+    let mut service = BucketedService::new(config, &config.model);
     let mut q = EventQueue::new();
 
     // Output directory
@@ -108,13 +110,16 @@ pub fn run(config: &Config, output_subdir: Option<&str>) -> Result<RunOutputs> {
                     Some(r) => r.clone(),
                     None => continue,
                 };
-                let stats = cluster.route_and_admit(&req, now);
+                let stats = service.route_and_admit(&req, now)?;
                 rt_writer.write(&stats.decision)?;
+                let strict_bucket = config.cluster.bucket_index_for_input_len(req.input_len)?;
                 inflight.insert(
                     req_id,
                     InflightInfo {
                         arrival: req.arrival,
+                        bucket: stats.bucket,
                         instance: stats.instance,
+                        length_bucket_match: stats.bucket as usize == strict_bucket,
                         total_blocks: req.hash_ids.len() as u32,
                         l0_hit_blocks: stats.l0_hit_blocks,
                         l1_hit_blocks: stats.l1_hit_blocks,
@@ -125,20 +130,23 @@ pub fn run(config: &Config, output_subdir: Option<&str>) -> Result<RunOutputs> {
                         probe_overhead_s: stats.probe_overhead_s,
                     },
                 );
-                let inst = &mut cluster.instances[stats.instance as usize];
+                let inst = &mut service.buckets[stats.bucket as usize].cluster.instances
+                    [stats.instance as usize];
                 if !inst.tick_scheduled {
                     inst.tick_scheduled = true;
                     let when = stats.ready_at.max(now);
                     q.schedule(
                         when,
                         Event::BatchTick {
+                            bucket: stats.bucket,
                             instance: stats.instance,
                         },
                     );
                 }
             }
-            Event::BatchTick { instance } => {
-                let inst = &mut cluster.instances[instance as usize];
+            Event::BatchTick { bucket, instance } => {
+                let inst =
+                    &mut service.buckets[bucket as usize].cluster.instances[instance as usize];
                 inst.tick_scheduled = false;
                 let result = inst.step(now);
                 for (rid, ttft, end) in result.completed {
@@ -148,7 +156,9 @@ pub fn run(config: &Config, output_subdir: Option<&str>) -> Result<RunOutputs> {
                             arrival: info.arrival,
                             ttft,
                             e2e: end - info.arrival,
+                            bucket: info.bucket,
                             instance: info.instance,
+                            length_bucket_match: info.length_bucket_match,
                             total_blocks: info.total_blocks,
                             l0_hit_blocks: info.l0_hit_blocks,
                             l1_hit_blocks: info.l1_hit_blocks,
@@ -163,24 +173,28 @@ pub fn run(config: &Config, output_subdir: Option<&str>) -> Result<RunOutputs> {
                     }
                 }
                 if let Some(next) = result.next_tick {
-                    let inst = &mut cluster.instances[instance as usize];
+                    let inst =
+                        &mut service.buckets[bucket as usize].cluster.instances[instance as usize];
                     if !inst.tick_scheduled {
                         inst.tick_scheduled = true;
-                        q.schedule(next.max(now), Event::BatchTick { instance });
+                        q.schedule(next.max(now), Event::BatchTick { bucket, instance });
                     }
                 }
             }
             Event::Sample => {
-                for inst in &cluster.instances {
-                    let busy = if inst.queue_len() > 0 { 1 } else { 0 };
-                    ts_writer.write(&TimeseriesRow {
-                        t: now,
-                        instance: inst.id,
-                        queue_len: inst.queue_len(),
-                        kv_blocks_used: inst.kv_blocks_used,
-                        kv_blocks_total: inst.hbm_block_budget,
-                        busy,
-                    })?;
+                for bucket in &service.buckets {
+                    for inst in &bucket.cluster.instances {
+                        let busy = if inst.queue_len() > 0 { 1 } else { 0 };
+                        ts_writer.write(&TimeseriesRow {
+                            t: now,
+                            bucket: bucket.id,
+                            instance: inst.id,
+                            queue_len: inst.queue_len(),
+                            kv_blocks_used: inst.kv_blocks_used,
+                            kv_blocks_total: inst.hbm_block_budget,
+                            busy,
+                        })?;
+                    }
                 }
             }
             Event::Stop => break,
@@ -217,6 +231,8 @@ pub fn ablate_fixed_placement_with_parallelism(
     evict_policies: &[ReplayEvictPolicy],
     jobs: usize,
 ) -> Result<Vec<AblationRow>> {
+    base.cluster
+        .require_legacy_single_pool("fixed-placement ablation")?;
     let mut out = Vec::new();
     for &policy in evict_policies {
         if policy != ReplayEvictPolicy::Lru {
diff --git a/src/main.rs b/src/main.rs
index c0b6e6f..af59237 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -50,9 +50,14 @@ struct ConfigOverrides {
 }
 
 impl ConfigOverrides {
-    fn apply(&self, cfg: &mut Config) {
+    fn apply(&self, cfg: &mut Config) -> Result<()> {
         if let Some(n) = self.num_instances {
-            cfg.cluster.num_instances = n;
+            if !cfg.cluster.buckets.is_empty() {
+                anyhow::bail!(
+                    "--num-instances does not support cluster.buckets until Task 2 lands"
+                );
+            }
+            cfg.cluster.num_instances = Some(n);
         }
         if let Some(m) = self.max_requests {
             cfg.sim.max_requests = Some(m);
@@ -78,6 +83,7 @@ impl ConfigOverrides {
         if let Some(hi) = self.input_length_max {
             cfg.sim.input_length_max = Some(hi);
         }
+        Ok(())
     }
 }
 
@@ -204,7 +210,8 @@ fn main() -> Result<()> {
 
 fn load(config: &PathBuf, overrides: &ConfigOverrides) -> Result<Config> {
     let mut cfg = Config::from_yaml_path(config)?;
-    overrides.apply(&mut cfg);
+    overrides.apply(&mut cfg)?;
+    cfg.cluster.validate()?;
     Ok(cfg)
 }
 
