feat(stage-8c): full-attention layer + decoder + Model + ForCausalLM for qwen3_5

Completes the single-GPU dense path for Qwen3-Next (Qwen3.6's
architecture). The four new modules wrap the substantive
`linear_attn.rs` (landed previously) with the rest of the
transformer:

- `arch/qwen3_5/rope.rs` — text-side rotary embedding. MRoPE is
  simplified to plain RoPE (the three position grids collapse to one
  for text-only inference); uses candle's `rope_slow` for the
  GLM-style rotate-half rotation.
- `arch/qwen3_5/mlp.rs` — Qwen3_5MLP (SwiGLU: gate/up/down, bias=False).
- `arch/qwen3_5/full_attn.rs` — Qwen3_5Attention with the two
  Qwen3-Next quirks:
  - `q_proj` widened to `2 * num_heads * head_dim`; second half
    sigmoid'd and multiplied into the attention output before `o_proj`.
  - q_norm/k_norm use the `(1+w)*x` RmsNorm variant.
- `arch/qwen3_5/decoder.rs` — Qwen3_5DecoderLayer dispatching on
  `layer_types[i]` to either Full attention or GatedDeltaNet.

`arch/qwen3_5/mod.rs` gets the real `Qwen3_5Model` (embedding + layer
stack + final norm) and `Qwen3_5ForCausalLM` (model + lm_head). The
forward returns `[B, 1, vocab]` to match `qwen3_dense`; the harness's
`squeeze_to_vocab` handles either shape.

Switch: `candle.rs::load_arch_dense` for `model_type=qwen3_5` now
builds a `ShardedVarBuilder` instead of a plain VarBuilder. The
sharded backend falls through to the unsharded path when
`world_size=1`, so single-GPU load is zero-cost; this lets the
forthcoming `tp_qwen3_5.rs` reuse the same load functions without a
second copy.

Verified: cargo build CPU + --features cuda inside the patched
container; clippy clean on both; 32 lib tests still pass. The
ForCausalLM forward no longer bails — but numerical correctness vs
the Python reference hasn't been validated yet (that's the next
step, with the Tbilisi probe).

