Is base a direct parent of `it`?

#57
by olisicky - opened

Thank you for gemma4 models, they are amazing! I have a question, though.

We measured weight-space distance between base and instruct releases across four model pairs. Llama-3.1-8B and Qwen3.5-35B-A3B both come out at cosine ~0.999 with weight norms shrinking ~0.4%. Both Gemma 4 pairs — the dense 31B and the 26B-A4B MoE — come out at cosine 0.81–0.84 with weight norms growing 1.2–2.1×, and with q_norm/k_norm bit-identical between the two releases.

Is the released base checkpoint the direct parent of the it checkpoint, or is there an unreleased stage in between? And relatedly: for adding a new language to Gemma 4, is SFT on the it checkpoint the intended path, given that (it - base) doesn't look like a usable task vector?

Thank you very much,

Ondřej

Google org

Hi @olisicky
Gemma4 instruction-tuned models as being obtained from the corresponding pre-trained (PT/base) models through a post-training pipeline, similar to Gemma 3. The intended flow is PT/base → post-training → IT. The IT checkpoint includes the instruction-tuning/post-training stage, including the additional training for thinking mode, safety, factuality, attribution, and refusals.
For adding a new language, we recommend starting from the instruction-tuned checkpoint if you want to retain the model's existing instruction-following behavior while adapting it to the new language. The PT/base checkpoint is more appropriate for continued pre-training or further model development.
You can start from the -it checkpoint and fine-tune it for the target language/task.
For ref :: https://ai.google.dev/gemma/docs/tune

Thanks

hi @pannaga10

thanks for reply. I tested also gemma3 model which shows similar small drift (as llama31, qwen3.5) from pt version and works well for task vector extraction etc. The big difference for gemma-4-26B-A4B and dense version is interesting, though. Have you tried some merge techniques or something similar, please?

Thanks

Ondřej

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