@@ -268,7 +275,8 @@ fn cmd_ablate(
             auto_target_ttft_mean,
             probe_mode.as_str()
         );
-        base.cluster.num_instances = chosen;
+        base.cluster.num_instances = Some(chosen);
+        base.cluster.buckets.clear();
     }
 
     eprintln!(
@@ -283,7 +291,7 @@ fn cmd_ablate(
             .map(ReplayEvictPolicy::as_str)
             .collect::<Vec<_>>()
             .join(","),
-        base.cluster.num_instances,
+        base.cluster.total_instances(),
         if jobs == 0 {
             "auto".to_string()
         } else {
@@ -310,6 +318,9 @@ fn auto_select_instances(
     probe: RouterMode,
     target_ttft_mean: f64,
 ) -> Result<u32> {
+    base.cluster
+        .require_legacy_single_pool("auto-instances calibration")?;
+
     #[derive(serde::Serialize)]
     struct CalibRow {
         num_instances: u32,
@@ -330,7 +341,7 @@ fn auto_select_instances(
 
     for &n in candidates {
         let mut cfg = base.clone();
-        cfg.cluster.num_instances = n;
+        cfg.cluster.num_instances = Some(n);
         cfg.cluster.router.mode = probe;
         // Isolate calibration output so ablation runs don't overwrite it.
         cfg.sim.output_dir = out_root
@@ -429,7 +440,7 @@ fn cmd_validate(path: &PathBuf, overrides: &ConfigOverrides) -> Result<()> {
     let hbm_blocks = (cfg.hardware.hbm_bytes / block_bytes) as u64;
     let dram_blocks = (cfg.hardware.dram_bytes / block_bytes) as u64;
     eprintln!("per-instance HBM blocks = {hbm_blocks}, DRAM blocks = {dram_blocks}");
-    eprintln!("num_instances = {}", cfg.cluster.num_instances);
+    eprintln!("num_instances = {}", cfg.cluster.total_instances());
     // Sample prefill times at a few prompt lengths.
     eprintln!("prefill_time samples:");
     for &n in &[256, 1024, 4096, 16384, 65536, 131072] {
@@ -460,9 +471,10 @@ fn cmd_oracle(
     out_path: Option<&std::path::Path>,
 ) -> Result<()> {
     let cfg = load(path, overrides)?;
+    cfg.cluster.require_legacy_single_pool("oracle analysis")?;
     let block_bytes = cfg.model.kv_block_bytes() as f64;
     let per_instance_blocks = (cfg.hardware.hbm_bytes / block_bytes).max(1.0) as u64;
-    let aggregate_blocks = per_instance_blocks * cfg.cluster.num_instances as u64;
+    let aggregate_blocks = per_instance_blocks * cfg.cluster.total_instances() as u64;
     let capacity = match (capacity_blocks, per_instance) {
         (Some(_), true) => {
             return Err(anyhow::anyhow!(
@@ -518,7 +530,7 @@ fn cmd_oracle(
         records.len(),
         capacity,
         per_instance_blocks,
-        cfg.cluster.num_instances,
+        cfg.cluster.total_instances(),
         if per_instance {
             ", per-instance mode"
         } else {
@@ -541,3 +553,123 @@ fn cmd_oracle(
     eprintln!("[oracle] wrote {}", target.display());
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use kvcache_simulator::config::{
+        BucketConfig, CalibrationConfig, ClusterConfig, GlobalRouterConfig, HardwareConfig,
+        MetaStoreConfig, ModelConfig, RouterConfig, SimConfig,
+    };
+
+    fn bucketed_config(out_dir: &str) -> Config {
+        Config {
+            model: ModelConfig {
+                name: "test".into(),
+                num_layers: 4,
+                num_kv_heads: 2,
+                head_dim: 64,
+                dtype_bytes: 2,
+                block_size_tokens: 16,
+                flops_per_token_prefill: Some(1.0e9),
+                attn_quadratic_coeff: Some(64.0),
+                ..Default::default()
+            },
+            hardware: HardwareConfig {
+                gpu_flops: 1.0e14,
+                gpu_fp8_flops: 0.0,
+                gpu_fp4_flops: 0.0,
+                gpu_mem_bw: 1.0e12,
+                hbm_bytes: 1.0e9,
+                dram_bytes: 4.0e9,
+                host_dram_bw: 5.0e11,
+                pcie_bw: 32.0e9,
+                pcie_latency_us: 1.0,
+                rdma_bw: 12.0e9,
+                rdma_latency_us: 5.0,
+                intra_node_tp_bw: 9.0e11,
+                intra_node_tp_latency_us: 2.0,
+                tp_degree: 1,
+                max_batch_slots: 32,
+                prefill_chunk_tokens: 1024,
+            },
+            calibration: CalibrationConfig::default(),
+            cluster: ClusterConfig {
+                num_instances: None,
+                buckets: vec![
+                    BucketConfig {
+                        name: "short".into(),
+                        input_length_min: 0,
+                        input_length_max: 64,
+                        num_instances: 2,
+                    },
+                    BucketConfig {
+                        name: "long".into(),
+                        input_length_min: 65,
+                        input_length_max: 128,
+                        num_instances: 1,
+                    },
+                ],
+                global_router: GlobalRouterConfig::default(),
+                meta_store: MetaStoreConfig {
+                    ttl_seconds: 1000.0,
+                },
+                router: RouterConfig {
+                    mode: RouterMode::Random,
+                    precise_probe_latency_us: 10.0,
+                    precise_probe_topk: 4,
+                    load_alpha: 0.1,
+                    score_alpha: 1.0,
+                    score_beta: 0.1,
+                    prefix_k: 8,
+                    affinity_fan_out: 0,
+                },
+            },
+            sim: SimConfig {
+                trace_path: "unused.jsonl".into(),
+                max_requests: None,
+                output_dir: out_dir.into(),
+                sample_interval_s: 0.0,
+                seed: 7,
+                input_length_min: None,
+                input_length_max: None,
+            },
+        }
+    }
+
+    #[test]
+    fn num_instances_override_rejects_bucketed_configs() {
+        let mut cfg = bucketed_config(std::env::temp_dir().to_str().unwrap());
+        let overrides = ConfigOverrides {
+            num_instances: Some(8),
+            ..ConfigOverrides::default()
+        };
+
+        let err = overrides.apply(&mut cfg).unwrap_err();
+        assert!(err.to_string().contains("--num-instances"));
+        assert!(err.to_string().contains("cluster.buckets"));
+    }
+
+    #[test]
+    fn auto_instances_rejects_bucketed_configs() {
+        let cfg = bucketed_config(std::env::temp_dir().to_str().unwrap());
+
+        let err = auto_select_instances(&cfg, &[4, 8], RouterMode::Random, 1.0).unwrap_err();
+        assert!(err.to_string().contains("auto-instances"));
+        assert!(err.to_string().contains("cluster.buckets"));
+    }
+
+    #[test]
+    fn oracle_rejects_bucketed_configs() {
+        let tmp = std::env::temp_dir().join("kvcache_sim_main_tests");
+        let _ = std::fs::remove_dir_all(&tmp);
+        std::fs::create_dir_all(&tmp).unwrap();
+        let path = tmp.join("bucketed.yaml");
+        let cfg = bucketed_config(tmp.to_str().unwrap());
+        std::fs::write(&path, serde_yaml::to_string(&cfg).unwrap()).unwrap();
+
+        let err = cmd_oracle(&path, &ConfigOverrides::default(), None, false, None).unwrap_err();
+        assert!(err.to_string().contains("oracle analysis"));
+        assert!(err.to_string().contains("cluster.buckets"));
+    }
+}
diff --git a/src/metrics/per_request.rs b/src/metrics/per_request.rs
index b4ccdd8..9d2a792 100644
--- a/src/metrics/per_request.rs
+++ b/src/metrics/per_request.rs
@@ -8,7 +8,9 @@ pub struct PerRequestRow {
     pub arrival: f64,
     pub ttft: f64,
     pub e2e: f64,
+    pub bucket: u32,
     pub instance: u32,
+    pub length_bucket_match: bool,
     pub total_blocks: u32,
     pub l0_hit_blocks: u32,
     pub l1_hit_blocks: u32,
diff --git a/src/metrics/timeseries.rs b/src/metrics/timeseries.rs
index cb670e0..3851465 100644
--- a/src/metrics/timeseries.rs
+++ b/src/metrics/timeseries.rs
@@ -5,6 +5,7 @@ use std::path::Path;
 #[derive(Debug, Clone, Serialize)]
 pub struct TimeseriesRow {
     pub t: f64,
+    pub bucket: u32,
     pub instance: u32,
     pub queue_len: u32,
     pub kv_blocks_used: u32,
diff --git a/src/oracle.rs b/src/oracle.rs
index 064a2d2..590736f 100644
--- a/src/oracle.rs
+++ b/src/oracle.rs
@@ -196,7 +196,12 @@ fn build_next_use(records: &[RequestRecord]) -> Vec<Vec<u32>> {
 /// Implementation: lazy-deletion max-heap keyed by next-use index. Each
 /// cache entry has a version; the heap may contain stale entries from
 /// previous insertions, which we skip on pop.
-fn run_belady(records: &[RequestRecord], next_use: &[Vec<u32>], capacity: usize, mask: &[bool]) -> u64 {
+fn run_belady(
+    records: &[RequestRecord],
+    next_use: &[Vec<u32>],
+    capacity: usize,
+    mask: &[bool],
+) -> u64 {
     if capacity == 0 {
         return 0;
     }
@@ -327,10 +332,7 @@ mod tests {
         // req 0 populates blocks [1,2,3] but is not counted.
         // req 1 has prefix [1,2,3,4] — the first 3 blocks are cache hits
         // because req 0 populated them, even though req 0 is masked out.
-        let recs = vec![
-            req(0, 0.0, vec![1, 2, 3]),
-            req(1, 1.0, vec![1, 2, 3, 4]),
-        ];
+        let recs = vec![req(0, 0.0, vec![1, 2, 3]), req(1, 1.0, vec![1, 2, 3, 4])];
         let mask = vec![false, true];
         let out = analyze(&recs, 100, Some(&mask));
         // Only req 1 is counted: total = 4, hits = 3 (prefix [1,2,3] hit)
diff --git a/src/replay.rs b/src/replay.rs
index 13a7cb5..a26b724 100644
--- a/src/replay.rs
+++ b/src/replay.rs
@@ -496,6 +496,8 @@ pub fn replay_fixed_placement(
     placements: &[PlacementEntry],
     policy: ReplayEvictPolicy,
 ) -> Result<ReplaySummary> {
+    cfg.cluster
+        .require_legacy_single_pool("fixed-placement replay")?;
     if records.len() != placements.len() {
         return Err(anyhow!(
             "records/placements length mismatch: {} vs {}",
@@ -519,7 +521,7 @@ pub fn replay_fixed_placement(
     let block_bytes = cfg.model.kv_block_bytes() as f64;
     let l0_cap = (cfg.hardware.hbm_bytes / block_bytes).max(1.0) as usize;
     let l1_cap = (cfg.hardware.dram_bytes / block_bytes).max(1.0) as usize;
-    let num_instances = cfg.cluster.num_instances as usize;
+    let num_instances = cfg.cluster.total_instances() as usize;
     let mut caches: Vec<ReplayInstanceCache> = (0..num_instances)
         .map(|_| ReplayInstanceCache::new(policy, l0_cap, l1_cap))
         .collect();
diff --git a/src/router/adaptive_affinity.rs b/src/router/adaptive_affinity.rs
index 7b94239..8b8c7cb 100644
--- a/src/router/adaptive_affinity.rs
+++ b/src/router/adaptive_affinity.rs
@@ -57,7 +57,7 @@ pub struct AdaptiveAffinityRouter {
 