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

crates/neuron/src/harness/arch/qwen3_5/decoder.rs

@@ -0,0 +1,117 @@
+//! Qwen3-Next decoder layer.
+//!
+//! Standard pre-norm transformer block (LN → attention → residual →
+//! LN → MLP → residual) where the attention slot dispatches on the
+//! per-layer `layer_types[i]` value in the config:
+//!
+//! - `"full_attention"` → [`Qwen3_5Attention`] (GQA causal + output
+//!   gate + RoPE + KV cache).
+//! - `"linear_attention"` → [`GatedDeltaNet`] (recurrent delta rule +
+//!   causal conv + per-head state).
+//!
+//! In Qwen3.6-27B every 4th layer is full_attention; the rest are
+//! linear_attention. `full_attention_interval` in the config is a
+//! hint; `layer_types` is authoritative.
+
+use anyhow::Result;
+use candle_core::{Module, Tensor};
+use candle_nn::var_builder::ShardedVarBuilder;
+use std::sync::Arc;
+
+use super::TextConfig;
+use super::full_attn::Qwen3_5Attention;
+use super::linear_attn::GatedDeltaNet;
+use super::mlp::Qwen3_5MLP;
+use super::rmsnorm::Qwen3_5RmsNorm;
+use super::rope::RotaryEmbedding;
+
+/// One of the two attention flavours sitting in a decoder layer's
+/// attention slot. Full-attention layers need the rotary table and
+/// take an attention mask; linear-attention layers carry their own
+/// recurrent state and ignore the mask.
+enum AttentionKind {
+    Full(Qwen3_5Attention),
+    Linear(GatedDeltaNet),
+}
+
+pub struct Qwen3_5DecoderLayer {
+    input_layernorm: Qwen3_5RmsNorm,
+    post_attention_layernorm: Qwen3_5RmsNorm,
+    mlp: Qwen3_5MLP,
+    attention: AttentionKind,
+}
+
+impl Qwen3_5DecoderLayer {
+    pub fn load(
+        cfg: &TextConfig,
+        rotary: Arc<RotaryEmbedding>,
+        layer_idx: usize,
+        vb: &ShardedVarBuilder,
+    ) -> Result<Self> {
+        let layer_type = cfg
+            .layer_types
+            .get(layer_idx)
+            .map(String::as_str)
+            .ok_or_else(|| {
+                anyhow::anyhow!(
+                    "layer_types[{layer_idx}] missing (have {} entries)",
+                    cfg.layer_types.len()
+                )
+            })?;
+
+        let attention = match layer_type {
+            "full_attention" => {
+                AttentionKind::Full(Qwen3_5Attention::load(cfg, rotary, &vb.pp("self_attn"))?)
+            }
+            "linear_attention" => {
+                AttentionKind::Linear(GatedDeltaNet::load(cfg, &vb.pp("linear_attn"))?)
+            }
+            other => anyhow::bail!(
+                "unknown layer_type '{other}' for layer {layer_idx} (expected \
+                 'full_attention' or 'linear_attention')"
+            ),
+        };
+
+        let mlp = Qwen3_5MLP::load(cfg, &vb.pp("mlp"))?;
+        let input_layernorm =
+            Qwen3_5RmsNorm::load(&vb.pp("input_layernorm"), cfg.hidden_size, cfg.rms_norm_eps)?;
+        let post_attention_layernorm = Qwen3_5RmsNorm::load(
+            &vb.pp("post_attention_layernorm"),
+            cfg.hidden_size,
+            cfg.rms_norm_eps,
+        )?;
+
+        Ok(Self {
+            input_layernorm,
+            post_attention_layernorm,
+            mlp,
+            attention,
+        })
+    }
+
+    pub fn forward(
+        &mut self,
+        x: &Tensor,
+        attn_mask: Option<&Tensor>,
+        offset: usize,
+    ) -> candle_core::Result<Tensor> {
+        let h = self.input_layernorm.forward(x)?;
+        let attn_out = match &mut self.attention {
+            AttentionKind::Full(attn) => attn.forward(&h, attn_mask, offset)?,
+            // Linear attention ignores attn_mask + offset; its causal
+            // structure is baked into the recurrent state lifecycle.
+            AttentionKind::Linear(net) => net.forward(&h)?,
+        };
+        let x = (x + attn_out)?;
+        let h2 = self.post_attention_layernorm.forward(&x)?;
+        let h2 = self.mlp.forward(&h2)?;
+        x + h2
+    }
+
+    pub fn clear_kv_cache(&mut self) {
+        match &mut self.attention {
+            AttentionKind::Full(attn) => attn.clear_kv_cache(),
+            AttentionKind::Linear(net) => net.clear_kv_cache(),
+        }
+    }
+}

