refactor(neuron): phase 3 — TP forward + NCCL state move onto device worker

Third slice of the per-device CUDA context-ownership refactor planned at
~/.claude/plans/plan-the-per-device-worker-abstract-micali.md. The
leader's `NcclState`, every `Comm::all_reduce` issued by the TP layers,
the leader-side KV cache reset, and the TP forward step itself now all
run on the per-device worker thread — the same OS thread that bound
the leader's `CudaContext` at startup.

What this phase changes:

- `Job` gains `NcclInit`, `NcclSanity`, `CloneLeaderComm` (Phase 3
  bridge — Phase 4 removes), `TransferInTp`, `DropTp`, `TpClearKv`,
  `TpForwardLogits`. Plus a new `TpHandle(u64)` opaque key.
- `DeviceWorkerState` gains `nccl: NcclState` and
  `tp_models: HashMap<TpHandle, Box<TpLeaderModel>>` (+ counter).
- `WorkerPool` loses its `leader_nccl` field; gains a
  `leader_worker: Arc<DeviceWorkerHandle>` passed at construction.
  `init_nccl`, `nccl_sanity_check`, `load_dense_shard`,
  `generate_step`, `clear_kv_cache` all route their leader-side ops
  through `Job::Nccl*` / `Job::Tp*` instead of spawn_blocking against
  a Mutex-wrapped state. `generate_step` returns `Vec<f32>` instead
  of a device-resident `Tensor` — the worker copies logits to CPU
  before reply so the async caller can sample on a CPU candle
  tensor with zero device-context touch.
- `TpLoadedModel.leader_model: Arc<Mutex<TpLeaderModel>>` → opaque
  `leader_handle: TpHandle`. The boxed `TpLeaderModel` lives in the
  worker thread's slab; both the model's CUDA tensors and the
  embedded `Arc<Comm>` clones release on the same thread that
  allocated them (the Drop semantics constraint cudarc forces).
- `Job::CloneLeaderComm` is a Phase 3 bridge: the TP shard load still
  runs in spawn_blocking and needs the leader's `Arc<Comm>` to build
  the row-parallel layers' AllReduce ops. The Job clones the Comm
  out of the worker's NcclState and ships it back as `SendComm`.
  Phase 4 deletes this bridge when the load itself moves onto the
  worker.
- `Job::NcclInit` and `Job::NcclSanity` are ungated by `cuda` so the
  no-cuda `NcclState` stubs (which reply with `cuda_feature_not_enabled`)
  still flow through the same channel uniformly; the cuda-only
  TP variants (CloneLeaderComm, Transfer/Drop/Clear/Forward Tp)
  remain gated.

What this phase doesn't touch (yet):

- TP shard load itself — still spawn_blocking, bridged via
  `CloneLeaderComm`. Phase 4 moves it to `Job::TpLoadShard` and
  reads `state.nccl.comm()` directly inside the worker.
- Single-GPU model loads — still spawn_blocking, transferred via
  `Job::TransferIn`. Phase 4 moves them.
- `device_vram_mb` / `cuda_mem_mb` / `log_construction_complete`
  helpers — still present, used inside spawn_blocking load closures.
  Phase 4 cleanup folds them into `dispatch.rs`.

`tp/mod.rs::WorkerPool::spawn` gained a required
`leader_worker: Arc<DeviceWorkerHandle>` argument. Three external
callers were updated: `CandleHarness::load_tp` (passes the cached
device worker), `main.rs::tp_smoke` (spawns a fresh worker), and
the two `tp_worker_lifecycle*.rs` integration tests.

Public API unchanged. fmt + clippy clean; 37 lib tests + all
integration tests pass. CUDA-only TP integration smoke deferred to
the next deploy on beast.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/neuron/src/harness/candle.rs

@@ -163,16 +163,22 @@ pub struct TpLoadedModel {
     pub model_id: String,
     pub tokenizer: Tokenizer,
     pub devices: Vec<u32>,
-    /// One end-to-end gate: the pool's RPC stream isn't safe to use
+    /// One end-to-end gate: the pool's RPC stream to the subprocess
-    /// concurrently and the leader shard's KV cache mutates with every
+    /// workers isn't safe to use concurrently. After Phase 3 the
-    /// step. The same Mutex covers both for the simplest correctness
+    /// leader's `TpLeaderModel` lives in the worker thread's slab,
-    /// story.
+    /// so this Mutex no longer covers the leader's KV cache; it just
+    /// serialises subprocess RPC traffic on the pool's
+    /// `Vec<Worker>` channels.
     pub pool: tokio::sync::Mutex<super::tp::WorkerPool>,
-    pub leader_model: Arc<tokio::sync::Mutex<super::tp::TpLeaderModel>>,
+    /// Handle into the leader device worker's TP slab. The boxed
-    /// Candle device for rank 0. Mirrors what `leader_model.device()`
+    /// `TpLeaderModel` (with its embedded `Arc<Comm>` clones and
-    /// would return, but stored separately so the request path can
+    /// per-rank CUDA tensors) lives on the worker thread; we hold an
-    /// query VRAM without locking the leader (which would contend with
+    /// opaque index. Forward / clear_kv / unload all route through
-    /// the in-flight forward).
+    /// `Job::Tp*` against this handle.
+    pub leader_handle: super::device_worker::TpHandle,
+    /// Candle device for rank 0. Mirrors what
+    /// `TpLeaderModel::device()` would return, kept on the struct so
+    /// the request path can name the device without an RPC.
     pub leader_device: Device,
     /// Same poisoning gate as [`LoadedModel::poisoned`]. A TP forward
     /// failure (CUDA OOM on any rank, NCCL desync, illegal address) is
@@ -180,9 +186,8 @@ pub struct TpLoadedModel {
     /// reliably reset without restarting the worker subprocesses.
     pub poisoned: AtomicBool,
     /// Worker thread for the leader's CUDA device. Owns the leader's
-    /// `CudaContext` for the daemon's lifetime. VRAM queries route
+    /// `CudaContext`, `NcclState`, and the boxed `TpLeaderModel`
-    /// through it; in later refactor phases the forward, kv-cache
+    /// referenced by `leader_handle`.
-    /// clear, and shard unload route through it too.
     pub worker: Arc<super::device_worker::DeviceWorkerHandle>,
 }
 
@@ -1642,6 +1647,16 @@ impl Harness for CandleHarness {
                         anyhow::bail!("cannot unload '{model_id}': inference still in flight");
                     }
                 };
+                // Drop the leader's TpLeaderModel on the device worker
+                // thread (CUDA tensors and Arc<Comm> clones release on
+                // the same OS thread that allocated them).
+                if let Err(e) = tp.worker.drop_tp(tp.leader_handle).await {
+                    tracing::warn!(
+                        model = %model_id,
+                        error = %e,
+                        "TP unload: DropTp RPC failed (leader model may leak in worker slab)"
+                    );
+                }
                 let mut pool = tp.pool.into_inner();
                 if let Err(e) = pool.unload_model(model_id).await {
                     tracing::warn!(model = %model_id, error = %e, "TP unload RPC failed");
@@ -1715,9 +1730,14 @@ impl CandleHarness {
 