 impl AdaptiveAffinityRouter {
     pub fn new(config: &Config) -> Self {
-        let n = config.cluster.num_instances as usize;
+        let n = config.cluster.total_instances() as usize;
         let configured_fan_out = config.cluster.router.affinity_fan_out;
         let max_fan_out = if configured_fan_out > 0 {
             configured_fan_out.max(2).min(n)
@@ -242,13 +242,13 @@ impl Router for AdaptiveAffinityRouter {
 
         self.observe(fp, now);
 
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "adaptive_affinity",
-            chosen: instances[chosen_idx].id,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "adaptive_affinity",
+            instances[chosen_idx].id,
+            0.0,
             candidates,
             reason,
-        }
+        )
     }
 }
diff --git a/src/router/cache_affinity.rs b/src/router/cache_affinity.rs
index 5e09d33..a42a1af 100644
--- a/src/router/cache_affinity.rs
+++ b/src/router/cache_affinity.rs
@@ -205,13 +205,13 @@ impl Router for CacheAffinityRouter {
             }
         }
 
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "cache_affinity",
-            chosen: instances[best_idx].id,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "cache_affinity",
+            instances[best_idx].id,
+            0.0,
             candidates,
-            reason: "argmin(α·q − γ·l0_hit − δ·meta_only) + rendezvous tiebreak",
-        }
+            "argmin(α·q − γ·l0_hit − δ·meta_only) + rendezvous tiebreak",
+        )
     }
 }
diff --git a/src/router/cache_load.rs b/src/router/cache_load.rs
index 142e00c..3101c6f 100644
--- a/src/router/cache_load.rs
+++ b/src/router/cache_load.rs
@@ -77,13 +77,13 @@ impl Router for CacheLoadRouter {
             });
         }
 
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "cache_load",
-            chosen: instances[best_idx].id,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "cache_load",
+            instances[best_idx].id,
+            0.0,
             candidates,
-            reason: "least-loaded 1/4, then best local L0 prefix",
-        }
+            "least-loaded 1/4, then best local L0 prefix",
+        )
     }
 }
diff --git a/src/router/cache_score.rs b/src/router/cache_score.rs
index ab23882..a727f32 100644
--- a/src/router/cache_score.rs
+++ b/src/router/cache_score.rs
@@ -99,13 +99,13 @@ impl Router for CacheScoreRouter {
             }
         }
 
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "cache_score",
-            chosen: instances[best_idx].id,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "cache_score",
+            instances[best_idx].id,
+            0.0,
             candidates,
-            reason: "argmin 2^(α·load + β·miss)",
-        }
+            "argmin 2^(α·load + β·miss)",
+        )
     }
 }
diff --git a/src/router/cache_score_ttl.rs b/src/router/cache_score_ttl.rs
index c67ad1d..d411247 100644
--- a/src/router/cache_score_ttl.rs
+++ b/src/router/cache_score_ttl.rs
@@ -74,13 +74,13 @@ impl Router for CacheScoreTtlRouter {
             }
         }
 