crates/neuron/src/harness/arch/qwen3_5/full_attn.rs

@@ -0,0 +1,179 @@
+//! Qwen3-Next's `full_attention` layer.
+//!
+//! Standard GQA causal attention with two Qwen3-Next-specific quirks:
+//!
+//! 1. **Output gate (`attn_output_gate=True`).** `q_proj` is widened
+//!    to `num_heads * head_dim * 2`. The second half is reshaped to
+//!    `(B, L, num_heads * head_dim)` and fed through a sigmoid; the
+//!    attention output is pointwise-multiplied by this gate before
+//!    `o_proj`. Effectively a per-head per-position attenuation on
+//!    the attention output.
+//!
+//! 2. **`(1 + w) * x` RmsNorm** on q and k (see `rmsnorm::Qwen3_5RmsNorm`).
+//!    candle_nn's RmsNorm applies `w * x`; the upstream Qwen3-Next
+//!    checkpoints expect the `(1 + w)` form.
+//!
+//! Otherwise: GQA with `num_attention_heads / num_key_value_heads`
+//! repeat, q_norm + k_norm on the head dim, GLM-style rotary (see
+//! `rope::RotaryEmbedding`), and the usual causal mask.
+
+use anyhow::{Context, Result};
+use candle_core::{Module, Tensor};
+use candle_nn::Linear;
+use candle_nn::kv_cache::ConcatKvCache;
+use candle_nn::var_builder::ShardedVarBuilder;
+use candle_transformers::utils::repeat_kv;
+use std::sync::Arc;
+
+use super::TextConfig;
+use super::rmsnorm::Qwen3_5RmsNorm;
+use super::rope::RotaryEmbedding;
+
+pub struct Qwen3_5Attention {
+    q_proj: Linear,
+    k_proj: Linear,
+    v_proj: Linear,
+    o_proj: Linear,
+    q_norm: Qwen3_5RmsNorm,
+    k_norm: Qwen3_5RmsNorm,
+    num_heads: usize,
+    num_kv_heads: usize,
+    num_kv_groups: usize,
+    head_dim: usize,
+    hidden_size: usize,
+    rotary: Arc<RotaryEmbedding>,
+    kv_cache: ConcatKvCache,
+}
+
+impl Qwen3_5Attention {
+    pub fn load(
+        cfg: &TextConfig,
+        rotary: Arc<RotaryEmbedding>,
+        vb: &ShardedVarBuilder,
+    ) -> Result<Self> {
+        let head_dim = cfg.head_dim;
+        let num_heads = cfg.num_attention_heads;
+        let num_kv_heads = cfg.num_key_value_heads;
+        if num_kv_heads == 0 || !num_heads.is_multiple_of(num_kv_heads) {
+            anyhow::bail!(
+                "num_attention_heads ({num_heads}) must be a positive multiple of \
+                 num_key_value_heads ({num_kv_heads})"
+            );
+        }
+        let num_kv_groups = num_heads / num_kv_heads;
+
+        // q_proj is 2x wide: the extra `num_heads * head_dim` slice is
+        // the gate (see attn_output_gate notes above).
+        let q_proj = load_linear_no_bias(vb, "q_proj", cfg.hidden_size, num_heads * head_dim * 2)?;
+        let k_proj = load_linear_no_bias(vb, "k_proj", cfg.hidden_size, num_kv_heads * head_dim)?;
+        let v_proj = load_linear_no_bias(vb, "v_proj", cfg.hidden_size, num_kv_heads * head_dim)?;
+        let o_proj = load_linear_no_bias(vb, "o_proj", num_heads * head_dim, cfg.hidden_size)?;
+
+        let q_norm = Qwen3_5RmsNorm::load(&vb.pp("q_norm"), head_dim, cfg.rms_norm_eps)?;
+        let k_norm = Qwen3_5RmsNorm::load(&vb.pp("k_norm"), head_dim, cfg.rms_norm_eps)?;
+
+        let hidden_size = head_dim * num_heads;
+        let kv_cache = ConcatKvCache::new(2);
+
+        Ok(Self {
+            q_proj,
+            k_proj,
+            v_proj,
+            o_proj,
+            q_norm,
+            k_norm,
+            num_heads,
+            num_kv_heads,
+            num_kv_groups,
+            head_dim,
+            hidden_size,
+            rotary,
+            kv_cache,
+        })
+    }
+
+    pub fn forward(
+        &mut self,
+        x: &Tensor,
+        attn_mask: Option<&Tensor>,
+        offset: usize,
+    ) -> candle_core::Result<Tensor> {
+        let (b, l, _) = x.dims3()?;
+
+        // 1. q_proj — widened output, split into (query, gate).
+        let q_raw = self
+            .q_proj
+            .forward(x)?
+            .reshape((b, l, self.num_heads, self.head_dim * 2))?;
+        let q = q_raw.narrow(3, 0, self.head_dim)?;
+        let gate = q_raw.narrow(3, self.head_dim, self.head_dim)?;
+        // Flatten the gate's head dim back into hidden_size for the
+        // post-attention pointwise multiply.
+        let gate = gate
+            .contiguous()?
+            .reshape((b, l, self.num_heads * self.head_dim))?;
+
+        // 2. q_norm + k_norm + reshape to (B, H, L, D).
+        let q = self.q_norm.forward(&q.contiguous()?)?;
+        let q = q.transpose(1, 2)?.contiguous()?; // (B, H, L, D)
+
+        let k = self
+            .k_proj
+            .forward(x)?
+            .reshape((b, l, self.num_kv_heads, self.head_dim))?;
+        let k = self.k_norm.forward(&k.contiguous()?)?;
+        let k = k.transpose(1, 2)?.contiguous()?;
+
+        let v = self
+            .v_proj
+            .forward(x)?
+            .reshape((b, l, self.num_kv_heads, self.head_dim))?
+            .transpose(1, 2)?
+            .contiguous()?;
+
+        // 3. RoPE on q, k.
+        let (q, k) = self.rotary.apply(&q, &k, offset)?;
+
+        // 4. KV cache.
+        let (k, v) = self.kv_cache.append(&k, &v)?;
+
+        // 5. GQA repeat (cheap shape op).
+        let k = repeat_kv(k, self.num_kv_groups)?.contiguous()?;
+        let v = repeat_kv(v, self.num_kv_groups)?.contiguous()?;
+
+        // 6. Scaled dot-product + causal mask.
+        let scale = 1.0_f64 / (self.head_dim as f64).sqrt();
+        let mut scores = (q.matmul(&k.transpose(2, 3)?)? * scale)?;
+        if let Some(m) = attn_mask {
+            scores = scores.broadcast_add(m)?;
+        }
+        let probs = candle_nn::ops::softmax_last_dim(&scores)?;
+        let ctx = probs.matmul(&v)?; // (B, H, L, D)
+
+        // 7. Reshape back, apply the output gate, project.
+        let ctx = ctx
+            .transpose(1, 2)?
+            .contiguous()?
+            .reshape((b, l, self.hidden_size))?;
+        let gate_sig = candle_nn::ops::sigmoid(&gate)?;
+        let gated = (ctx * gate_sig)?;
+        self.o_proj.forward(&gated)
+    }
+
+    pub fn clear_kv_cache(&mut self) {
+        self.kv_cache.reset();
+    }
+}
+
+fn load_linear_no_bias(
+    vb: &ShardedVarBuilder,
+    name: &str,
+    in_dim: usize,
+    out_dim: usize,
+) -> Result<Linear> {
+    let weight = vb
+        .pp(name)
+        .get((out_dim, in_dim), "weight")
+        .with_context(|| format!("load '{}/{name}/weight'", vb.prefix()))?;
+    Ok(Linear::new(weight, None))
+}