         // 2. Spawn the worker pool. Rank 0 stays in-process; ranks
         //    1..tp_size are subprocesses, one per device after the
-        //    leader's own.
+        //    leader's own. The leader's device worker thread is
+        //    spawned (or reused) here and passed into the pool so
+        //    `init_nccl`, the load, every TP forward, and KV-cache
+        //    clears all dispatch from the same OS thread.
         let exe = std::env::current_exe().context("resolve current_exe for worker spawn")?;
-        let mut pool = super::tp::WorkerPool::spawn(&exe, tp_size, &devices).await?;
+        let leader_worker = self.ensure_device_worker(devices[0]).await?;
+        let mut pool =
+            super::tp::WorkerPool::spawn(&exe, tp_size, &devices, leader_worker.clone()).await?;
 
         // 3. NCCL handshake across all ranks.
         let leader_device_idx = devices[0];
@@ -1727,8 +1747,11 @@ impl CandleHarness {
         let leader_device = candle_core::Device::new_cuda(leader_device_idx as usize)
             .context("Device::new_cuda for TP leader")?;
 
-        // 5. Load this rank's shard on every rank.
+        // 5. Load this rank's shard on every rank. After Phase 3
-        let leader_model = pool
+        //    `load_dense_shard` transfers the freshly-built
+        //    `TpLeaderModel` into the device worker's TP slab and
+        //    returns the resulting handle.
+        let leader_handle = pool
             .load_dense_shard(
                 &spec.model_id,
                 &config_json,
@@ -1743,21 +1766,18 @@ impl CandleHarness {
         let tokenizer = Tokenizer::from_file(&tokenizer_path)
             .map_err(|e| anyhow::anyhow!("load tokenizer: {e}"))?;
 
-        // 7. Worker thread for the leader's CUDA device. TP always
-        // runs on CUDA — the harness rejects TP without the cuda
-        // feature earlier in this function — so we always have a
-        // device to own.
-        let worker = self.ensure_device_worker(devices[0]).await?;
-
         let tp_loaded = StdArc::new(TpLoadedModel {
             model_id: spec.model_id.clone(),
             tokenizer,
             devices: devices.clone(),
             pool: TMutex::new(pool),
-            leader_model,
+            leader_handle,
             leader_device: leader_device.clone(),
             poisoned: AtomicBool::new(false),
-            worker,
+            // Same `leader_worker` we passed into the pool above —
+            // single `Arc` shared between WorkerPool and
+            // TpLoadedModel so they reference the same thread.
+            worker: leader_worker,
         });
 
         let mut models = self.models.write().await;
@@ -1932,14 +1952,14 @@ impl CandleHarness {
             async move {
                 let mut failure: Option<String> = None;
                 let mut pool = acquire_pool_lock(&tp_for_task.pool, &model_id).await;
-                let leader_arc = tp_for_task.leader_model.clone();
+                let leader_handle = tp_for_task.leader_handle;
 
                 let mut all_tokens: Vec<u32> = Vec::new();
                 let mut decoded_prefix = String::new();
                 let mut finish_reason = "length".to_string();
 
                 'work: {
-                    if let Err(e) = pool.clear_kv_cache(&model_id, leader_arc.clone()).await {
+                    if let Err(e) = pool.clear_kv_cache(&model_id, leader_handle).await {
                         failure = Some(format!("clear_kv_cache: {e:#}"));
                         break 'work;
                     }
@@ -1957,8 +1977,8 @@ impl CandleHarness {
                     };
 
                     // Prefill — every rank embeds the prompt, offset = 0.
-                    let logits = match pool
+                    let logits_vec = match pool
-                        .generate_step(&model_id, leader_arc.clone(), prompt_tokens.clone(), 0)
+                        .generate_step(&model_id, leader_handle, prompt_tokens.clone(), 0)
                         .await
                     {
                         Ok(l) => l,
@@ -1974,11 +1994,18 @@ impl CandleHarness {
                         vram_free_mb = post_prefill_vram_free_mb,
                         "TP chat_completion (stream): prefill complete"
                     );
+                    let logits = match Tensor::new(logits_vec.as_slice(), &Device::Cpu) {
+                        Ok(t) => t,
+                        Err(e) => {
+                            failure = Some(format!("prefill build cpu logits: {e:#}"));
+                            break 'work;
+                        }
+                    };
                     let mut next_token =
                         match sample_with_penalty(&logits, &all_tokens, &mut logits_processor) {
                             Ok(t) => t,
                             Err(e) => {
-                                let health = logits_health(&logits);
+                                let health = logits_health_slice(&logits_vec);
                                 tracing::warn!(
                                     model = %model_id,
                                     ?health,
@@ -2010,10 +2037,10 @@ impl CandleHarness {
                         }
 
                         for index in 0..max_new.saturating_sub(1) {
-                            let logits = match pool
+                            let logits_vec = match pool
                                 .generate_step(
                                     &model_id,
-                                    leader_arc.clone(),
+                                    leader_handle,
                                     vec![next_token],
                                     prompt_len + index,
                                 )
@@ -2025,6 +2052,14 @@ impl CandleHarness {
                                     break 'work;
                                 }
                             };
+                            let logits = match Tensor::new(logits_vec.as_slice(), &Device::Cpu) {
+                                Ok(t) => t,
+                                Err(e) => {
+                                    failure =
+                                        Some(format!("decode build cpu logits {index}: {e:#}"));
+                                    break 'work;
+                                }
+                            };
                             next_token = match sample_with_penalty(
                                 &logits,
                                 &all_tokens,
@@ -2032,7 +2067,7 @@ impl CandleHarness {
                             ) {
                                 Ok(t) => t,
                                 Err(e) => {
-                                    let health = logits_health(&logits);
+                                    let health = logits_health_slice(&logits_vec);
                                     tracing::warn!(
                                         model = %model_id,
                                         step = index,
@@ -2180,20 +2215,19 @@ async fn chat_completion_tp_inner(
         "TP chat_completion: starting"
     );
 
-    // Acquire the pool lock for the duration of the request. The
+    // Acquire the pool lock for the duration of the request. After
-    // leader_model's own Mutex is acquired step-by-step inside
+    // Phase 3 the leader's TpLeaderModel lives in the device worker
-    // pool.generate_step (so spawn_blocking can grab it without
+    // thread, so the pool lock now serialises only subprocess RPC
-    // holding the pool lock across the blocking_lock call).
+    // traffic — but holding it for the whole request still keeps
-    // `acquire_pool_lock` warns periodically while we wait so a
+    // concurrent chat_completions against the same TP model from
-    // stuck holder doesn't make the queueing requests look like
+    // interleaving prefill/decode jobs.
-    // silence in the journal.
     let mut pool = acquire_pool_lock(&tp.pool, &model_id).await;
-    let leader_arc = tp.leader_model.clone();
+    let leader_handle = tp.leader_handle;
 
     // Reset every rank's KV cache so this request doesn't attend
     // over the previous request's tokens.
     let clear_start = std::time::Instant::now();
-    pool.clear_kv_cache(&model_id, leader_arc.clone())
+    pool.clear_kv_cache(&model_id, leader_handle)
         .await
         .map_err(InferenceError::Other)?;
     tracing::debug!(
@@ -2219,8 +2253,8 @@ async fn chat_completion_tp_inner(
 
