SenseTime's Loop-9B runs its own layers twice, and gets a deeper model for free ๐
Here's a question I keep coming back to when I pick models for my homelab: do you really need more parameters to get more thinking, or can you get extra depth out of the weights you already have? On October 30, SenseTime's OpenSenseNova team released an open model that bets on the second answer, and I love how simple the core idea is. It's called SenseNova-Loop-9B-Preview.
The trick: run the middle of the network twice
A normal transformer pushes your input through each layer once. Loop-9B has 32 distinct decoder layers, but it treats them differently. Layers 0 to 3 run once, then the block of layers 4 to 27 is applied twice in a row with the same shared weights, and finally layers 28 to 31 and the output head finish the job. The model card writes it nicely as B(B(h)): the second pass takes the first pass's output as its input.
The result is 56 decoder-layer executions per forward pass while only 32 layers' worth of weights are stored. SenseTime describes it as roughly 9B total parameters, but about 14.6B "inference parameters" because of the repeated computation. In other words, you pay for depth with compute, not with memory.
Keeping two streams in balance
Reusing layers sounds easy, but feeding a block its own output can get unstable. To manage that, the looped section uses what SenseTime calls two-stream manifold-constrained hyper-connections (mHC). Instead of a single residual stream, there are two, and learned mappings decide how information moves and mixes between them, with those mappings constrained using Sinkhorn normalization. The decoder also interleaves linear attention and full attention in a repeating four-layer pattern, which is a familiar recipe for keeping long contexts affordable.
Built for long documents and agents
This isn't just an architecture demo. The model takes text and images and produces text, and it's aimed at document understanding, coding and tool use. The documented training recipes mix long sequences, images, code and tool trajectories, with training sequences up to 262,144 tokens. Multi-token prediction (MTP) is already available for faster inference, and two speculative-decoding variants, Dspark and Dflash2, are still in training. Weights are on Hugging Face under the Apache 2.0 license, and the model card shows how to load it with Transformers on a single GPU.
Honest about the limits
This is clearly labelled a preview. The technical report is still marked "TBD", benchmark results are published as charts by the team itself, and independent tests don't exist yet. Running the looped block twice also means more compute per token than a plain 9B model, so "free depth" is really "free in memory, not in time". I'd want to see real numbers on my own hardware before drawing big conclusions.
Why I like it
Looped and recurrent-depth models are one of the most interesting directions for anyone who runs AI at home or in a classroom. Memory is usually the hard wall, and this design spends compute instead. If ideas like this mature, a small open model could quietly "think deeper" on hardware schools and hobbyists already own.
Sources
- SenseNova-Loop-9B-Preview on Hugging Face
- OpenSenseNova/SenseNova-Loop on GitHub
- TechNode: SenseTime open-sources a 9B model that reuses layers
I'm curious whether three or four passes would keep helping, or whether two is the sweet spot.
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