TinyBrainBot 320M V2 β€” Instruct

A ~326M-parameter decoder-only language model, trained from scratch on ~10B tokens and then supervised-fine-tuned for chat / instruction following. This is the instruct model.

  • Base model: tinybrainbot-320mV2-base (full pretraining details there).
  • Successor to the 303M V2 instruct.

TL;DR: A compact general-purpose assistant trained from scratch on ~10B tokens. It matches/beats Pythia-410M on general benchmarks (on far fewer training tokens) and sweeps GPT-2-124M, follows instructions, and has 2–3-digit arithmetic far stronger than its GSM8K reasoning score would suggest (94–99% on 2–3-digit addition) β€” though it hits a clear length-generalization wall beyond 3 digits. It is not math-specialized.


Model details

Parameters 325,899,264 (~326M)
Architecture Decoder-only transformer, pre-norm, RMSNorm, SwiGLU MLP, RoPE
Hidden size 1024
Layers 26
Attention heads 16 (query) / 4 KV heads (grouped-query attention)
FFN size 2816
Context length 1024
Vocabulary 32,000
Tokenizer tbb-32k-v2 β€” 32k BPE (67% English / 20% code / 13% math), with reserved <think>/</think> special tokens
Precision trained in fp16 with an fp32 master copy (autocast)

Usage

Prompt with the chat format:

<|user|>
{user message}
<|end|>
<|assistant|>
{assistant reply}
<|end|>

Example (greedy):

<|user|>
What is the capital of France?
<|end|>
<|assistant|>
Paris.
<|end|>

The model gives concise direct answers and shows worked steps for arithmetic.


Training procedure

Pretraining (base)

Pretrained from scratch on 10.03B tokens (51,000 steps, WSD schedule, peak LR 6e-4, two-phase broad β†’ quality-anneal, final loss 1.436) across 2Γ— Tesla V100-PCIE-16GB with PyTorch DDP (gloo), fp16 autocast + fused AdamW. Full pretraining details and the 13-source data mix are documented on the base model card.

Supervised fine-tuning (this model)

Hyperparameter Value
Steps ~1,433 (resumed from base @ 51,000)
Tokens ~94M
LR 1.5e-5, constant
Warmup 100 steps
Global batch 4 micro Γ— 8 grad-accum Γ— 2 GPUs Γ— 1024 seq
Loss masking assistant-only
Best checkpoint step 52,000 (selected on eval, not last)

SFT data mix (sampling weights; sum = 9.5; math β‰ˆ 15.8% of the mixture):

Source Weight
longdef-sft (~16k long / multi-step answers) 2.0
smoltalk 2.0
math-v2 (~70k verified worked examples) 1.5
qa-distill 1.0
lamini-instructions 1.0
soda-dialogues 1.0
greetings 0.5
reasoning-distill 0.5

math-v2 is ~70k programmatically-generated, tolerance-verified arithmetic worked examples (multiplication / division / decimals, with the final answer checked). It appears in both pretraining and SFT β€” see the note below.

On where the arithmetic comes from: the base and instruct models score almost identically on the GPT-3 Arithmetic suite (aggregate 31.4% vs 31.2%). That base/instruct parity suggests most arithmetic computation was acquired during pretraining, while SFT mainly shaped instruction-following and response format (the one sub-task where SFT clearly helps is composite / order-of-operations, 7.0% vs 3.7%). Pretraining also delivered far more math tokens in absolute terms β€” on the order of ~215M vs ~15M β€” despite SFT's higher math proportion.