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "cache_score_ttl",
-            chosen: instances[best_idx].id,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "cache_score_ttl",
+            instances[best_idx].id,
+            0.0,
             candidates,
-            reason: "argmin 2^(alpha*load + beta*meta_store_miss)",
-        }
+            "argmin 2^(alpha*load + beta*meta_store_miss)",
+        )
     }
 }
diff --git a/src/router/estimated_ttft.rs b/src/router/estimated_ttft.rs
index bc206bc..e4b8a69 100644
--- a/src/router/estimated_ttft.rs
+++ b/src/router/estimated_ttft.rs
@@ -89,13 +89,13 @@ impl Router for EstimatedTtftRouter {
             }
         }
 
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "estimated_ttft",
-            chosen: best,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "estimated_ttft",
+            best,
+            0.0,
             candidates,
-            reason: "argmin(drain + scheduler + kv_prepare + prefill + first_token_tail)",
-        }
+            "argmin(drain + scheduler + kv_prepare + prefill + first_token_tail)",
+        )
     }
 }
diff --git a/src/router/global_bucket.rs b/src/router/global_bucket.rs
new file mode 100644
index 0000000..2522e7d
--- /dev/null
+++ b/src/router/global_bucket.rs
@@ -0,0 +1,371 @@
+use anyhow::{anyhow, Result};
+use serde::Serialize;
+
+use crate::config::{Config, GlobalRouterMode};
+use crate::trace::RequestRecord;
+
+pub type BucketId = u32;
+
+#[derive(Debug, Clone, Serialize)]
+pub struct BucketView {
+    pub id: BucketId,
+    pub input_length_min: u32,
+    pub input_length_max: u32,
+    pub num_instances: u32,
+    pub total_queue_len: u32,
+    pub total_load_blocks: u32,
+    pub predicted_prefix: u32,
+}
+
+#[derive(Debug, Clone, Serialize)]
+pub struct BucketCandidate {
+    pub bucket: BucketId,
+    pub input_length_min: u32,
+    pub input_length_max: u32,
+    pub num_instances: u32,
+    pub total_queue_len: u32,
+    pub total_load_blocks: u32,
+    pub predicted_prefix: u32,
+    pub matches_input_len: bool,
+    pub score: f64,
+}
+
+#[derive(Debug, Clone, Serialize)]
+pub struct GlobalRouteDecision {
+    pub req_id: u64,
+    pub mode: &'static str,
+    pub chosen_bucket: BucketId,
+    pub candidates: Vec<BucketCandidate>,
+    pub reason: &'static str,
+}
+
+impl GlobalRouteDecision {
+    pub fn single_bucket(req_id: u64, chosen_bucket: BucketId) -> Self {
+        Self {
+            req_id,
+            mode: "single_pool",
+            chosen_bucket,
+            candidates: Vec::new(),
+            reason: "single pool uses bucket 0",
+        }
+    }
+}
+
+pub trait GlobalRouter: Send {
+    fn name(&self) -> &'static str;
+    fn route(
+        &mut self,
+        req: &RequestRecord,
+        buckets: &[BucketView],
+        now: f64,
+    ) -> Result<GlobalRouteDecision>;
+}
+
+struct StrictInputLengthRouter {
+    reported_mode: &'static str,
+    reason: &'static str,
+}
+
+impl StrictInputLengthRouter {
+    fn new(reported_mode: &'static str, reason: &'static str) -> Self {
+        Self {
+            reported_mode,
+            reason,
+        }
+    }
+}
+
+impl GlobalRouter for StrictInputLengthRouter {
+    fn name(&self) -> &'static str {
+        self.reported_mode
+    }
+
+    fn route(
+        &mut self,
+        req: &RequestRecord,
+        buckets: &[BucketView],
+        _now: f64,
+    ) -> Result<GlobalRouteDecision> {
+        let candidates = buckets
+            .iter()
+            .map(|view| BucketCandidate {
+                bucket: view.id,
+                input_length_min: view.input_length_min,
+                input_length_max: view.input_length_max,
+                num_instances: view.num_instances,
+                total_queue_len: view.total_queue_len,
+                total_load_blocks: view.total_load_blocks,
+                predicted_prefix: view.predicted_prefix,
+                matches_input_len: view.input_length_min <= req.input_len
+                    && req.input_len <= view.input_length_max,
+                score: if view.input_length_min <= req.input_len
+                    && req.input_len <= view.input_length_max
+                {
+                    0.0
+                } else {
+                    f64::INFINITY
+                },
+            })
+            .collect::<Vec<_>>();
+
+        let matches = candidates
+            .iter()
+            .filter(|candidate| candidate.matches_input_len)
+            .map(|candidate| candidate.bucket)
+            .collect::<Vec<_>>();
+
+        let chosen_bucket = match matches.as_slice() {
+            [bucket] => *bucket,
+            [] => {
+                return Err(anyhow!(
+                    "cluster.global_router.mode={} has no bucket for input_length={}",
+                    self.reported_mode,
+                    req.input_len
+                ));
+            }
+            _ => {
+                return Err(anyhow!(
+                    "cluster.global_router.mode={} matched multiple buckets for input_length={}",
+                    self.reported_mode,
+                    req.input_len
+                ));
+            }
+        };
+
+        Ok(GlobalRouteDecision {
+            req_id: req.req_id,
+            mode: self.reported_mode,
+            chosen_bucket,
+            candidates,
+            reason: self.reason,
+        })
+    }
+}
+
+struct BucketScoreRouter {
+    length_penalty_weight: f64,
+    load_weight: f64,
+    cache_weight: f64,
+}
+
+impl BucketScoreRouter {
+    fn new(full: &Config) -> Self {
+        Self {
+            length_penalty_weight: full.cluster.global_router.length_penalty_weight,
+            load_weight: full.cluster.global_router.load_weight,
+            cache_weight: full.cluster.global_router.cache_weight,
+        }
+    }
+
+    fn length_penalty(&self, req: &RequestRecord, bucket: &BucketView) -> f64 {
+        if req.input_len < bucket.input_length_min {
+            (bucket.input_length_min - req.input_len) as f64
+        } else if req.input_len > bucket.input_length_max {
+            (req.input_len - bucket.input_length_max) as f64
+        } else {
+            0.0
+        }
+    }
+}
+
+impl GlobalRouter for BucketScoreRouter {
+    fn name(&self) -> &'static str {
+        "bucket_score"
+    }
+
+    fn route(
+        &mut self,
+        req: &RequestRecord,
+        buckets: &[BucketView],
+        _now: f64,
+    ) -> Result<GlobalRouteDecision> {
+        let mut chosen_bucket = None;
+        let mut best_score = f64::INFINITY;
+        let mut candidates = Vec::with_capacity(buckets.len());
+
+        for bucket in buckets {
+            let length_penalty = self.length_penalty(req, bucket);
+            let miss = req
+                .hash_ids
+                .len()
+                .saturating_sub(bucket.predicted_prefix as usize) as f64;
+            let score = self.length_penalty_weight * length_penalty
+                + self.load_weight * bucket.total_queue_len as f64
+                + self.cache_weight * miss;
