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*This model was contributed to Hugging Face Transformers on 2026-05-02.*

# DeepSeek-V4

[DeepSeek-V4](https://huggingface.co/deepseek-ai) is the next-generation MoE language model from DeepSeek
([paper](https://huggingface.co/deepseek-ai/DeepSeek-V4-Flash/blob/main/DeepSeek_V4.pdf)). The architecture replaces
DeepSeek-V3's Multi-head Latent Attention (MLA) with a hybrid local + long-range design, swaps residual connections
for Manifold-Constrained Hyper-Connections (mHC), and bootstraps the first few MoE layers with a static
token-id → expert-id hash table.

This implementation covers `DeepSeek-V4-Flash`, `DeepSeek-V4-Pro`, and their `-Base` pretrained siblings. All four
share the same architecture; they differ only in width / depth / expert count and weights.

## Architecture (paper §2)

### Hybrid attention (§2.3)

Each decoder block is one of three attention types, dispatched by `config.layer_types[i]`:

* **Sliding-window full attention** (`"sliding_attention"`): only the local window of `sliding_window` tokens, no
  long-range branch. Matches V3's "Full Attention" style for the bootstrap layers.
* **Compressed Sparse Attention** (`"compressed_sparse_attention"`, **CSA** — paper §2.3.1): a low-compression
  pool (`compress_rate_csa`, default `m=4`) with overlapping windows, plus a **Lightning Indexer** (eqs. 13–17)
  that scores queries against the pool and gathers the top `index_topk` blocks per query before they reach core
  attention.
* **Heavily Compressed Attention** (`"heavily_compressed_attention"`, **HCA** — paper §2.3.2): a high-compression
  pool (`compress_rate_hca`, default `m'=128`) with non-overlapping windows. No indexer — every pooled entry
  contributes to attention.

All three types share the same backbone:

* **Shared K=V Multi-Query Attention**: `num_key_value_heads = 1`; `kv_proj` produces a single KV head and the same
  tensor is read as both key and value.
* **Partial RoPE** (interleaved-pair, paper §2.3.3 "Partial Rotary Positional Embedding") on the trailing
  `qk_rope_head_dim = head_dim * partial_rotary_factor` channels of each head. The same rotation is applied with
  position `-i` to the attention output's rope slice (eq. 26) so the contribution of each KV entry stays a function
  of the *relative* distance to the query.
* **Per-head learnable attention sink** (eq. 27).
* **Grouped low-rank output projection** (§2.3.1 "Grouped Output Projection"): `o_groups` head-groups → `o_lora_rank`
  per group → `hidden_size`, computed by [`DeepseekV4GroupedLinear`] (`o_a_proj`) followed by `o_b_proj`. Cuts the
  per-token cost of the wide attention output without losing expressivity.
* **Shared sliding-window K=V branch** of size `sliding_window` ("Additional Branch of Sliding Window Attention",
  §2.3.1) preserves local fine-grained dependencies; the long-range compressor's output is concatenated with this
  branch's KVs before core attention.

### Manifold-Constrained Hyper-Connections (§2.2)

Residual connections are replaced by mHC (Xie et al., 2026): `hc_mult` parallel residual streams kept in shape
`[B, S, hc_mult, D]` throughout each block. Two [`DeepseekV4HyperConnection`] modules — `attn_hc` and `ffn_hc` — mix
streams in and out around the attention / MLP sublayers via a `(pre, post, comb)` triplet. The `comb` matrix is a
doubly-stochastic projection produced by `hc_sinkhorn_iters` Sinkhorn–Knopp iterations on the manifold, making
signal propagation non-expansive across deep stacks. A final [`DeepseekV4HyperHead`] collapses the `hc_mult`
streams down to a single sequence before the model norm.

### MoE schedule (§2.1)

Routing is configured per layer by `config.mlp_layer_types`, with values from `{"hash_moe", "moe"}`:

* `"hash_moe"`: expert indices come from a frozen `tid2eid[input_ids]` lookup populated from the V4 checkpoint.
  The learned gate `weight` still produces the per-expert scores that weight the selected experts; only
  *which-experts* is static. Used for the first few bootstrap layers (default 3, override via legacy
  `num_hash_layers`).
* `"moe"`: standard top-k routed MoE. The expert affinity uses **Sqrt(Softplus(·))** instead of V3's Sigmoid
  ("we change the activation function that computes the affinity scores from Sigmoid(·) into Sqrt(Softplus(·))",
  paper §2.1), and V3's `n_group` / `topk_group` constraint is dropped. The auxiliary-loss-free strategy
  (DeepSeek's `noaux_tc`) is preserved via the `e_score_correction_bias` buffer that biases the top-k argmax
  without flowing gradients.