     // Prefill: every rank embeds the whole prompt, offset = 0.
     let prefill_start = std::time::Instant::now();
-    let logits = pool
+    let logits_vec = pool
-        .generate_step(&model_id, leader_arc.clone(), prompt_tokens.clone(), 0)
+        .generate_step(&model_id, leader_handle, prompt_tokens.clone(), 0)
         .await
         .map_err(InferenceError::Other)?;
     let (post_prefill_vram_free_mb, _) = tp.query_vram().await;
@@ -2231,6 +2265,11 @@ async fn chat_completion_tp_inner(
         vram_free_mb = post_prefill_vram_free_mb,
         "TP chat_completion: prefill complete"
     );
+    // Wrap the CPU-side logits in a CPU candle Tensor for sampling.
+    // No device touch on the async caller's thread — sampling reads
+    // from CPU memory only.
+    let logits = Tensor::new(logits_vec.as_slice(), &Device::Cpu)
+        .map_err(|e| InferenceError::Other(anyhow::anyhow!("build cpu logits: {e}")))?;
     let mut next_token = match sample_with_penalty(&logits, &generated, &mut logits_processor) {
         Ok(t) => t,
         Err(e) => {
@@ -2239,7 +2278,7 @@ async fn chat_completion_tp_inner(
             // this WARN sits just above that and carries the actual
             // numerical state so an operator can tell at a glance
             // whether it was a NaN cascade, an Inf, or something else.
-            let health = logits_health(&logits);
+            let health = logits_health_slice(&logits_vec);
             tracing::warn!(
                 model = %model_id,
                 ?health,
@@ -2256,19 +2295,22 @@ async fn chat_completion_tp_inner(
         let decode_start = std::time::Instant::now();
         for index in 0..max_new.saturating_sub(1) {
             let step_start = std::time::Instant::now();
-            let logits = pool
+            let logits_vec = pool
                 .generate_step(
                     &model_id,
-                    leader_arc.clone(),
+                    leader_handle,
                     vec![next_token],
                     prompt_len + index,
                 )
                 .await
                 .map_err(InferenceError::Other)?;
+            let logits = Tensor::new(logits_vec.as_slice(), &Device::Cpu).map_err(|e| {
+                InferenceError::Other(anyhow::anyhow!("build cpu logits step {index}: {e}"))
+            })?;
             next_token = match sample_with_penalty(&logits, &generated, &mut logits_processor) {
                 Ok(t) => t,
                 Err(e) => {
-                    let health = logits_health(&logits);
+                    let health = logits_health_slice(&logits_vec);
                     tracing::warn!(
                         model = %model_id,
                         step = index,

crates/neuron/src/harness/device_worker/dispatch.rs

@@ -14,7 +14,12 @@
 //! `tp_models: HashMap<TpHandle, Box<TpLeaderModel>>`.
 
 use crate::harness::candle::ModelArch;
+#[cfg(feature = "cuda")]
+use crate::harness::device_worker::jobs::TpHandle;
 use crate::harness::device_worker::jobs::{ArchHandle, Job};
+#[cfg(feature = "cuda")]
+use crate::harness::tp::TpLeaderModel;
+use crate::harness::tp::nccl_state::NcclState;
 use std::collections::HashMap;
 use std::sync::Arc;
 use std::sync::atomic::{AtomicBool, Ordering};
@@ -40,6 +45,20 @@ struct DeviceWorkerState {
     /// increments and returns the new value. Wraps at u64::MAX after
     /// ~10^19 model loads — not a practical concern.
     next_handle: u64,
+    /// Leader's NCCL state. Populated by `Job::NcclInit`; the
+    /// underlying `Comm`'s libnccl handle lives bound to this thread
+    /// for its entire lifetime. Subprocess workers maintain their own
+    /// `NcclState` in their own processes — that's not visible from
+    /// here.
+    #[allow(dead_code)] // Read only via methods on NcclState
+    nccl: NcclState,
+    /// TP leader model slab. Same lifecycle as `models`; separate
+    /// namespace so `ArchHandle` and `TpHandle` can't collide.
+    #[cfg(feature = "cuda")]
+    tp_models: HashMap<TpHandle, Box<TpLeaderModel>>,
+    /// Counter for minting fresh `TpHandle`s.
+    #[cfg(feature = "cuda")]
+    next_tp_handle: u64,
     #[cfg(feature = "cuda")]
     #[allow(dead_code)]
     /// `None` only if `CudaContext::new()` failed — in that case the
@@ -128,15 +147,93 @@ pub(crate) fn run(device_index: u32, rx: Receiver<Job>, poisoned: Arc<AtomicBool
                 let result = forward_logits(&mut state, handle, &tokens, offset);
                 let _ = reply.send(result);
             }
+            Job::NcclInit {
+                cfg,
+                comm_id_hex,
+                reply,
+            } => {
+                let resp = state.nccl.init(cfg, &comm_id_hex);
+                let _ = reply.send(resp);
+            }
+            Job::NcclSanity { reply } => {
+                let resp = state.nccl.sanity_check();
+                let _ = reply.send(resp);
+            }
+            #[cfg(feature = "cuda")]
+            Job::CloneLeaderComm { reply } => {
+                let result = match state.nccl.comm() {
+                    Some(comm) => Ok(crate::harness::tp::nccl_state::SendComm(comm)),
+                    None => Err(anyhow::anyhow!(
+                        "CloneLeaderComm: NcclState has no Comm; call NcclInit first"
+                    )),
+                };
+                let _ = reply.send(result);
+            }
+            #[cfg(feature = "cuda")]
+            Job::TransferInTp { model, reply } => {
+                let handle = TpHandle(state.next_tp_handle);
+                state.next_tp_handle = state.next_tp_handle.wrapping_add(1);
+                state.tp_models.insert(handle, model);
+                tracing::debug!(
+                    device_index,
+                    tp_handle = handle.0,
+                    slab_size = state.tp_models.len(),
+                    "device worker: TP model transferred in"
+                );
+                let _ = reply.send(Ok(handle));
+            }
+            #[cfg(feature = "cuda")]
+            Job::DropTp { handle, reply } => {
+                let removed = state.tp_models.remove(&handle);
+                let was_present = removed.is_some();
+                drop(removed);
+                tracing::debug!(
+                    device_index,
+                    tp_handle = handle.0,
+                    was_present,
+                    slab_size = state.tp_models.len(),
+                    "device worker: TP model dropped"
+                );
+                let _ = reply.send(());
+            }
+            #[cfg(feature = "cuda")]
+            Job::TpClearKv { handle, reply } => {
+                let result = match state.tp_models.get_mut(&handle) {
+                    Some(model) => {
+                        model.clear_kv_cache();
+                        Ok(())
+                    }
+                    None => Err(anyhow::anyhow!(
+                        "TpClearKv: no TP model for handle {}",
+                        handle.0
+                    )),
+                };
+                let _ = reply.send(result);
+            }
+            #[cfg(feature = "cuda")]
+            Job::TpForwardLogits {
+                handle,
+                tokens,
+                offset,
+                reply,
+            } => {
+                let result = tp_forward_logits(&mut state, handle, &tokens, offset);
+                let _ = reply.send(result);
+            }
             // Handled by the matches!() check above; reaching here
             // means a Shutdown slipped past which is a bug.
             Job::Shutdown => unreachable!("Shutdown should break above"),
         }
     }
 