+
+            candidates.push(BucketCandidate {
+                bucket: bucket.id,
+                input_length_min: bucket.input_length_min,
+                input_length_max: bucket.input_length_max,
+                num_instances: bucket.num_instances,
+                total_queue_len: bucket.total_queue_len,
+                total_load_blocks: bucket.total_load_blocks,
+                predicted_prefix: bucket.predicted_prefix,
+                matches_input_len: bucket.input_length_min <= req.input_len
+                    && req.input_len <= bucket.input_length_max,
+                score,
+            });
+
+            let better = score < best_score
+                || (score == best_score && chosen_bucket.is_none_or(|best| bucket.id < best));
+            if better {
+                best_score = score;
+                chosen_bucket = Some(bucket.id);
+            }
+        }
+
+        Ok(GlobalRouteDecision {
+            req_id: req.req_id,
+            mode: self.name(),
+            chosen_bucket: chosen_bucket.ok_or_else(|| anyhow!("no buckets available"))?,
+            candidates,
+            reason: "weighted length/load/cache bucket score",
+        })
+    }
+}
+
+pub fn build(full: &Config) -> Box<dyn GlobalRouter> {
+    match full.cluster.global_router.mode {
+        GlobalRouterMode::StrictInputLength => Box::new(StrictInputLengthRouter::new(
+            "strict_input_length",
+            "unique bucket range contains input_length",
+        )) as Box<dyn GlobalRouter>,
+        GlobalRouterMode::BucketScore => {
+            Box::new(BucketScoreRouter::new(full)) as Box<dyn GlobalRouter>
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::config::{
+        ClusterConfig, GlobalRouterConfig, MetaStoreConfig, RouterConfig, RouterMode,
+    };
+
+    fn cfg() -> Config {
+        Config {
+            model: crate::config::ModelConfig::default(),
+            hardware: crate::config::HardwareConfig {
+                gpu_flops: 1.0,
+                gpu_fp8_flops: 0.0,
+                gpu_fp4_flops: 0.0,
+                gpu_mem_bw: 1.0,
+                hbm_bytes: 1.0,
+                dram_bytes: 1.0,
+                host_dram_bw: 1.0,
+                pcie_bw: 1.0,
+                pcie_latency_us: 1.0,
+                rdma_bw: 1.0,
+                rdma_latency_us: 1.0,
+                intra_node_tp_bw: 1.0,
+                intra_node_tp_latency_us: 1.0,
+                tp_degree: 1,
+                max_batch_slots: 1,
+                prefill_chunk_tokens: 1,
+            },
+            calibration: crate::config::CalibrationConfig::default(),
+            cluster: ClusterConfig {
+                num_instances: None,
+                buckets: Vec::new(),
+                global_router: GlobalRouterConfig {
+                    mode: GlobalRouterMode::BucketScore,
+                    length_penalty_weight: 1.0,
+                    load_weight: 1.0,
+                    cache_weight: 1.0,
+                },
+                meta_store: MetaStoreConfig { ttl_seconds: 1.0 },
+                router: RouterConfig {
+                    mode: RouterMode::LeastLoaded,
+                    precise_probe_latency_us: 1.0,
+                    precise_probe_topk: 1,
+                    load_alpha: 1.0,
+                    score_alpha: 1.0,
+                    score_beta: 1.0,
+                    prefix_k: 8,
+                    affinity_fan_out: 1,
+                },
+            },
+            sim: crate::config::SimConfig {
+                trace_path: String::new(),
+                max_requests: None,
+                output_dir: String::new(),
+                sample_interval_s: 0.0,
+                seed: 0,
+                input_length_min: None,
+                input_length_max: None,
+            },
+        }
+    }
+
+    fn req(input_len: u32) -> RequestRecord {
+        RequestRecord {
+            req_id: 1,
+            chat_id: 0,
+            parent_chat_id: -1,
+            turn: 0,
+            arrival: 0.0,
+            input_len,
+            output_len: 16,
+            hash_ids: vec![10, 11, 12],
+        }
+    }
+
+    #[test]
+    fn bucket_score_prefers_matching_bucket_when_load_is_equal() {
+        let mut router = BucketScoreRouter::new(&cfg());
+        let buckets = vec![
+            BucketView {
+                id: 0,
+                input_length_min: 0,
+                input_length_max: 32,
+                num_instances: 2,
+                total_queue_len: 1,
+                total_load_blocks: 0,
+                predicted_prefix: 0,
+            },
+            BucketView {
+                id: 1,
+                input_length_min: 33,
+                input_length_max: 96,
+                num_instances: 2,
+                total_queue_len: 1,
+                total_load_blocks: 0,
+                predicted_prefix: 0,
+            },
+        ];
+        let decision = router.route(&req(24), &buckets, 0.0).unwrap();
+        assert_eq!(decision.chosen_bucket, 0);
+    }
+
+    #[test]
+    fn bucket_score_can_override_length_match_when_load_gap_is_large() {
+        let mut full = cfg();
+        full.cluster.global_router.load_weight = 5.0;
+        full.cluster.global_router.cache_weight = 1.0;
+        full.cluster.global_router.length_penalty_weight = 1.0;
+        let mut router = BucketScoreRouter::new(&full);
+        let buckets = vec![
+            BucketView {
+                id: 0,
+                input_length_min: 0,
+                input_length_max: 32,
+                num_instances: 2,
+                total_queue_len: 20,
+                total_load_blocks: 0,
+                predicted_prefix: 0,
+            },
+            BucketView {
+                id: 1,
+                input_length_min: 33,
+                input_length_max: 96,
+                num_instances: 2,
+                total_queue_len: 0,
+                total_load_blocks: 0,
+                predicted_prefix: 2,
+            },
+        ];
+        let decision = router.route(&req(24), &buckets, 0.0).unwrap();
+        assert_eq!(decision.chosen_bucket, 1);
+    }
+}
diff --git a/src/router/least_loaded.rs b/src/router/least_loaded.rs
index efc0ed8..272cb93 100644
--- a/src/router/least_loaded.rs
+++ b/src/router/least_loaded.rs
@@ -41,13 +41,13 @@ impl Router for LeastLoadedRouter {
                 best = inst.id;
             }
         }
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "least_loaded",
-            chosen: best,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "least_loaded",
+            best,
+            0.0,
             candidates,
-            reason: "argmin(kv_used + alpha * queue_len)",
-        }
+            "argmin(kv_used + alpha * queue_len)",
+        )
     }
 }
diff --git a/src/router/least_tokens.rs b/src/router/least_tokens.rs
index effdad7..2c06427 100644
--- a/src/router/least_tokens.rs
+++ b/src/router/least_tokens.rs
@@ -61,13 +61,13 @@ impl Router for LeastTokensRouter {
             }
         }
 