crates/neuron/src/harness/arch/qwen3_5/mlp.rs

@@ -0,0 +1,53 @@
+//! SwiGLU MLP block for Qwen3-Next.
+//!
+//! Identical to plain Qwen3's MLP: `down(silu(gate(x)) * up(x))` with
+//! no bias on any of the three projections.
+
+use anyhow::{Context, Result};
+use candle_core::{Module, Tensor};
+use candle_nn::Linear;
+use candle_nn::var_builder::ShardedVarBuilder;
+
+use super::TextConfig;
+
+pub struct Qwen3_5MLP {
+    gate_proj: Linear,
+    up_proj: Linear,
+    down_proj: Linear,
+}
+
+impl Qwen3_5MLP {
+    pub fn load(cfg: &TextConfig, vb: &ShardedVarBuilder) -> Result<Self> {
+        let h = cfg.hidden_size;
+        let i = cfg.intermediate_size;
+        let gate_proj = load_linear_no_bias(vb, "gate_proj", h, i)?;
+        let up_proj = load_linear_no_bias(vb, "up_proj", h, i)?;
+        let down_proj = load_linear_no_bias(vb, "down_proj", i, h)?;
+        Ok(Self {
+            gate_proj,
+            up_proj,
+            down_proj,
+        })
+    }
+}
+
+impl Module for Qwen3_5MLP {
+    fn forward(&self, x: &Tensor) -> candle_core::Result<Tensor> {
+        let lhs = candle_nn::ops::silu(&self.gate_proj.forward(x)?)?;
+        let rhs = self.up_proj.forward(x)?;
+        self.down_proj.forward(&(lhs * rhs)?)
+    }
+}
+
+fn load_linear_no_bias(
+    vb: &ShardedVarBuilder,
+    name: &str,
+    in_dim: usize,
+    out_dim: usize,
+) -> Result<Linear> {
+    let weight = vb
+        .pp(name)
+        .get((out_dim, in_dim), "weight")
+        .with_context(|| format!("load '{}/{name}/weight'", vb.prefix()))?;
+    Ok(Linear::new(weight, None))
+}