+    #[cfg(feature = "cuda")]
+    let tp_slab_size = state.tp_models.len();
+    #[cfg(not(feature = "cuda"))]
+    let tp_slab_size = 0_usize;
     tracing::info!(
         device_index,
         slab_size = state.models.len(),
+        tp_slab_size,
         "device worker exiting; dropping remaining models"
     );
     // Drops every model in the slab on this thread before the function
@@ -193,6 +290,9 @@ fn init_state(device_index: u32) -> DeviceWorkerState {
             device,
             models: HashMap::new(),
             next_handle: 1,
+            nccl: NcclState::new(),
+            tp_models: HashMap::new(),
+            next_tp_handle: 1,
             ctx,
         }
     }
@@ -203,6 +303,7 @@ fn init_state(device_index: u32) -> DeviceWorkerState {
             device: candle_core::Device::Cpu,
             models: HashMap::new(),
             next_handle: 1,
+            nccl: NcclState::new(),
         }
     }
 }
@@ -231,6 +332,38 @@ fn query_vram(_state: &DeviceWorkerState) -> anyhow::Result<(u64, u64)> {
     Ok((0, 0))
 }
 
+/// TP-equivalent of [`forward_logits`]: looks up the leader's
+/// [`TpLeaderModel`] in the slab, runs its forward, copies the
+/// `[vocab]` logits to a CPU `Vec<f32>`. The leader's `Arc<Comm>`
+/// clones embedded in the TP layers' AllReduce ops fire from this
+/// thread — same thread that bound the CUDA context and that holds
+/// the `Comm` in `state.nccl`.
+#[cfg(feature = "cuda")]
+fn tp_forward_logits(
+    state: &mut DeviceWorkerState,
+    handle: TpHandle,
+    tokens: &[u32],
+    offset: usize,
+) -> anyhow::Result<Vec<f32>> {
+    use candle_core::{DType, Tensor};
+
+    let input = Tensor::new(tokens, &state.device)?.unsqueeze(0)?;
+
+    let model = state
+        .tp_models
+        .get_mut(&handle)
+        .ok_or_else(|| anyhow::anyhow!("TpForwardLogits: no model for handle {}", handle.0))?;
+
+    let logits = model.forward(&input, offset)?;
+    // ForCausalLM forward returns [B, 1, V] after the trailing
+    // .i((.., l - 1.., ..))?.apply(lm_head); squeeze both leading
+    // singleton dims to a rank-1 [V] tensor for sampling.
+    let logits = logits.squeeze(0)?.squeeze(0)?;
+    let logits = logits.to_dtype(DType::F32)?.flatten_all()?;
+    let values = logits.to_vec1::<f32>()?;
+    Ok(values)
+}
+
 /// Forward step + copy the `[vocab]` logits to a CPU `Vec<f32>` ready
 /// for sampling on the async caller. The model's `device()` (CUDA or
 /// CPU) determines where the kernel runs; this fn doesn't care.
@@ -297,6 +430,38 @@ fn drain_poisoned(job: Job, device_index: u32) {
         Job::ForwardLogits { reply, .. } => {
             let _ = reply.send(Err(err()));
         }
+        Job::NcclInit { reply, .. } => {
+            let _ = reply.send(crate::harness::tp::rpc::WorkerResponse::Error {
+                kind: "device_worker_poisoned".into(),
+                message: format!("device worker {device_index} poisoned"),
+            });
+        }
+        Job::NcclSanity { reply } => {
+            let _ = reply.send(crate::harness::tp::rpc::WorkerResponse::Error {
+                kind: "device_worker_poisoned".into(),
+                message: format!("device worker {device_index} poisoned"),
+            });
+        }
+        #[cfg(feature = "cuda")]
+        Job::CloneLeaderComm { reply } => {
+            let _ = reply.send(Err(err()));
+        }
+        #[cfg(feature = "cuda")]
+        Job::TransferInTp { reply, .. } => {
+            let _ = reply.send(Err(err()));
+        }
+        #[cfg(feature = "cuda")]
+        Job::DropTp { reply, .. } => {
+            let _ = reply.send(());
+        }
+        #[cfg(feature = "cuda")]
+        Job::TpClearKv { reply, .. } => {
+            let _ = reply.send(Err(err()));
+        }
+        #[cfg(feature = "cuda")]
+        Job::TpForwardLogits { reply, .. } => {
+            let _ = reply.send(Err(err()));
+        }
         Job::Shutdown => {
             // Filtered by the matches!() guard in run(); reaching
             // here would be a logic error.

crates/neuron/src/harness/device_worker/jobs.rs

@@ -19,6 +19,15 @@ use tokio::sync::oneshot;
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct ArchHandle(pub u64);
 
+/// Opaque handle to a `TpLeaderModel` stored in the worker thread's
+/// state slab. Same shape as [`ArchHandle`] but in a separate
+/// namespace so the two slabs can coexist without ambiguity. Phase 3
+/// introduces it; Phase 4 may unify the two slabs after the TP forward
+/// path proves out.
+#[cfg(feature = "cuda")]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct TpHandle(pub u64);
+
 /// One unit of work for the device worker.
 ///
 /// Phase 1 had only `QueryVram` and `Shutdown`. Phase 2 adds the
@@ -73,6 +82,74 @@ pub enum Job {
         offset: usize,
         reply: oneshot::Sender<Result<Vec<f32>>>,
     },
+    /// Initialize the leader's NCCL communicator. The worker's
+    /// `NcclState` mints the `Comm` here so its underlying
+    /// `ncclComm_t` and `CudaContext` live on the same thread as
+    /// every later `Comm::all_reduce` call. Reply is the worker
+    /// response shape used by the subprocess workers (`InitOk` on
+    /// success, `Error` on failure) so the calling
+    /// `WorkerPool::init_nccl` orchestration stays uniform.
+    ///
+    /// Available on both cuda and no-cuda builds — the dispatch
+    /// handler calls `NcclState::init` which has a no-cuda stub that
+    /// replies with `cuda_feature_not_enabled`. Keeping the Job
+    /// variant ungated lets `WorkerPool::init_nccl` stay uniform.
+    NcclInit {
+        cfg: crate::harness::tp::worker::WorkerConfig,
+        comm_id_hex: String,
+        reply: oneshot::Sender<crate::harness::tp::rpc::WorkerResponse>,
+    },
+    /// Run NCCL's all_reduce sanity check on the leader's rank 0.
+    /// Same response shape as `NcclInit`; also available on both
+    /// builds via the no-cuda `NcclState::sanity_check` stub.
+    NcclSanity {
+        reply: oneshot::Sender<crate::harness::tp::rpc::WorkerResponse>,
+    },
+    /// Clone the leader's `Arc<Comm>` out of the worker's `NcclState`
+    /// so a spawn_blocking-based load (Phase 3 bridge) can hand it to
+    /// the row-parallel layers. Wrapped in `SendComm` because
+    /// `Arc<Comm>` is `!Send` at the type level (the NCCL contract
+    /// requires serialised access, which we provide structurally).
+    /// Phase 4 eliminates this when `TpLoadShard` becomes a Job and
+    /// the load runs entirely on the worker thread.
+    #[cfg(feature = "cuda")]
+    CloneLeaderComm {
+        reply: oneshot::Sender<Result<crate::harness::tp::nccl_state::SendComm>>,
+    },
+    /// Move a freshly-built `TpLeaderModel` into the worker's tp slab.
+    /// Returns a `TpHandle` the caller stores on `TpLoadedModel`.
+    #[cfg(feature = "cuda")]
+    TransferInTp {
+        model: Box<crate::harness::tp::TpLeaderModel>,
+        reply: oneshot::Sender<Result<TpHandle>>,
+    },
+    /// Drop the TP leader model on the worker thread. CUDA tensors
+    /// and `Arc<Comm>` clones held inside the model release on the
+    /// thread that allocated them.
+    #[cfg(feature = "cuda")]
+    DropTp {
+        handle: TpHandle,
+        reply: oneshot::Sender<()>,
+    },
+    /// Reset the leader's KV cache for a TP model. Mirrors `ClearKv`
+    /// for single-GPU.
+    #[cfg(feature = "cuda")]
+    TpClearKv {
+        handle: TpHandle,
+        reply: oneshot::Sender<Result<()>>,
+    },
+    /// Run one TP forward step on the leader's shard. Returns CPU-
+    /// side logits as a `Vec<f32>` so the async caller can sample
+    /// without holding a device tensor. The caller is also
+    /// responsible for fan-out to subprocess ranks and drain — only
+    /// the leader's forward moves into the worker thread.
+    #[cfg(feature = "cuda")]
+    TpForwardLogits {
+        handle: TpHandle,
+        tokens: Vec<u32>,
+        offset: usize,
+        reply: oneshot::Sender<Result<Vec<f32>>>,
+    },
     /// Tell the worker to break its dispatch loop and exit. Any jobs
     /// queued after this in the channel reply `Err` to their oneshot
     /// senders (the senders are dropped on the worker's exit, which