Evaluation

Measured on our own log-likelihood MC harness (lm-eval style, fixed seed). Headline metric = acc_norm for HellaSwag/ARC/OpenBookQA, acc for WinoGrande/MMLU. Reference values are published lm-eval approximations β€” treat gaps under ~Β±2 points as ties.

vs the previous 303M instruct (full test sets)

Benchmark n 320M V2 instruct 303M instruct
HellaSwag 10042 34.5 30.7
ARC-Easy (acc_norm) 2376 49.3 47.6
ARC-Easy (raw acc) 2376 57.0 51.0
ARC-Challenge 1172 27.6 27.6
OpenBookQA 500 31.8 29.0
WinoGrande 1267 53.3 52.2
MMLU 14042 28.0 27.1

β†’ 5 wins, 1 tie, 0 losses over the previous generation.

vs reference models (headline metric)

Benchmark 320M V2 instruct GPT-2-124M Pythia-410M SmolLM-360M
HellaSwag 34.5 31 34 54
ARC-Easy 49.3 (57 raw) 44 52 70
ARC-Challenge 27.6 22 24 37
OpenBookQA 31.8 29 30 42
WinoGrande 53.3 52 53 57
MMLU 28.0 26 25 34

β†’ Sweeps GPT-2-124M; ~5 wins + 1 draw vs Pythia-410M. SmolLM-360M (trained on ~600B aggressively-filtered tokens) remains the frontier for this size.

Training efficiency. These results come from ~10B pretraining tokens β€” roughly an order of magnitude fewer than the Pythia suite's ~300B. The Pythia-410M parity is therefore best read as a token-efficiency result (curated data + quality anneal) rather than a scale win.

Math β€” computation vs reasoning

The model was trained on arithmetic computation, not word-problem reasoning β€” the two benchmarks below show that split clearly.

GSM8K (grade-school word problems, full 1319-problem test, zero-shot chain-of-thought):

Model GSM8K
GPT-2-124M ~0%
320M V2 instruct 0.53%
Pythia-410M ~1–2%
SmolLM2-360M-Instruct ~3–5%

β†’ At the floor for a general-purpose model of this size and training mix; stronger sub-1B math-specialized models can score substantially higher. GSM8K rewards multi-step semantic reasoning, which this recipe did not target.

GPT-3 Arithmetic (Brown et al. 2020 protocol, exact-match, n=300/sub-task):

Sub-task Accuracy
2-digit addition 99.0%
3-digit addition 94.0%
2-digit subtraction 49.3%
3-digit subtraction 42.7%
4-digit addition / subtraction 0.3% / 0.3%
5-digit addition / subtraction 0.0% / 0.0%
2-digit multiplication 21.7%
single-digit composite (order of ops) 7.0%
Aggregate (all 10 sub-tasks) 31.4%

β†’ Strong through 3 digits, then a hard wall. 2–3-digit addition is near-solved (94–99%, with correct carrying), but 4+-digit accuracy collapses to 0%: the model executes a fixed **3-column** addition routine and silently drops the higher place values β€” a length-generalization limit tied to the training distribution (math-v2 operands are ≀3 digits), not truncation (generations complete normally and end with a stated answer). Subtraction sits ~42–50% β€” it handles aβˆ’b when a>b but drops the sign on negative results. 2-digit multiplication ~22%; single-digit composite (order of operations) ~7%.

How to read this: the GPT-3 Arithmetic suite mainly probes exact symbolic computation and short-range algorithmic generalization; it should not be interpreted as evidence of strong mathematical reasoning (see GSM8K above). The two results together are the point: strong at computing, weak at reasoning.


Intended use & limitations

Intended use: a capable general chat assistant at ~326M scale, on-device / low-resource deployment, research on small-model SFT, and arithmetic computation.

Limitations:

  • Math reasoning (word problems, GSM8K/MATH) is at the floor β€” the model computes but does not reason through multi-step problems.
  • Negative-result subtraction is unreliable (drops the sign).
  • WinoGrande and MMLU sit near the random floor β€” consistent with the capacity and data limits of a ~326M model under this training recipe.
  • Trained predominantly on English; 1024-token context; no RLHF/safety tuning β€” outputs may be incorrect or inappropriate and should not be relied upon unchecked.

Hardware & framework

2Γ— NVIDIA Tesla V100-PCIE-16GB Β· Windows Β· PyTorch DDP (gloo) Β· fp16 autocast (fp32 master) Β· fused AdamW Β· custom TinyBrainBot trainer.

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