crates/neuron/src/harness/arch/qwen3_5/mod.rs

@@ -11,66 +11,77 @@
 //!
 //! ## Status
 //!
-//! **Scaffold only.** `Config` deserialisation is real (so the dispatch
-//! in `candle.rs::load_arch_dense` can route based on `model_type`
-//! and the operator's diagnostic surfaces "qwen3_5" in the supported
-//! set); the actual forward pass is `unimplemented!()`. Filling this
-//! in is the substantive Stage 8c work.
+//! **Single-GPU dense path is real**. Both attention flavours
+//! (`full_attention` with the output-gated GQA causal attention and
+//! `linear_attention` with the Gated DeltaNet recurrent block) are
+//! implemented. The model loads from upstream safetensors via the
+//! existing `load_arch_dense` dispatch and runs forward end to end.
 //!
-//! ## What the architecture needs (open work)
+//! Numerical correctness vs the reference Python is **not yet
+//! validated** — the structural code path is right, weight tensor
+//! names match the upstream layout, shapes flow through cleanly, but
+//! the Tbilisi probe (and any other downstream test) is the next
+//! step. Likely places a bug would surface:
+//! - Per-rank vs per-token-position offsets in the recurrent delta
+//!   rule (`linear_attn.rs`).
+//! - Off-by-one in the conv state continuation across decode steps.
+//! - RoPE phase mismatch from MRoPE simplification (we treat the
+//!   three position grids as collapsed, which is correct only for
+//!   text-only inference).
 //!
-//! Confirmed from `Qwen/Qwen3.6-27B/config.json`:
-//! - Real hyperparams nested under `text_config: {...}`. The
-//!   architecture is text-side; the multimodal vision tower is
-//!   separate (`image_token_id`, `language_model_only=false`).
-//! - `hidden_size: 5120`, `head_dim: 256`, `intermediate_size: 17408`,
-//!   `num_attention_heads`, `num_key_value_heads`, etc. — bigger
-//!   head_dim than plain Qwen3.
-//! - `attn_output_gate: true` — a sigmoid gate multiplied into the
-//!   attention output before the projection. ~10 LoC addition vs the
-//!   plain Qwen3 attention.
-//! - `layer_types: ["linear_attention", "linear_attention",
-//!   "linear_attention", "full_attention", ...]` with
-//!   `full_attention_interval: 4` — every 4th layer is full
-//!   attention, the rest are linear-attention. The full-attention
-//!   layers shape like a Qwen3 attention; the linear-attention
-//!   layers are the hard part.
+//! ## Submodules
 //!
-//! ## Linear-attention layer
+//! - [`rmsnorm`] — `Qwen3_5RmsNorm` (`(1+w)*x` variant), the
+//!   `Qwen3_5RmsNormGated` used after the delta rule, and the
+//!   `l2norm` helper.
+//! - [`rope`] — text-side rotary embedding (mrope simplified, GLM
+//!   rotate-half).
+//! - [`mlp`] — SwiGLU MLP (gate/up/down, no bias).
+//! - [`full_attn`] — `Qwen3_5Attention` with the output-gate
+//!   widening on `q_proj`.
+//! - [`linear_attn`] — `GatedDeltaNet` recurrent delta-rule block
+//!   (causal depthwise Conv1d → silu → split → L2norm → per-token
+//!   delta rule → RMSNormGated → out_proj).
+//! - [`decoder`] — `Qwen3_5DecoderLayer` dispatching to one of the
+//!   two attention flavours per layer index.
 //!
-//! Candle has nothing we can reuse — has to be written against the
-//! reference Python in the Qwen3-Next HF repo. Likely Lightning
-//! Attention-2 (state-space-ish recurrence) given the
-//! `linear_attention` tag and Qwen3's prior `qwen3-omni` work. Needs:
-//! - A persistent recurrent state per layer (replaces the explicit
-//!   KV cache for full attention).
-//! - Per-token update + readout primitives, fused if possible.
-//! - Numerical-correctness validation against the Python reference
-//!   on a fixed prompt before trusting any output downstream.
+//! ## Open work
 //!
-//! ## TP-2 (the immediate motivator)
+//! - **TP variant.** `harness/tp/tp_qwen3_5.rs` is the next step.
+//!   Sharding strategy diverges by layer type:
+//!   - Full-attention layers: column-parallel q/k/v (including the
+//!     gate half of `q_proj`) + row-parallel `o_proj`, mirroring
+//!     `tp_qwen3.rs`.
+//!   - Linear-attention layers: the recurrent state is per-V-head, so
+//!     V-head-dimension sharding works cleanly — split `num_v_heads`
+//!     across ranks (`num_v_heads / world_size` per rank), shard
+//!     `in_proj_qkv` / `in_proj_z` / `in_proj_b` / `in_proj_a` along
+//!     the V-head dim, and row-parallel `out_proj`. The `A_log` /
+//!     `dt_bias` per-head params shard with the heads.
 //!
-//! Beast's 2x RTX 5090 needs tensor-parallel to fit Qwen3.6-27B.
-//! TP-aware analogue lives at `harness/tp/tp_qwen3_5.rs` (not yet
-//! created — added alongside the dense impl). Sharding strategy
-//! diverges by layer type:
-//! - Full-attention layers: column-parallel q/k/v + row-parallel o,
-//!   same as `tp_qwen3.rs`. With `attn_output_gate`, the gate weight
-//!   is also column-parallel (one gate scalar per head).
-//! - Linear-attention layers: the recurrent state is per-token, not
-//!   per-head, so head-dim sharding doesn't apply. Options are
-//!   (a) replicate the linear-attention layers across ranks (cheap
-//!   but wastes ~half the per-rank VRAM since 3 of every 4 layers
-//!   replicate), or (b) shard along the recurrent-state dimension
-//!   if the formulation allows. Decision deferred until the linear
-//!   attention is actually implemented and profiled.
+//! - **Chunked delta-rule prefill.** `linear_attn.rs` runs the
+//!   per-token recurrent path for prefill too — correct but O(L).
+//!   Porting `torch_chunk_gated_delta_rule` (chunk_size=64) speeds
+//!   prefill substantially with no surface change.
 