crates/neuron/src/harness/device_worker/mod.rs

@@ -49,6 +49,8 @@ use std::sync::mpsc::{self, Sender};
 use std::thread::JoinHandle;
 use tokio::sync::oneshot;
 
+#[cfg(feature = "cuda")]
+pub use jobs::TpHandle;
 pub use jobs::{ArchHandle, Job};
 
 /// Errors returned by `DeviceWorkerHandle` submit methods.
@@ -277,6 +279,192 @@ impl DeviceWorkerHandle {
         }
     }
 
+    /// Initialise the leader's NCCL communicator. The reply uses
+    /// `WorkerResponse` (same shape subprocess workers use over stdio
+    /// RPC) so `WorkerPool::init_nccl`'s aggregation treats leader +
+    /// subprocess responses uniformly. Available on no-cuda builds
+    /// too — the dispatch handler calls the no-cuda `NcclState::init`
+    /// stub which replies `cuda_feature_not_enabled`.
+    pub async fn nccl_init(
+        &self,
+        cfg: crate::harness::tp::worker::WorkerConfig,
+        comm_id_hex: String,
+    ) -> Result<crate::harness::tp::rpc::WorkerResponse, WorkerError> {
+        if self.poisoned.load(Ordering::Acquire) {
+            return Err(WorkerError::Poisoned {
+                device_index: self.device_index,
+            });
+        }
+        let (reply_tx, reply_rx) = oneshot::channel();
+        self.tx
+            .send(Job::NcclInit {
+                cfg,
+                comm_id_hex,
+                reply: reply_tx,
+            })
+            .map_err(|_| WorkerError::Gone {
+                device_index: self.device_index,
+            })?;
+        reply_rx.await.map_err(|_| WorkerError::Gone {
+            device_index: self.device_index,
+        })
+    }
+
+    /// Run an NCCL sanity all_reduce on the leader's rank 0.
+    /// Available on no-cuda builds; replies with an error response.
+    pub async fn nccl_sanity(
+        &self,
+    ) -> Result<crate::harness::tp::rpc::WorkerResponse, WorkerError> {
+        if self.poisoned.load(Ordering::Acquire) {
+            return Err(WorkerError::Poisoned {
+                device_index: self.device_index,
+            });
+        }
+        let (reply_tx, reply_rx) = oneshot::channel();
+        self.tx
+            .send(Job::NcclSanity { reply: reply_tx })
+            .map_err(|_| WorkerError::Gone {
+                device_index: self.device_index,
+            })?;
+        reply_rx.await.map_err(|_| WorkerError::Gone {
+            device_index: self.device_index,
+        })
+    }
+
+    /// Clone the leader's `Arc<Comm>` so a spawn_blocking-based load
+    /// (Phase 3 bridge) can pass it to the row-parallel layers.
+    /// Phase 4 eliminates this once the TP load runs on this thread.
+    #[cfg(feature = "cuda")]
+    pub async fn clone_leader_comm(
+        &self,
+    ) -> Result<crate::harness::tp::nccl_state::SendComm, WorkerError> {
+        if self.poisoned.load(Ordering::Acquire) {
+            return Err(WorkerError::Poisoned {
+                device_index: self.device_index,
+            });
+        }
+        let (reply_tx, reply_rx) = oneshot::channel();
+        self.tx
+            .send(Job::CloneLeaderComm { reply: reply_tx })
+            .map_err(|_| WorkerError::Gone {
+                device_index: self.device_index,
+            })?;
+        match reply_rx.await {
+            Ok(result) => result.map_err(WorkerError::from),
+            Err(_) => Err(WorkerError::Gone {
+                device_index: self.device_index,
+            }),
+        }
+    }
+
+    /// Move a freshly-built `TpLeaderModel` into the worker's TP slab.
+    #[cfg(feature = "cuda")]
+    pub async fn transfer_in_tp(
+        &self,
+        model: Box<crate::harness::tp::TpLeaderModel>,
+    ) -> Result<TpHandle, WorkerError> {
+        if self.poisoned.load(Ordering::Acquire) {
+            return Err(WorkerError::Poisoned {
+                device_index: self.device_index,
+            });
+        }
+        let (reply_tx, reply_rx) = oneshot::channel();
+        self.tx
+            .send(Job::TransferInTp {
+                model,
+                reply: reply_tx,
+            })
+            .map_err(|_| WorkerError::Gone {
+                device_index: self.device_index,
+            })?;
+        match reply_rx.await {
+            Ok(result) => result.map_err(WorkerError::from),
+            Err(_) => Err(WorkerError::Gone {
+                device_index: self.device_index,
+            }),
+        }
+    }
+
+    /// Drop the TP model at `handle` on the worker thread.
+    #[cfg(feature = "cuda")]
+    pub async fn drop_tp(&self, handle: TpHandle) -> Result<(), WorkerError> {
+        let (reply_tx, reply_rx) = oneshot::channel();
+        self.tx
+            .send(Job::DropTp {
+                handle,
+                reply: reply_tx,
+            })
+            .map_err(|_| WorkerError::Gone {
+                device_index: self.device_index,
+            })?;
+        match reply_rx.await {
+            Ok(()) => Ok(()),
+            Err(_) => Err(WorkerError::Gone {
+                device_index: self.device_index,
+            }),
+        }
+    }
+
+    /// Reset the leader's KV cache for a TP model.
+    #[cfg(feature = "cuda")]
+    pub async fn tp_clear_kv(&self, handle: TpHandle) -> Result<(), WorkerError> {
+        if self.poisoned.load(Ordering::Acquire) {
+            return Err(WorkerError::Poisoned {
+                device_index: self.device_index,
+            });
+        }
+        let (reply_tx, reply_rx) = oneshot::channel();
+        self.tx
+            .send(Job::TpClearKv {
+                handle,
+                reply: reply_tx,
+            })
+            .map_err(|_| WorkerError::Gone {
+                device_index: self.device_index,
+            })?;
+        match reply_rx.await {
+            Ok(result) => result.map_err(WorkerError::from),
+            Err(_) => Err(WorkerError::Gone {
+                device_index: self.device_index,
+            }),
+        }
+    }
+
+    /// Run one TP forward step on the leader's shard. Returns CPU-side
+    /// logits as `Vec<f32>` ready for sampling. The caller is
+    /// responsible for fan-out / drain of the subprocess workers
+    /// concurrently with this call.
+    #[cfg(feature = "cuda")]
+    pub async fn tp_forward_logits(
+        &self,
+        handle: TpHandle,
+        tokens: Vec<u32>,
+        offset: usize,
+    ) -> Result<Vec<f32>, WorkerError> {
+        if self.poisoned.load(Ordering::Acquire) {
+            return Err(WorkerError::Poisoned {
+                device_index: self.device_index,
+            });
+        }
+        let (reply_tx, reply_rx) = oneshot::channel();
+        self.tx
+            .send(Job::TpForwardLogits {
+                handle,
+                tokens,
+                offset,
+                reply: reply_tx,
+            })
+            .map_err(|_| WorkerError::Gone {
+                device_index: self.device_index,
+            })?;
+        match reply_rx.await {
+            Ok(result) => result.map_err(WorkerError::from),
+            Err(_) => Err(WorkerError::Gone {
+                device_index: self.device_index,
+            }),
+        }
+    }
+
     /// Send `Job::Shutdown` and join the thread. Idempotent — calling
     /// twice is a no-op the second time.
     pub fn shutdown(&self) -> anyhow::Result<()> {