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "least_tokens",
-            chosen: best,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "least_tokens",
+            best,
+            0.0,
             candidates,
-            reason: "argmin(waiting_prefill_tokens)",
-        }
+            "argmin(waiting_prefill_tokens)",
+        )
     }
 }
diff --git a/src/router/lineage_affinity.rs b/src/router/lineage_affinity.rs
index f8d51e6..0e9035b 100644
--- a/src/router/lineage_affinity.rs
+++ b/src/router/lineage_affinity.rs
@@ -53,7 +53,7 @@ pub struct LineageAffinityRouter {
 
 impl LineageAffinityRouter {
     pub fn new(config: &Config) -> Self {
-        let n = config.cluster.num_instances as usize;
+        let n = config.cluster.total_instances() as usize;
         let configured_fan_out = config.cluster.router.affinity_fan_out;
         let max_fan_out = if configured_fan_out > 0 {
             configured_fan_out.max(2).min(n)
@@ -231,13 +231,13 @@ impl Router for LineageAffinityRouter {
         self.request_home
             .insert(req.chat_id, instances[chosen.idx].id);
 
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "lineage_affinity",
-            chosen: instances[chosen.idx].id,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "lineage_affinity",
+            instances[chosen.idx].id,
+            0.0,
             candidates,
             reason,
-        }
+        )
     }
 }
diff --git a/src/router/min_pd.rs b/src/router/min_pd.rs
index f801639..2033dee 100644
--- a/src/router/min_pd.rs
+++ b/src/router/min_pd.rs
@@ -90,13 +90,13 @@ impl Router for MinPdRouter {
             }
         }
 
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "min_pd",
-            chosen: best,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "min_pd",
+            best,
+            0.0,
             candidates,
-            reason: "argmin(P*D), P=local-L0 miss tokens, D=ongoing reqs",
-        }
+            "argmin(P*D), P=local-L0 miss tokens, D=ongoing reqs",
+        )
     }
 }
diff --git a/src/router/mod.rs b/src/router/mod.rs
index 7773e3d..ef68dd5 100644
--- a/src/router/mod.rs
+++ b/src/router/mod.rs
@@ -6,6 +6,7 @@ pub mod cache_load;
 pub mod cache_score;
 pub mod cache_score_ttl;
 pub mod estimated_ttft;
+pub mod global_bucket;
 pub mod least_loaded;
 pub mod least_tokens;
 pub mod lineage_affinity;
@@ -23,6 +24,8 @@ use crate::instance::Instance;
 use crate::trace::RequestRecord;
 use crate::types::InstanceId;
 
+pub use global_bucket::{BucketCandidate, BucketId, BucketView, GlobalRouteDecision, GlobalRouter};
+
 #[derive(Debug, Clone, Serialize)]
 pub struct CandidateInfo {
     pub instance: InstanceId,
@@ -34,11 +37,25 @@ pub struct CandidateInfo {
 #[derive(Debug, Clone, Serialize)]
 pub struct RouteDecision {
     pub req_id: u64,
+    pub global_mode: &'static str,
     pub mode: &'static str,
+    pub global_reason: &'static str,
+    pub local_reason: &'static str,
+    pub chosen_bucket: BucketId,
     pub chosen: InstanceId,
     pub probe_overhead_s: f64,
+    pub bucket_candidates: Vec<BucketCandidate>,
     pub candidates: Vec<CandidateInfo>,
-    pub reason: &'static str,
+}
+
+impl RouteDecision {
+    pub fn with_global(mut self, decision: &GlobalRouteDecision) -> Self {
+        self.global_mode = decision.mode;
+        self.global_reason = decision.reason;
+        self.chosen_bucket = decision.chosen_bucket;
+        self.bucket_candidates = decision.candidates.clone();
+        self
+    }
 }
 
 pub trait Router: Send {
@@ -63,6 +80,28 @@ pub(crate) fn local_l0_scores(req: &RequestRecord, instances: &[Instance]) -> Ve
         .collect()
 }
 
+pub fn local_route_decision(
+    req_id: u64,
+    mode: &'static str,
+    chosen: InstanceId,
+    probe_overhead_s: f64,
+    candidates: Vec<CandidateInfo>,
+    reason: &'static str,
+) -> RouteDecision {
+    RouteDecision {
+        req_id,
+        global_mode: "single_pool",
+        mode,
+        global_reason: "single pool uses bucket 0",
+        local_reason: reason,
+        chosen_bucket: 0,
+        chosen,
+        probe_overhead_s,
+        bucket_candidates: Vec::new(),
+        candidates,
+    }
+}
+
 pub fn build(full: &Config, seed: u64) -> Box<dyn Router> {
     use crate::config::RouterMode::*;
     let cfg = &full.cluster.router;
@@ -122,6 +161,10 @@ pub fn build(full: &Config, seed: u64) -> Box<dyn Router> {
     }
 }
 
+pub fn build_global(full: &Config) -> Box<dyn GlobalRouter> {
+    global_bucket::build(full)
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -181,7 +224,9 @@ mod tests {
             hardware: test_hardware(),
             calibration: CalibrationConfig::default(),
             cluster: ClusterConfig {
-                num_instances: 2,
+                num_instances: Some(2),
+                buckets: Vec::new(),
+                global_router: Default::default(),
                 meta_store: MetaStoreConfig {
                     ttl_seconds: 1000.0,
                 },
@@ -351,7 +396,7 @@ mod tests {
         let mut router = PrefixAffinityRouter::new(&cfg);
         let decision = router.route(&req, &instances, &meta, 0.0);
 
-        assert_eq!(decision.reason, "affinity fallback: min(drain+fetch)");
+        assert_eq!(decision.local_reason, "affinity fallback: min(drain+fetch)");
         assert_eq!(decision.chosen, 1);
     }
 
diff --git a/src/router/precise_aware.rs b/src/router/precise_aware.rs
index 9940815..e0d90cd 100644
--- a/src/router/precise_aware.rs
+++ b/src/router/precise_aware.rs
@@ -62,13 +62,13 @@ impl Router for PreciseRouter {
             }
         }
 
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "precise",
-            chosen: best,
-            probe_overhead_s: n as f64 * self.probe_latency_s,
+        crate::router::local_route_decision(
+            req.req_id,
+            "precise",
+            best,
+            n as f64 * self.probe_latency_s,
             candidates,
-            reason: "exact-probe all instances' L0 cache",
-        }
+            "exact-probe all instances' L0 cache",
+        )
     }
 }
diff --git a/src/router/prefix_affinity.rs b/src/router/prefix_affinity.rs
index 8f99b7a..bde3443 100644
--- a/src/router/prefix_affinity.rs
+++ b/src/router/prefix_affinity.rs
@@ -51,7 +51,7 @@ pub struct PrefixAffinityRouter {
 
 impl PrefixAffinityRouter {
     pub fn new(config: &Config) -> Self {
-        let n = config.cluster.num_instances as usize;
+        let n = config.cluster.total_instances() as usize;
         let cfg_fan = config.cluster.router.affinity_fan_out;
         // fan_out: if configured, use it; otherwise auto = max(2, n/8).
         let fan_out = if cfg_fan > 0 {
@@ -166,13 +166,13 @@ impl Router for PrefixAffinityRouter {
             reason = "prefix affinity: top-K min drain";
         }
 