-use anyhow::Result;
-use candle_core::Tensor;
+use anyhow::{Context, Result};
+use candle_core::{DType, Device, IndexOp, Module, Tensor};
+use candle_nn::Embedding;
+use candle_nn::Linear;
+use candle_nn::var_builder::ShardedVarBuilder;
 use serde::Deserialize;
+use std::sync::Arc;
 
+pub mod decoder;
+pub mod full_attn;
 pub mod linear_attn;
+pub mod mlp;
 pub mod rmsnorm;
+pub mod rope;
+
+use decoder::Qwen3_5DecoderLayer;
+use rmsnorm::Qwen3_5RmsNorm;
+use rope::RotaryEmbedding;
 
 /// `model_type` we deserialise from `config.json`. Const so the
 /// dispatch in `candle.rs::load_arch_dense` can pattern-match without
@@ -159,41 +170,131 @@ fn default_hidden_act() -> String {
     "silu".into()
 }
 
-/// Stub model. Fields are intentionally empty — filling in the
-/// concrete architecture is the substantive Stage 8c work. The struct
-/// exists so the `ModelArch::Qwen3_5Dense(_)` variant has a payload
-/// and dispatch wiring compiles end-to-end.
-///
-/// To extend: add embed_tokens, decoder layers, final norm, and
-/// lm_head fields here; implement `new`, `forward`, `clear_kv_cache`
-/// in terms of them. Mirror the layout of `qwen3_dense::ModelForCausalLM`
-/// (in candle-transformers) as a starting point.
-pub struct Qwen3_5ForCausalLM {
-    #[allow(dead_code)]
-    config: Config,
+/// Qwen3-Next base transformer (embedding + decoder stack + final
+/// norm). Public so a TP variant in `harness/tp/tp_qwen3_5.rs` can
+/// also build on it later — for now only `Qwen3_5ForCausalLM` is the
+/// loaded handle.
+pub struct Qwen3_5Model {
+    embed_tokens: Embedding,
+    layers: Vec<Qwen3_5DecoderLayer>,
+    norm: Qwen3_5RmsNorm,
+    device: Device,
+    dtype: DType,
 }
 
-impl Qwen3_5ForCausalLM {
-    pub fn new(config: Config, _vb: candle_nn::VarBuilder) -> Result<Self> {
-        // TODO(stage-8c): build embed_tokens, decoder layers (dispatching
-        // on layer_types), final RmsNorm, lm_head from the VarBuilder.
-        // For now we accept the construction so the load path can be
-        // exercised end-to-end (config parse + safetensors mmap), and
-        // bail at forward time with a clear marker.
-        Ok(Self { config })
-    }
+impl Qwen3_5Model {
+    pub fn load(cfg: &TextConfig, vb: &ShardedVarBuilder) -> Result<Self> {
+        let dtype = vb.dtype();
+        let device = vb.device().clone();
 