Routed experts use a **clamped SwiGLU** (`gate.clamp(max=swiglu_limit)`, `up.clamp(min=-swiglu_limit, max=swiglu_limit)`,
then `act_fn(gate) * up`) on top of the standard Mixtral `[num_experts, 2 * moe_intermediate_size, hidden_size]`
expert weight layout. A single shared expert (a plain SwiGLU MLP at `moe_intermediate_size` width) runs in parallel
on every token.

### Attention mask layout

Each `DeepseekV4Attention` layer extends the standard sliding-window-causal mask along the key axis with a
`block_bias` returned by its compressor, then feeds the concatenated mask to `eager_attention_forward`. The
sliding-section (left, `[S, S]`) is the same for every layer type; the compressor-section (right) differs by
layer type and is the actual "novel" piece introduced by V4.

The diagrams below were produced with a tiny config (`sliding_window=8`, CSA `m=4`, HCA `m'=8`, `index_topk=2`)
on a 16-token input so the full per-layer-type mask fits on screen. Green = the query/key diagonal in the
sliding section, dark = a visible standard KV position, light = masked, amber = a compressor / indexer slot
the query is allowed to attend to. Columns past the dashed line are appended by the compressor via
`cat([sliding_causal_mask, block_bias], dim=-1)`.

**Sliding-only layer (`"sliding_attention"`).** No compressor, no right-padding — the mask is the plain
sliding-window-causal mask of shape `[S, S]` (window = 8). For `i ≥ window` the lower-left triangle is cut
off, recovering the local-only attention pattern.

<img alt="DeepSeek-V4 sliding attention mask" src="https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/deepseek_v4/deepseek_v4_mask_layer0_sliding_attention.svg" />

**CSA layer (`"compressed_sparse_attention"`).** The compressor flattens its per-query gathered output to
`[B, 1, S·k, D]` and right-pads the mask by `S·k` columns. For query `t`, only the `k` slots at columns
`[S + t·k, S + (t+1)·k)` carry the indexer's picks; all other compressor columns are `-inf`. Queries before
the first window has closed (`t < m − 1`) get nothing — the indexer's `-1` sentinel propagates straight to
the mask. As `t` grows, more compressed entries are ready and the indexer can fill all `k` slots.

<img alt="DeepSeek-V4 CSA attention mask" src="https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/deepseek_v4/deepseek_v4_mask_layer1_compressed_sparse_attention.svg" />

**HCA layer (`"heavily_compressed_attention"`).** No indexer — every cached compressed entry is potentially
visible. Right-padded by `T_total = entry_count["compressor"]` columns. Query `t` may only see entry `w` once
its source window has closed, i.e. `w < (t + 1) // m`. With `m=8` here, entries 0 (covers positions `0..7`)
and 1 (covers `8..15`) only become visible at `t ≥ 7` and `t ≥ 15` respectively.

<img alt="DeepSeek-V4 HCA attention mask" src="https://huggingface.co/datasets/huggingface/documentation-images/resolve/main/deepseek_v4/deepseek_v4_mask_layer2_heavily_compressed_attention.svg" />

These diagrams are reproducible end-to-end via:

```bash
python docs/source/en/imgs/deepseek_v4/visualize_attention_masks.py \
    --svg docs/source/en/imgs/deepseek_v4
```

The script runs a forward pass on this tiny config, wraps each attention layer to capture the exact
post-`cat([attention_mask, block_bias])` mask, remaps CSA's `[S, S·k]` flat-slot mask back to a
`[S, T_entries]` entry-visibility view (so each `C_w` column is a compressed *entry*, not a gather slot),
and writes the three SVGs above. It also prints an ANSI grid to stdout for quick terminal inspection and
dumps the indexer's per-query top-k picks so warm-up sentinels and pick choices are auditable.

### Cache layers

Each non-sliding attention block needs to thread compressor / indexer state across forward calls. V4 ships two
cache layer types that auto-register with `LAYER_TYPE_CACHE_MAPPING`:

* `DeepseekV4HCACache`: sliding-window K=V + HCA compressor buffer / pool / count (no overlap, no indexer).
* `DeepseekV4CSACache`: sliding-window K=V + CSA compressor (with overlap state) + parallel indexer
  buffer / pool / count / overlap at `index_head_dim`.

`DynamicCache(config=…)` builds the right cache layer per `config.layer_types[i]`.

## DeepseekV4Config

[[autodoc]] DeepseekV4Config

## DeepseekV4Model

[[autodoc]] DeepseekV4Model
    - forward

## DeepseekV4ForCausalLM

[[autodoc]] DeepseekV4ForCausalLM
    - forward