crates/neuron/src/harness/tp/mod.rs

@@ -212,10 +212,15 @@ pub struct WorkerPool {
     /// Path to the neuron binary used to launch workers.
     #[allow(dead_code)]
     exe: PathBuf,
-    /// Leader's own NCCL rank-0 state. Defaults to empty; populated by
+    /// The leader's per-device CUDA worker thread. Phase 3 moved the
-    /// `init_nccl()`. Held here so the leader can participate in
+    /// leader's `NcclState` (rank-0 NCCL Comm) into this thread, so
-    /// collectives (rank 0) without spawning a fourth subprocess.
+    /// every NCCL op (init, sanity, all_reduce inside forward) issues
-    leader_nccl: nccl_state::NcclState,
+    /// from one OS thread for the daemon's lifetime. The handle is
+    /// also used by `load_dense_shard` to clone the leader's
+    /// `Arc<Comm>` for the row-parallel layers' AllReduce ops; in
+    /// Phase 4 the load itself moves onto the worker and that bridge
+    /// goes away.
+    pub(crate) leader_worker: std::sync::Arc<super::device_worker::DeviceWorkerHandle>,
 }
 
 impl WorkerPool {
@@ -228,7 +233,12 @@ impl WorkerPool {
     /// sibling-binary path from `env!("CARGO_BIN_EXE_neuron")`).
     /// `cuda_devices` is one entry per rank including rank 0. Worker
     /// `i` (rank `i`) gets `cuda_devices[i]` as its `--cuda-device`.
-    pub async fn spawn(binary: &Path, world_size: u32, cuda_devices: &[u32]) -> Result<Self> {
+    pub async fn spawn(
+        binary: &Path,
+        world_size: u32,
+        cuda_devices: &[u32],
+        leader_worker: std::sync::Arc<super::device_worker::DeviceWorkerHandle>,
+    ) -> Result<Self> {
         if world_size < 2 {
             anyhow::bail!(
                 "WorkerPool::spawn called with world_size={world_size}; \
@@ -289,7 +299,7 @@ impl WorkerPool {
             world_size,
             workers,
             exe,
-            leader_nccl: nccl_state::NcclState::new(),
+            leader_worker,
         })
     }
 
@@ -321,27 +331,26 @@ impl WorkerPool {
         }
 
         // 2. Leader rank 0 calls Comm::from_rank on its own device.
-        //    Runs on spawn_blocking because NCCL's init blocks until
+        //    Phase 3 moved this from spawn_blocking onto the leader's
-        //    every rank has called in — that's exactly the workers
+        //    device worker thread (`Job::NcclInit`); the underlying
-        //    above. The leader's NcclState is moved through the
+        //    `Comm` now lives on the same OS thread for its entire
-        //    blocking task and returned to the pool.
+        //    lifetime, including every later `Comm::all_reduce` issued
+        //    by the row-parallel layers during forward.
+        //
+        //    NCCL's init blocks until every rank has called in — the
+        //    subprocess workers above and the leader's device worker
+        //    here. The Job's reply unblocks when the leader's
+        //    Comm::from_rank returns.
         let leader_cfg = worker::WorkerConfig {
             rank: 0,
             world_size: self.world_size,
             cuda_device: leader_cuda_device,
         };
-        let comm_id_for_leader = comm_id.clone();
+        let leader_resp = self
-        // Swap out the leader's NcclState into a fresh empty one so we
+            .leader_worker
-        // can move it into spawn_blocking; restore after the task
+            .nccl_init(leader_cfg, comm_id.clone())
-        // returns. (NcclState isn't Clone — it owns a real NCCL Comm.)
-        let mut leader_state = std::mem::take(&mut self.leader_nccl);
-        let (returned_state, leader_resp) = tokio::task::spawn_blocking(move || {
-            let resp = leader_state.init(leader_cfg, &comm_id_for_leader);
-            (leader_state, resp)
-        })
             .await
-        .context("leader NCCL init task panicked")?;
+            .map_err(|e| anyhow::anyhow!("leader NCCL init via device worker: {e}"))?;
-        self.leader_nccl = returned_state;
         match leader_resp {
             rpc::WorkerResponse::InitOk => {}
             rpc::WorkerResponse::Error { kind, message } => {
@@ -387,16 +396,16 @@ impl WorkerPool {
             w.send_only(&WorkerRequest::NcclSanityCheck).await?;
         }
 