-    pub fn forward(&mut self, _input: &Tensor, _offset: usize) -> Result<Tensor> {
+        let embed_vb = vb.pp("model.embed_tokens");
+        let embed_weight = embed_vb
+            .get((cfg.vocab_size, cfg.hidden_size), "weight")
+            .with_context(|| format!("load '{}/weight'", embed_vb.prefix()))?;
+        let embed_tokens = Embedding::new(embed_weight, cfg.hidden_size);
+
+        let rotary = Arc::new(RotaryEmbedding::new(dtype, cfg, &device)?);
+
+        if cfg.layer_types.len() != cfg.num_hidden_layers {
             anyhow::bail!(
-            "Qwen3-Next ({}) forward not implemented yet (Stage 8c, TP-2 motivator)",
-            self.config.model_type
-        )
+                "config.text_config.layer_types must have num_hidden_layers ({}) entries; \
+                 got {}",
+                cfg.num_hidden_layers,
+                cfg.layer_types.len()
+            );
+        }
+
+        let vb_l = vb.pp("model.layers");
+        let mut layers = Vec::with_capacity(cfg.num_hidden_layers);
+        for i in 0..cfg.num_hidden_layers {
+            layers.push(Qwen3_5DecoderLayer::load(
+                cfg,
+                rotary.clone(),
+                i,
+                &vb_l.pp(i),
+            )?);
+        }
+
+        let norm = Qwen3_5RmsNorm::load(&vb.pp("model.norm"), cfg.hidden_size, cfg.rms_norm_eps)?;
+
+        Ok(Self {
+            embed_tokens,
+            layers,
+            norm,
+            device,
+            dtype,
+        })
+    }
+
+    pub fn embed_weight(&self) -> &Tensor {
+        self.embed_tokens.embeddings()
     }
 
     pub fn clear_kv_cache(&mut self) {
-        // No-op for the stub. The real impl needs a `clear_kv_cache`
-        // that resets the per-layer KV cache (full-attention layers)
-        // and the per-layer recurrent state (linear-attention layers).
+        for l in &mut self.layers {
+            l.clear_kv_cache();
+        }
+    }
+
+    fn causal_mask(&self, b: usize, tgt: usize, offset: usize) -> candle_core::Result<Tensor> {
+        let minf = f32::NEG_INFINITY;
+        let mask: Vec<_> = (0..tgt)
+            .flat_map(|i| (0..(tgt + offset)).map(move |j| if j <= i + offset { 0. } else { minf }))
+            .collect();
+        Tensor::from_slice(&mask, (b, 1, tgt, tgt + offset), &self.device)?.to_dtype(self.dtype)
+    }
+
+    pub fn forward(&mut self, input: &Tensor, offset: usize) -> candle_core::Result<Tensor> {
+        let (b, l) = input.dims2()?;
+        let mut h = self.embed_tokens.forward(input)?;
+        // Causal mask only needed for L > 1 prefill; full-attention
+        // layers consume it via broadcast_add. Linear-attention layers
+        // ignore the mask.
+        let causal = if l == 1 {
+            None
+        } else {
+            Some(self.causal_mask(b, l, offset)?)
+        };
+        for layer in &mut self.layers {
+            h = layer.forward(&h, causal.as_ref(), offset)?;
+        }
+        self.norm.forward(&h)
+    }
+}
+
+pub struct Qwen3_5ForCausalLM {
+    base: Qwen3_5Model,
+    lm_head: Linear,
+}
+
+impl Qwen3_5ForCausalLM {
+    pub fn new(config: Config, vb: ShardedVarBuilder) -> Result<Self> {
+        let cfg = &config.text_config;
+        let base = Qwen3_5Model::load(cfg, &vb)?;
+        let lm_head = if cfg.tie_word_embeddings {
+            Linear::new(base.embed_weight().clone(), None)
+        } else {
+            let weight = vb
+                .pp("lm_head")
+                .get((cfg.vocab_size, cfg.hidden_size), "weight")
+                .with_context(|| format!("load '{}/lm_head/weight'", vb.prefix()))?;
+            Linear::new(weight, None)
+        };
+        Ok(Self { base, lm_head })
+    }
+
+    /// `input`: token-id tensor of shape `(B, L)`. Returns logits at
+    /// the last position, shape `(B, 1, vocab_size)` — same contract
+    /// as `qwen3::ModelForCausalLM::forward` so the harness's
+    /// `squeeze_to_vocab` helper handles both uniformly.
+    pub fn forward(&mut self, input: &Tensor, offset: usize) -> candle_core::Result<Tensor> {
+        let (_, l) = input.dims2()?;
+        let hidden = self.base.forward(input, offset)?;
+        hidden.i((.., l - 1.., ..))?.apply(&self.lm_head)
+    }
+
+    pub fn clear_kv_cache(&mut self) {
+        self.base.clear_kv_cache();
     }
 }
 

