Huginn-3.5B: Efficient Latent Reasoning Model

• Huginn-3.5B is a large-scale language model that utilizes depth-recurrent transformer blocks with latent representations for efficient reasoning.
• It integrates a latent reward model and latent thinking optimization to score and select the most promising latent trajectories with minimal overhead.
• Empirical findings reveal that while the approach enhances compute efficiency, the interpretability of latent steps remains challenging as recurrence depth increases.

Huginn-3.5B is a large-scale LLM architecture that departs from classical transformer stacks by embedding intermediate reasoning steps as latent representations rather than natural language. It combines a depth-recurrent transformer design with a compact recurrent latent reasoning core, and introduces specialized mechanisms—most notably, the Latent Reward Model (LRM) and Latent Thinking Optimization (LTO)—to detect and optimize “correct” reasoning trajectories in latent space. This approach targets the efficiency and reliability of complex problem solving while minimizing the overhead of explicit chain-of-thought token generation (Du et al., 30 Sep 2025, Lu et al., 2 Jul 2025).

1. Model Structure and Depth-Recurrent Design

Huginn-3.5B consists of approximately 3.5 billion parameters and employs a decoder-only transformer backbone. Unlike a conventional deep stack of unique transformer layers, Huginn-3.5B uses a “bank” of a small number of unique blocks: 2 Prelude, 4 Recurrent, and 2 Coda transformer blocks. The core of the architecture is the cycling of the 4 recurrent blocks over R passes at inference time (typical values R = 16–128), producing an effectively deep network without increasing parameter count. Each recurrent pass operates on the same parameters, implementing parameter sharing via weight tying.

The feed-forward inner dimension is approximately 17,920, with hidden dimension $d = 5280$ and $H = 55$ attention heads. Positional encodings and input embeddings are reused identically at each recurrence; rotary embeddings or separate recurrence-specific encodings are not introduced. The entire unrolling sequence comprises Prelude → (R₁ → R₂ → R₃ → R₄) × R → Coda. At the first recurrence, a Gaussian noise seed is injected: $n \sim \mathcal{N}(0,\sigma^2 I)$.

The forward pass pseudo-code illustrates this mechanism:

def Huginn_forward(x, R): e = x @ W_E s = P1(e) s = P2(s) s = R1(s, normal(0, σ²)) for r in range(1, R): s = R2(s) s = R3(s) s = R4(s) s = R1(s) s = C1(s) s = C2(s) logits = RMSNorm(s) @ W_U return logits

This setup implements the recurrent application of transformer blocks—providing deep computation per forward pass—without added parameters or distinct layers for each depth (Lu et al., 2 Jul 2025).

2. Latent Reasoning Pipeline

On top of the recurrent transformer backbone, Huginn-3.5B applies a latent thinking pipeline. Given a prompt $x$, the model first samples an initial latent state $h_0 \sim \mathcal{N}(0, \sigma^2 I)$ in $\mathbb{R}^{L \times d}$, where $L$ is the output sequence length. A lightweight recurrent cell (such as a small RNN or shallow transformer) evolves this state over $T$ discrete steps (typical $T=32$):

$$h_t = \mathrm{RecurCell}(h_{t-1}, \mathrm{Enc}(x)) \ \ \text{for } t = 1,\dots,T$$

Here, $\mathrm{Enc}(x)$ refers to a fixed or pooled embedding of $x$. The trajectory $\{h_1, ..., h_T\}$ constitutes the “latent chain of thought.” After $T$ steps, a decoding head—often a shallow attention or linear projection—maps $h_T$ to the output token distribution. No additional bottleneck or projection is inserted beyond the initial Gaussian sampling, recurrent cell updates, and the final decoding step.

The recurrent nature is functionally equivalent to unrolling a small-state RNN $T$ times for each input, but operated in high-dimensional latent space ($d = 5280$). Output token count $L \lessapprox 128$ (task-dependent).

3. Latent Reward Model (LRM): Structure and Training

To evaluate correctness of a latent reasoning trajectory, Huginn-3.5B leverages the Latent Reward Model. LRM receives as input a sequence of mean-pooled latent thoughts $\{v_1, ..., v_t\}$, where $v_i = \mathrm{MeanPool}_L(h_i) \in \mathbb{R}^d$.

The LRM stack consists of:

• A 2-layer transformer encoder (hidden size $d=5280$, $H=55$ heads, MLP inner size $17920$, sinusoidal positional encodings) ingesting the sequence of $t$ latent vectors.
• Mean pooling over the $t$ outputs to yield a single $d$-vector.
• A 2-layer MLP (with ReLU) mapping this vector to a scalar logit $\ell$.
• The final reward estimate: $p(o=1|x,z) = \sigma(\ell)$, with $o$ the indicator for answer correctness.

LRM is trained with binary cross-entropy over sampled trajectories $\{z^{(j)}, y^{(j)}\}$, where $z^{(j)}$ are latent chains and $y^{(j)}$ model outputs:

$$\mathcal{L}_{\mathrm{cls}} = -\sum_{j=1}^k [ o_j \log p(o=1|z^{(j)}) + (1-o_j)\log (1-p(o=1|z^{(j)})) ]$$

The trained $p(o=1|x,z)$ is used directly as a reward in subsequent optimization.