-        // 2. Leader's own all_reduce, in spawn_blocking. NCCL operations
+        // 2. Leader's own all_reduce, on its device worker thread.
-        //    block until every rank participates.
+        //    NCCL operations block until every rank participates;
-        let mut leader_state = std::mem::take(&mut self.leader_nccl);
+        //    Job::NcclSanity returns once the leader's side completes
-        let (returned_state, leader_resp) = tokio::task::spawn_blocking(move || {
+        //    (which happens when every subprocess worker reaches its
-            let resp = leader_state.sanity_check();
+        //    all_reduce call too).
-            (leader_state, resp)
+        let leader_resp = self
-        })
+            .leader_worker
+            .nccl_sanity()
             .await
-        .context("leader NCCL sanity task panicked")?;
+            .map_err(|e| anyhow::anyhow!("leader NCCL sanity via device worker: {e}"))?;
-        self.leader_nccl = returned_state;
 
         let expected = self.world_size;
         let leader_sum = match leader_resp {
@@ -483,21 +492,24 @@ impl WorkerPool {
         leader_device: &candle_core::Device,
         dtype: candle_core::DType,
         quant: Option<String>,
-    ) -> Result<std::sync::Arc<tokio::sync::Mutex<TpLeaderModel>>> {
+    ) -> Result<super::device_worker::TpHandle> {
         use candle_nn::var_builder::ShardedSafeTensors;
-        use std::sync::Arc;
-        use tokio::sync::Mutex;
 
-        // Wrap the comm in SendComm immediately so it stays Send across
+        // Ask the leader's device worker for an `Arc<Comm>` clone.
-        // the await points in this method — bare Arc<Comm> would
+        // Phase 3 moved `NcclState` ownership onto the worker thread,
-        // poison the async fn's Send bound (Comm's raw NCCL pointer is
+        // so the spawn_blocking load below can no longer reach the
-        // !Send). The wrapper's safety contract is satisfied by the
+        // Comm directly. The reply is wrapped in `SendComm` because
-        // pool's outer Mutex serialising callers + the spawn_blocking
+        // the underlying `Arc<Comm>` is `!Send` at the type level;
-        // thread being the only place ops are issued.
+        // the safety contract (only one thread issues NCCL ops at a
-        let leader_comm =
+        // time) is preserved because the load runs on a single
-            nccl_state::SendComm(self.leader_nccl.comm().ok_or_else(|| {
+        // spawn_blocking thread and AllReduce ops fire only from the
-                anyhow::anyhow!("leader NCCL not initialised; call init_nccl first")
+        // device worker thread later. Phase 4 eliminates this bridge
-            })?);
+        // when the load itself moves onto the worker.
+        let leader_comm = self
+            .leader_worker
+            .clone_leader_comm()
+            .await
+            .map_err(|e| anyhow::anyhow!("clone leader Comm via device worker: {e}"))?;
         let world_size = self.world_size;
         let safetensors_str: Vec<String> = safetensors_paths
             .iter()
@@ -601,15 +613,32 @@ impl WorkerPool {
             }
         }
 
-        Ok(Arc::new(Mutex::new(leader_model)))
+        // Phase 3: move the leader's freshly-built `TpLeaderModel`
+        // into the device worker's TP slab. The model holds
+        // `Arc<Comm>` clones (in its AllReduce ops) plus CUDA
+        // tensors — both need to live on the device worker thread so
+        // every `Comm::all_reduce` and tensor op during forward
+        // dispatches from the same OS thread that bound the CUDA
+        // context.
+        let handle = self
+            .leader_worker
+            .transfer_in_tp(Box::new(leader_model))
+            .await
+            .map_err(|e| anyhow::anyhow!("transfer TP leader model into device worker: {e}"))?;
+        Ok(handle)
     }
 
     /// Run one forward step across every rank. The leader's forward
-    /// returns the last-position logits as a candle Tensor on the
+    /// runs on the device worker thread via `Job::TpForwardLogits` and
-    /// leader's device; the caller does sampling out-of-band. Workers
+    /// returns CPU-side `[vocab]` logits as `Vec<f32>`; the async
-    /// run their own forwards (the AllReduce inside row-parallel layers
+    /// caller wraps them in a CPU tensor for `apply_repeat_penalty` +
-    /// is what lets the leader's collective complete) and reply with
+    /// sampling without holding a device-resident tensor on a tokio
-    /// `GenerateStepOk` — they do not ship logits over the wire.
+    /// thread.
+    ///
+    /// Subprocess workers run their own forwards in parallel (the
+    /// AllReduce CustomOps inside row-parallel layers are what let
+    /// the leader's collective complete) and reply with
+    /// `GenerateStepOk` over the RPC stream — they do not ship logits.
     ///
     /// `tokens` is the input for this step (prompt for prefill, the
     /// previously-sampled token for decode). `offset` is the KV-cache
@@ -618,10 +647,10 @@ impl WorkerPool {
     pub async fn generate_step(
         &mut self,
         model_id: &str,
-        leader_model: std::sync::Arc<tokio::sync::Mutex<TpLeaderModel>>,
+        leader_handle: super::device_worker::TpHandle,
         tokens: Vec<u32>,
         offset: usize,
-    ) -> Result<candle_core::Tensor> {
+    ) -> Result<Vec<f32>> {
         let step_start = std::time::Instant::now();
         let tokens_len = tokens.len();
         tracing::debug!(
@@ -630,7 +659,7 @@ impl WorkerPool {
             offset,
             "WorkerPool::generate_step: fan-out"
         );
-        // 1. Fan-out to workers.
+        // 1. Fan-out to subprocess workers.
         for w in &mut self.workers {
             w.send_only(&WorkerRequest::GenerateStep {
                 model_id: model_id.to_string(),
@@ -640,35 +669,30 @@ impl WorkerPool {
             .await?;
         }
 
-        // 2. Leader's forward in spawn_blocking. The AllReduce CustomOps
+        // 2. Leader's forward on its device worker thread. The
-        //    inside the row-parallel layers block until every worker's
+        //    AllReduce CustomOps inside the row-parallel layers block
-        //    forward issues the matching collective.
+        //    until every subprocess worker's forward issues the
+        //    matching collective. Returning CPU-side `Vec<f32>` keeps
+        //    the device tensor from escaping the worker thread —
+        //    that's the invariant the whole refactor exists to
+        //    preserve.
         let leader_start = std::time::Instant::now();
-        let leader_result = tokio::task::spawn_blocking(move || -> Result<candle_core::Tensor> {
+        let leader_result = self
-            let mut model = leader_model.blocking_lock();
+            .leader_worker
-            let device = model.device().clone();
+            .tp_forward_logits(leader_handle, tokens, offset)
-            let input = candle_core::Tensor::new(tokens.as_slice(), &device)?.unsqueeze(0)?;
+            .await;
-            // ForCausalLM::forward returns [B, 1, V] — squeeze both
+        let leader_ok = leader_result.is_ok();
-            // leading dims to the rank-1 vocab logits the sampler wants.
-            let logits = model.forward(&input, offset)?.squeeze(0)?.squeeze(0)?;
-            Ok(logits)
-        })
-        .await
-        .context("leader forward task panicked");
-        let leader_ok = matches!(leader_result, Ok(Ok(_)));
         let leader_ms = leader_start.elapsed().as_millis();
-        // Surface the leader's own error at WARN. Previously this was
+        // Surface the leader's own error at WARN before draining
-        // silently coerced to `leader_ok=false` while only worker
+        // workers so the operator can correlate it with whatever the
-        // ranks' errors got logged — when both the leader and a worker
+        // subprocess workers logged. Previously this was silently
-        // fail together (the typical "CUDA context is now poisoned"
+        // coerced to a bool.
-        // pattern after an OOM), the operator could see only the
-        // worker side and had to guess what hit rank 0.
         if !leader_ok {
-            let detail = match &leader_result {
+            let detail = leader_result
-                Ok(Err(e)) => format!("{e:#}"),
+                .as_ref()
-                Err(e) => format!("task: {e:#}"),
+                .err()
-                Ok(Ok(_)) => unreachable!("leader_ok=false implies an error path"),
+                .map(|e| format!("{e:#}"))
-            };
+                .unwrap_or_default();
             tracing::warn!(
                 model = %model_id,
                 tokens = tokens_len,
@@ -707,7 +731,33 @@ impl WorkerPool {
             "WorkerPool::generate_step: workers drained"
         );
 