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "prefix_affinity",
-            chosen: instances[best_idx].id,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "prefix_affinity",
+            instances[best_idx].id,
+            0.0,
             candidates,
             reason,
-        }
+        )
     }
 }
diff --git a/src/router/random.rs b/src/router/random.rs
index c4bfb17..8f2deae 100644
--- a/src/router/random.rs
+++ b/src/router/random.rs
@@ -33,19 +33,19 @@ impl Router for RandomRouter {
     ) -> RouteDecision {
         let n = instances.len();
         let chosen = self.rng.gen_range(0..n) as InstanceId;
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "random",
+        crate::router::local_route_decision(
+            req.req_id,
+            "random",
             chosen,
-            probe_overhead_s: 0.0,
-            candidates: vec![CandidateInfo {
+            0.0,
+            vec![CandidateInfo {
                 instance: chosen,
                 predicted_prefix: 0,
                 load_blocks: instances[chosen as usize].kv_blocks_used,
                 queue_len: instances[chosen as usize].queue_len(),
             }],
-            reason: "uniform random",
-        }
+            "uniform random",
+        )
     }
 }
 
@@ -75,18 +75,18 @@ impl Router for RoundRobinRouter {
         let n = instances.len() as u32;
         let chosen = self.next % n;
         self.next = self.next.wrapping_add(1);
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "round_robin",
+        crate::router::local_route_decision(
+            req.req_id,
+            "round_robin",
             chosen,
-            probe_overhead_s: 0.0,
-            candidates: vec![CandidateInfo {
+            0.0,
+            vec![CandidateInfo {
                 instance: chosen,
                 predicted_prefix: 0,
                 load_blocks: instances[chosen as usize].kv_blocks_used,
                 queue_len: instances[chosen as usize].queue_len(),
             }],
-            reason: "round robin",
-        }
+            "round robin",
+        )
     }
 }
diff --git a/src/router/ttl_aware.rs b/src/router/ttl_aware.rs
index 04481d0..6206ae7 100644
--- a/src/router/ttl_aware.rs
+++ b/src/router/ttl_aware.rs
@@ -46,13 +46,13 @@ impl Router for TtlAwareRouter {
                 best = inst.id;
             }
         }
-        RouteDecision {
-            req_id: req.req_id,
-            mode: "ttl_aware",
-            chosen: best,
-            probe_overhead_s: 0.0,
+        crate::router::local_route_decision(
+            req.req_id,
+            "ttl_aware",
+            best,
+            0.0,
             candidates,
-            reason: "max meta_store prefix, tie -> least loaded",
-        }
+            "max meta_store prefix, tie -> least loaded",
+        )
     }
 }
diff --git a/src/sim/engine.rs b/src/sim/engine.rs
index 7a515e6..83651a3 100644
--- a/src/sim/engine.rs
+++ b/src/sim/engine.rs
@@ -91,11 +91,24 @@ mod tests {
         q.schedule(
             2.0,
             Event::BatchTick {
+                bucket: 0,
                 instance: 0 as InstanceId,
             },
         );
-        q.schedule(1.0, Event::BatchTick { instance: 1 });
-        q.schedule(1.5, Event::BatchTick { instance: 2 });
+        q.schedule(
+            1.0,
+            Event::BatchTick {
+                bucket: 0,
+                instance: 1,
+            },
+        );
+        q.schedule(
+            1.5,
+            Event::BatchTick {
+                bucket: 0,
+                instance: 2,
+            },
+        );
         let (t1, _) = q.pop().unwrap();
         let (t2, _) = q.pop().unwrap();
         let (t3, _) = q.pop().unwrap();
@@ -107,12 +120,24 @@ mod tests {
     #[test]
     fn equal_time_fifo() {
         let mut q = EventQueue::new();
-        q.schedule(1.0, Event::BatchTick { instance: 7 });
-        q.schedule(1.0, Event::BatchTick { instance: 8 });
+        q.schedule(
+            1.0,
+            Event::BatchTick {
+                bucket: 0,
+                instance: 7,
+            },
+        );
+        q.schedule(
+            1.0,
+            Event::BatchTick {
+                bucket: 1,
+                instance: 8,
+            },
+        );
         let (_, e1) = q.pop().unwrap();
         let (_, e2) = q.pop().unwrap();
         match (e1, e2) {
-            (Event::BatchTick { instance: a }, Event::BatchTick { instance: b }) => {
+            (Event::BatchTick { instance: a, .. }, Event::BatchTick { instance: b, .. }) => {
                 assert_eq!(a, 7);
                 assert_eq!(b, 8);
             }
diff --git a/src/sim/events.rs b/src/sim/events.rs
index e369fa2..c8847a3 100644
--- a/src/sim/events.rs
+++ b/src/sim/events.rs
@@ -1,5 +1,6 @@
 //! Event types for the discrete-event engine.
 
+use crate::router::BucketId;
 use crate::types::{InstanceId, ReqId};
 
 #[derive(Debug)]
@@ -7,7 +8,10 @@ pub enum Event {
     /// New trace request arrives at the cluster router.
     Arrival { req_id: ReqId },
     /// Per-instance scheduler tick (continuous batching).
-    BatchTick { instance: InstanceId },
+    BatchTick {
+        bucket: BucketId,
+        instance: InstanceId,
+    },
     /// Periodic time-series sample of all instances.
     Sample,
     /// Stop the simulation early (used internally).
diff --git a/tests/smoke.rs b/tests/smoke.rs
index 700408d..bd5ec38 100644
--- a/tests/smoke.rs
+++ b/tests/smoke.rs
@@ -6,7 +6,7 @@ use std::io::Write;
 
 use kvcache_simulator::config::*;
 use kvcache_simulator::driver;
-use kvcache_simulator::replay::ReplayEvictPolicy;
+use kvcache_simulator::replay::{self, PlacementEntry, ReplayEvictPolicy};
 
 fn base_config(trace_path: &str, out_dir: &str, mode: RouterMode) -> Config {
     Config {
@@ -41,7 +41,9 @@ fn base_config(trace_path: &str, out_dir: &str, mode: RouterMode) -> Config {
         },
         calibration: CalibrationConfig::default(),
         cluster: ClusterConfig {
-            num_instances: 4,
+            num_instances: Some(4),
+            buckets: Vec::new(),
+            global_router: Default::default(),
             meta_store: MetaStoreConfig {
                 ttl_seconds: 1000.0,
             },
@@ -68,6 +70,26 @@ fn base_config(trace_path: &str, out_dir: &str, mode: RouterMode) -> Config {
     }
 }
 