crates/neuron/src/harness/arch/qwen3_5/rope.rs

@@ -0,0 +1,67 @@
+//! Rotary position embedding for Qwen3-Next's full-attention layers.
+//!
+//! Qwen3.6 ships with MRoPE (multimodal RoPE) machinery in the
+//! reference Python — three position grids interleaved per
+//! `mrope_section`. For text-only inference all three grids carry the
+//! same position ids and the interleave is a no-op, so this module
+//! implements the plain (non-mrope) flavour: the standard inv_freq
+//! cosine/sine tables driven by `rope_theta` and `head_dim`.
+//!
+//! Rotation flavour: **GLM-style** rotate-half (the second half of the
+//! head dim is negated and swapped into the first). The reference
+//! Python uses `apply_rotary_pos_emb` with `rotate_half`; candle's
+//! `rope_slow` is the matching helper.
+
+use anyhow::Result;
+use candle_core::{DType, Device, Tensor};
+
+use super::TextConfig;
+
+#[derive(Debug, Clone)]
+pub struct RotaryEmbedding {
+    sin: Tensor,
+    cos: Tensor,
+}
+
+impl RotaryEmbedding {
+    pub fn new(dtype: DType, cfg: &TextConfig, dev: &Device) -> Result<Self> {
+        let dim = cfg.head_dim;
+        let max_seq_len = cfg.max_position_embeddings;
+        let inv_freq: Vec<f32> = (0..dim)
+            .step_by(2)
+            .map(|i| 1f32 / cfg.rope_theta.powf(i as f64 / dim as f64) as f32)
+            .collect();
+        let n = inv_freq.len();
+        let inv_freq = Tensor::from_vec(inv_freq, (1, n), dev)?.to_dtype(DType::F32)?;
+        let t = Tensor::arange(0u32, max_seq_len as u32, dev)?
+            .to_dtype(DType::F32)?
+            .reshape((max_seq_len, 1))?;
+        let freqs = t.matmul(&inv_freq)?;
+        Ok(Self {
+            sin: freqs.sin()?.to_dtype(dtype)?,
+            cos: freqs.cos()?.to_dtype(dtype)?,
+        })
+    }
+
+    /// Apply RoPE to q, k.
+    ///
+    /// `q`, `k` shape: `(B, H, L, head_dim)`. `offset` is the index
+    /// into the cached cos/sin table — the position of the first token
+    /// in the current step. `candle_nn::rotary_emb::rope_slow` does
+    /// the GLM-style `x*cos + rotate_half(x)*sin` rotation and
+    /// internally `cat`s cos/sin with themselves along the last dim,
+    /// so we hand it the `(seq_len, head_dim/2)` slice it expects.
+    pub fn apply(
+        &self,
+        q: &Tensor,
+        k: &Tensor,
+        offset: usize,
+    ) -> candle_core::Result<(Tensor, Tensor)> {
+        let (_, _, seq_len, _) = q.dims4()?;
+        let cos = self.cos.narrow(0, offset, seq_len)?;
+        let sin = self.sin.narrow(0, offset, seq_len)?;
+        let q_embed = candle_nn::rotary_emb::rope_slow(&q.contiguous()?, &cos, &sin)?;
+        let k_embed = candle_nn::rotary_emb::rope_slow(&k.contiguous()?, &cos, &sin)?;
+        Ok((q_embed, k_embed))
+    }
+}

crates/neuron/src/harness/candle.rs

@@ -617,12 +617,22 @@ impl CandleHarness {
                     })))
                 }
                 "qwen3_5" => {
-                    // Stage 8c scaffold: config parses, model
-                    // constructs, but forward bails. See
-                    // `arch/qwen3_5.rs` for the open architecture work.
+                    // Qwen3-Next needs a ShardedVarBuilder because its
+                    // load functions use the sharded backend (so they
+                    // can be reused unchanged by the future TP variant).
+                    // With world_size=1 the backend falls through to
+                    // the unsharded path, so there is no per-load cost.
                     let cfg: super::arch::qwen3_5::Config = serde_json::from_str(&cfg_text)
                         .context("parse Qwen3-Next (qwen3_5) config.json")?;
-                    let model = super::arch::qwen3_5::Qwen3_5ForCausalLM::new(cfg, vb)
+                    let sharded_vb = unsafe {
+                        candle_nn::var_builder::ShardedSafeTensors::var_builder(
+                            &safetensors_paths,
+                            dtype,
+                            &device_for_load,
+                        )
+                        .context("build ShardedVarBuilder for Qwen3-Next")?
+                    };
+                    let model = super::arch::qwen3_5::Qwen3_5ForCausalLM::new(cfg, sharded_vb)
                         .context("build Qwen3-Next dense model")?;
                     Ok(ModelArch::Qwen3_5Dense(model))
                 }