question regarding adding vision capabilities

[Arsalan (arsalanrzp)]

Hi,

I am building a Vision-Language Model (VLM) using BitNet b1.58 as the frozen text decoder, with a lightweight adapter module connecting a vision encoder to the decoder.

I have two questions regarding the choice of model variant:

1. For training: I am currently using microsoft/bitnet-b1.58-2B-4T-bf16 as the decoder backbone, keeping its weights frozen and only training the adapter. Is this the correct variant for this use case?

2. For deployment: Once the adapter is trained, would it be safe to swap the bf16 decoder for microsoft/bitnet-b1.58-2B-4T (the packed 1.58-bit variant) without retraining, given that both variants represent the same underlying model mathematically?

Thank you for your time and for open-sourcing this work.

[ssshhh (shifulegend)]

Great questions — I've been working on something closely related.

Q1 (training): Yes, bitnet-b1.58-2B-4T-bf16 is the right backbone for adapter training. The bf16 variant gives you stable gradients for the adapter layers while the frozen ternary decoder stays intact. Using the packed 1.58-bit variant during training would force dequantization on every forward pass, which slows things down without any accuracy benefit since the backbone is frozen anyway.

Q2 (deployment swap): Mathematically safe, yes — both represent the same weights. Practically, watch out for one thing: the bf16 and 1.58-bit variants may have subtly different numerical ranges in intermediate activations, especially at the adapter boundary (the projection MLP output feeds into the first decoder layer). Worth running a quick cosine similarity check on a few outputs before and after the swap to confirm alignment.

For reference: we built a working SigLIP → projector → KV injection pipeline on top of BitNet b1.58-2B-4T in a from-scratch C engine. The architecture follows: image → SigLIP ViT encoder → 2-layer MLP projector → soft-prefix tokens injected into the KV cache before the text decoder runs. The full pipeline is implemented and functional, though full image understanding requires a multimodal-trained backbone (BitNet as-is is text-only, as you noted). Implementation is at https://github.com/shifulegend/project-zero if the code is useful to reference.