+fn bucketed_config(trace_path: &str, out_dir: &str, mode: RouterMode) -> Config {
+    let mut cfg = base_config(trace_path, out_dir, mode);
+    cfg.cluster.num_instances = None;
+    cfg.cluster.buckets = vec![
+        BucketConfig {
+            name: "short".into(),
+            input_length_min: 0,
+            input_length_max: 64,
+            num_instances: 2,
+        },
+        BucketConfig {
+            name: "long".into(),
+            input_length_min: 65,
+            input_length_max: 128,
+            num_instances: 1,
+        },
+    ];
+    cfg
+}
+
 fn write_synthetic_trace(path: &std::path::Path) {
     // 5 distinct conversations, each with 8 turns. Within a conversation,
     // turn k+1 reuses the prefix of turn k (shared first ~10 blocks) and
@@ -272,3 +294,160 @@ fn ablation_parallel_matches_serial() {
         assert!((lhs.miss_rate - rhs.miss_rate).abs() < 1e-12);
     }
 }
+
+#[test]
+fn strict_bucket_run_emits_bucket_fields_in_outputs() {
+    let tmp = std::env::temp_dir().join("kvcache_sim_bucket_outputs");
+    let _ = std::fs::remove_dir_all(&tmp);
+    std::fs::create_dir_all(&tmp).unwrap();
+    let trace_path = tmp.join("trace.jsonl");
+
+    let mut f = std::fs::File::create(&trace_path).unwrap();
+    writeln!(
+        f,
+        "{}",
+        serde_json::json!({
+            "chat_id": 1,
+            "parent_chat_id": -1,
+            "timestamp": 0.0,
+            "input_length": 32,
+            "output_length": 16,
+            "type": "text",
+            "turn": 0,
+            "hash_ids": [1, 2]
+        })
+    )
+    .unwrap();
+    writeln!(
+        f,
+        "{}",
+        serde_json::json!({
+            "chat_id": 2,
+            "parent_chat_id": -1,
+            "timestamp": 0.1,
+            "input_length": 80,
+            "output_length": 16,
+            "type": "text",
+            "turn": 0,
+            "hash_ids": [3, 4, 5, 6, 7]
+        })
+    )
+    .unwrap();
+
+    let mut cfg = bucketed_config(
+        trace_path.to_str().unwrap(),
+        tmp.to_str().unwrap(),
+        RouterMode::LeastLoaded,
+    );
+    cfg.cluster.global_router.mode = GlobalRouterMode::StrictInputLength;
+    cfg.sim.sample_interval_s = 0.05;
+
+    let _ = driver::run(&cfg, Some("strict_bucket")).expect("bucketed run");
+
+    let per_request = std::fs::read_to_string(tmp.join("strict_bucket/per_request.csv")).unwrap();
+    assert!(per_request.contains("bucket"));
+    assert!(per_request.contains("length_bucket_match"));
+
+    let instances = std::fs::read_to_string(tmp.join("strict_bucket/instances.csv")).unwrap();
+    assert!(instances.contains("bucket"));
+
+    let routing_log = std::fs::read_to_string(tmp.join("strict_bucket/routing_log.jsonl")).unwrap();
+    assert!(routing_log.contains("\"chosen_bucket\""));
+    assert!(routing_log.contains("\"bucket_candidates\""));
+    assert!(routing_log.contains("\"global_reason\""));
+}
+
+#[test]
+fn bucketed_configs_are_rejected_by_legacy_fixed_placement_paths() {
+    let tmp = std::env::temp_dir().join("kvcache_sim_bucketed_reject");
+    let _ = std::fs::remove_dir_all(&tmp);
+    std::fs::create_dir_all(&tmp).unwrap();
+    let trace_path = tmp.join("trace.jsonl");
+    write_synthetic_trace(&trace_path);
+
+    let cfg = bucketed_config(
+        trace_path.to_str().unwrap(),
+        tmp.to_str().unwrap(),
+        RouterMode::Random,
+    );
+
+    let err =
+        driver::ablate_fixed_placement(&cfg, &[RouterMode::Random], &[ReplayEvictPolicy::Lru])
+            .expect_err("bucketed ablation should fail");
+    assert!(err.to_string().contains("cluster.buckets"));
+
+    let err = replay::replay_fixed_placement(
+        &cfg,
+        &[],
+        &Vec::<PlacementEntry>::new(),
+        ReplayEvictPolicy::Lru,
+    )
+    .expect_err("bucketed replay should fail");
+    assert!(err.to_string().contains("cluster.buckets"));
+}
+
+#[test]
+fn bucket_score_can_deviate_from_strict_length_bucket() {
+    let tmp = std::env::temp_dir().join("kvcache_sim_bucket_score");
+    let _ = std::fs::remove_dir_all(&tmp);
+    std::fs::create_dir_all(&tmp).unwrap();
+    let trace_path = tmp.join("trace.jsonl");
+
+    let mut f = std::fs::File::create(&trace_path).unwrap();
+    for req_id in 0..3 {
+        writeln!(
+            f,
+            "{}",
+            serde_json::json!({
+                "chat_id": req_id,
+                "parent_chat_id": -1,
+                "timestamp": 0.0,
+                "input_length": 24,
+                "output_length": 16,
+                "type": "text",
+                "turn": 0,
+                "hash_ids": [100 + req_id, 200 + req_id]
+            })
+        )
+        .unwrap();
+    }
+
+    let mut strict_cfg = bucketed_config(
+        trace_path.to_str().unwrap(),
+        tmp.to_str().unwrap(),
+        RouterMode::LeastLoaded,
+    );
+    strict_cfg.cluster.buckets = vec![
+        BucketConfig {
+            name: "short".into(),
+            input_length_min: 0,
+            input_length_max: 32,
+            num_instances: 1,
+        },
+        BucketConfig {
+            name: "long".into(),
+            input_length_min: 33,
+            input_length_max: 96,
+            num_instances: 1,
+        },
+    ];
+    strict_cfg.cluster.global_router.mode = GlobalRouterMode::StrictInputLength;
+
+    let mut score_cfg = strict_cfg.clone();
+    score_cfg.cluster.global_router.mode = GlobalRouterMode::BucketScore;
+    score_cfg.cluster.global_router.length_penalty_weight = 1.0;
+    score_cfg.cluster.global_router.load_weight = 10.0;
+    score_cfg.cluster.global_router.cache_weight = 0.0;
+
+    let _ = driver::run(&strict_cfg, Some("strict_score_cmp")).expect("strict run");
+    let _ = driver::run(&score_cfg, Some("bucket_score_cmp")).expect("bucket score run");
+
+    let strict_log =
+        std::fs::read_to_string(tmp.join("strict_score_cmp/routing_log.jsonl")).unwrap();
+    let score_log =
+        std::fs::read_to_string(tmp.join("bucket_score_cmp/routing_log.jsonl")).unwrap();
+
+    assert!(strict_log.contains("\"chosen_bucket\":0"));
+    assert!(score_log.contains("\"global_mode\":\"bucket_score\""));
+    assert!(score_log.contains("\"chosen_bucket\":1"));
+}