4. Latent Thinking Optimization (LTO): Trajectory Selection

LRM enables Latent Thinking Optimization—a procedure to preferentially select likely-correct latent trajectories at test time. The goal is to optimize a new sampling policy $\pi(z|x)$ that maximizes expected reward while keeping it close to the original policy $\mathrm{ref}(z|x)$ via a KL constraint:

$$\max_{\pi} \mathbb{E}_{z\sim \pi}[ r(x, z) ] - \beta D_{KL}(\pi(z|x)\|\mathrm{ref}(z|x))$$

With a discrete set of $N$ sampled candidate trajectories $\{z_i\}$, the optimal solution:

$$\pi_r(z_i|x) \propto \mathrm{ref}(z_i|x) \cdot \exp\left( \frac{r(x, z_i)}{\beta} \right)$$

Sampling is performed via an acceptance–rejection method: for each trajectory, the acceptance probability is

$$\phi_i = \exp \left( \frac{r(x, z_i) - r_{\max}}{\beta} \right)$$

where $r_{\max}$ is the largest reward among sampled candidates.

The process samples $N$ candidates, computes rewards, then accepts $M \leq N$ according to $\phi_i$. This produces i.i.d. samples from $\pi_r(\cdot|x)$.

A single end-to-end table for algorithmic workflow:

Step	Operation	Typical Values
Sample $N$ latent trajectories	Run recurrent latent generator and decode output	$N=20$, $T=32$, $L\leq128$
Score each trajectory with LRM	Mean-pooling and transformer-based classifier	LRM overhead $<1\%$
Acceptance–rejection selection	Apply $\phi_i$, output $M=1$ trajectory	$\beta=10^{-3}$
Output answer	Use decoded $y^*$ from selected trajectory

5. Latent Reasoning Efficiency and Supervision

Latent thinking achieves substantial inference cost reductions compared to explicit chain-of-thought prompting. Generating and decoding a base-model latent trajectory requires on the order of 10–40 seconds on a single A100 GPU; LRM-based scoring introduces an overhead of $\approx 0.07$ seconds per candidate, or $<1\%$ per trajectory—negligible due to parallelizability.

Chain-of-thought generation in output tokens typically doubles or triples inference time, whereas the Huginn-3.5B latent reasoning pipeline with LTO increases inference by at most $10\%$. This efficiency profile is preserved as the method is applied to larger or smaller LLMs by adapting only the LRM’s dimensions to match the backbone model.

On training, the LRM relies solely on answer correctness for supervision: no human annotation of latent steps is required. LRM is typically trained on 5–50 trajectories per question (dataset-dependent), and exhibits robustness to variations in KL weight $\beta \in [10^{-3}, 10^{-1}]$.

6. Empirical Findings and Comparative Analysis

Empirical studies on Huginn-3.5B demonstrate that correct-answer and incorrect-answer latent trajectories display highly discriminable patterns, as verified by the LRM’s classification performance (Du et al., 30 Sep 2025). The LTO procedure, when applied at test time, yields significant accuracy improvements across mathematics, programming, and commonsense reasoning tasks.

Results in “Latent Chain-of-Thought? Decoding the Depth-Recurrent Transformer” (Lu et al., 2 Jul 2025) indicate that, although Huginn-3.5B’s depth-recurrent mechanism enables deep latent computation without parameter growth, interpretability of latent steps remains limited. The use of probing techniques such as the Logit Lens and Coda Lens reveals that most latent steps do not correspond to explicit or human-interpretable sub-results; interpretability fluctuates with both layer index and decoding method. Moreover, increasing the recurrence depth beyond certain thresholds produces only marginal gains—suggesting diminishing returns relative to architectures that externalize reasoning via chains of verbalized tokens.

A plausible implication is that, while latent reasoning improves compute efficiency and can be effectively optimized with reward modeling, the inherent lack of stepwise interpretability remains a challenge, especially for users requiring transparency in decision making.

7. Applicability and Integration with General LLMs

The LRM/LTO pipeline is designed to be domain-agnostic and can be applied for plug-in reward-modeling across different LLMs. For architectures outside Huginn-3.5B (e.g., Llama-2, Mistral), the LRM is adapted to the model’s hidden dimensionality and attention head configuration. The acceptance–rejection LTO algorithm integrates at inference, requiring only access to the internal latent states and decoded outputs.

This approach supports scaling of “test-time thinking” with negligible human supervision cost and minimal compute overhead, generalizing across a variety of domains provided the base model exposes a suitable latent trajectory interface.

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Huginn-3.5B encapsulates design principles of depth-recurrence for parameter-efficient latent reasoning, and introduces machine-learned reward modeling in non-verbal latent spaces for selective trajectory optimization. The architecture’s significance lies in separating reasoning competence from output language modeling, potentially enabling new frameworks for efficient, robust LLM inference. Limitations include the current opacity of latent reasoning steps and diminishing empirical gains as recurrence depth increases without interpretability constraints (Du et al., 30 Sep 2025, Lu et al., 2 Jul 2025).