-        combine_leader_workers(leader_result, worker_errors, "GenerateStep")
+        // Combine the leader's Result + the workers' string-error
+        // list. Phase 3 inlines this because the upstream
+        // `combine_leader_workers` expects the spawn_blocking-shaped
+        // `Result<Result<T>>`; the new device-worker path produces a
+        // single `Result<T, WorkerError>` instead.
+        match leader_result {
+            Ok(values) => {
+                if worker_errors.is_empty() {
+                    Ok(values)
+                } else {
+                    anyhow::bail!(
+                        "GenerateStep: leader succeeded but workers failed: {}",
+                        worker_errors.join("; ")
+                    )
+                }
+            }
+            Err(e) => {
+                if worker_errors.is_empty() {
+                    Err(anyhow::Error::new(e).context("GenerateStep: leader forward failed"))
+                } else {
+                    Err(anyhow::Error::new(e).context(format!(
+                        "GenerateStep: leader forward failed and workers also failed: {}",
+                        worker_errors.join("; ")
+                    )))
+                }
+            }
+        }
     }
 
     /// Reset the KV cache for `model_id` on every rank. Called at the
@@ -716,7 +766,7 @@ impl WorkerPool {
     pub async fn clear_kv_cache(
         &mut self,
         model_id: &str,
-        #[cfg(feature = "cuda")] leader_model: std::sync::Arc<tokio::sync::Mutex<TpLeaderModel>>,
+        #[cfg(feature = "cuda")] leader_handle: super::device_worker::TpHandle,
     ) -> Result<()> {
         let start = std::time::Instant::now();
         tracing::debug!(model = %model_id, "WorkerPool::clear_kv_cache: fan-out");
@@ -728,13 +778,18 @@ impl WorkerPool {
         }
         #[cfg(feature = "cuda")]
         {
-            let mut m = leader_model.lock().await;
+            // Leader-side clear on the device worker thread —
-            m.clear_kv_cache();
+            // `TpLeaderModel::clear_kv_cache` is infallible but still
+            // routes through Job::TpClearKv so the cache reset runs
+            // on the same thread that owns the model's CUDA tensors.
+            if let Err(e) = self.leader_worker.tp_clear_kv(leader_handle).await {
+                anyhow::bail!("leader TP clear_kv_cache via device worker: {e}");
+            }
         }
         // Drain workers — same rationale as `generate_step`. The
-        // leader's clear_kv_cache is in-process and infallible, but we
+        // leader's clear_kv_cache is now async-via-channel but still
-        // still always drain so an error on one worker doesn't leave
+        // returns before the drain so the workers' KvCacheCleared
-        // pending responses for the others.
+        // replies are processed in order.
         let worker_errors = drain_workers(&mut self.workers, |r| match r {
             WorkerResponse::KvCacheCleared => Ok(()),
             WorkerResponse::Error { kind, message } => Err(format!("[{kind}]: {message}")),

crates/neuron/src/main.rs

@@ -118,7 +118,13 @@ async fn tp_smoke(tp_size: u32, cuda_devices: Vec<u32>) -> Result<()> {
         binary = %exe.display(),
         "tp-smoke: spawning worker pool"
     );
-    let mut pool = tp::WorkerPool::spawn(&exe, tp_size, &cuda_devices).await?;
+    // tp_smoke is a diagnostic tool; spawn the leader's device worker
+    // directly. (In the daemon path, CandleHarness::ensure_device_worker
+    // caches one per device.)
+    let leader_worker = neuron::harness::device_worker::DeviceWorkerHandle::spawn(leader_device)
+        .context("spawn leader device worker for tp-smoke")?;
+    let mut pool =
+        tp::WorkerPool::spawn(&exe, tp_size, &cuda_devices, leader_worker.clone()).await?;
 
     tracing::info!("tp-smoke: pinging every worker");
     let pongs = pool.ping_all().await?;

crates/neuron/tests/tp_worker_lifecycle.rs

@@ -5,6 +5,7 @@
 //! `Init` and `NcclSanityCheck` are stubbed in 7a-i, so this test
 //! runs on any host the workspace builds on.
 
+use neuron::harness::device_worker::DeviceWorkerHandle;
 use neuron::harness::tp::{WorkerPool, rpc::WorkerResponse};
 
 /// Path to the neuron binary built by cargo for this test process.
@@ -19,7 +20,8 @@ const NEURON_BIN: &str = env!("CARGO_BIN_EXE_neuron");
 async fn test_spawn_ping_shutdown() {
     // cuda_devices: rank 0 → device 0 (leader, unused here),
     //               rank 1 → device 1 (worker; not actually opened in 7a-i).
-    let mut pool = WorkerPool::spawn(NEURON_BIN.as_ref(), 2, &[0, 1])
+    let leader_worker = DeviceWorkerHandle::spawn(0).expect("spawn device worker");
+    let mut pool = WorkerPool::spawn(NEURON_BIN.as_ref(), 2, &[0, 1], leader_worker)
         .await
         .expect("spawn worker pool");
 
@@ -44,7 +46,8 @@ async fn test_spawn_ping_shutdown() {
 /// Three workers — exercise the loop in `ping_all` / `shutdown`.
 #[tokio::test]
 async fn test_spawn_three_workers() {
-    let mut pool = WorkerPool::spawn(NEURON_BIN.as_ref(), 3, &[0, 1, 2])
+    let leader_worker = DeviceWorkerHandle::spawn(0).expect("spawn device worker");
+    let mut pool = WorkerPool::spawn(NEURON_BIN.as_ref(), 3, &[0, 1, 2], leader_worker)
         .await
         .expect("spawn worker pool");
 

crates/neuron/tests/tp_worker_lifecycle_cuda.rs

@@ -25,7 +25,9 @@ async fn test_init_and_sanity_check_two_ranks() {
         .try_init();
 
     // 2 ranks: leader = rank 0 on device 0, worker = rank 1 on device 1.
-    let mut pool = WorkerPool::spawn(NEURON_BIN.as_ref(), 2, &[0, 1])
+    let leader_worker = neuron::harness::device_worker::DeviceWorkerHandle::spawn(0)
+        .expect("spawn leader device worker");
+    let mut pool = WorkerPool::spawn(NEURON_BIN.as_ref(), 2, &[0, 1], leader_worker)
         .await
         .expect("spawn worker pool");