Code name: Scratch. The KOS-V4 series is nicknamed Scratch LLM: it was trained completely from scratch by a small team on a fraction of the data and compute of commercial models. It is not a frontier model.

⚠️ Research use only. This model is provided for research purposes only and must not be used for any commercial, clinical, legal, or production-grade applications. The user assumes all risks associated with its use.

KOS-V4-Base — a from-scratch 3B medical foundation model

KOS-V4-Base (kos-v4-pretrain) is an open-weights 3B language model trained completely from scratch by a University of Kentucky College of Medicine team (Office for Research, Center for Clinical and Translational Sciences). It is a decoder-only transformer (Qwen3 architecture, bespoke 3B config) — the pretrained foundation that KOS-V4-Instruct is fine-tuned from. This is a base model: it completes text; it is not instruction-tuned and has no chat template.

The headline. On held-out medical bits-per-byte (5-domain mean 0.4309) KOS-V4-Base ranks 1st of 17 across a 17-model benchmark pool — beating from-scratch and trillion-token peers alike (BPB is tokenizer-agnostic, so this is a fair cross-model comparison; the distillation confound flatters the trillion-token externals, so this is conservative).

KOS-V4 training-token budget vs the comparator fleet

Training-token budget of KOS-V4 (180.3 B tokens) against the comparator fleet (log scale). Every peer was trained on 1.7–200× more data (0.3–36 T tokens); the biomedical specialists additionally continue-pretrain on a multi-trillion-token general base. KOS-V4 is the small dot in the lower-left — the from-scratch 3B on a fraction of the budget.

Core specifications

Attribute	Detail
Architecture	Decoder-only Transformer (`Qwen3ForCausalLM`), Grouped-Query Attention
Parameters	3.015 B
Hidden / Layers	3072 / 28
Attention	24 query / 8 KV heads (GQA 3:1), head_dim 128, per-head QK-RMSNorm
Feed-forward	SwiGLU, intermediate 8192
Vocabulary	32,000, custom medical byte-level BPE
Context length	24,576 (`max_position_embeddings` 65,536)
Position encoding	RoPE, θ = 25,000 (pin on export)
Precision	bfloat16
Objective	pure next-token cross-entropy — no auxiliary losses
Pretraining tokens	180.3 B (English medical/biomedical + web)

Quickstart (Hugging Face Transformers)

python

from transformers import AutoModelForCausalLM, AutoTokenizer
import torch

model_id = "Kentucky-Open-Science/KOS-V4-Base"
tok = AutoTokenizer.from_pretrained(model_id)
model = AutoModelForCausalLM.from_pretrained(model_id, torch_dtype=torch.bfloat16, device_map="auto")

prompt = "The patient presented with acute chest pain and shortness of breath. The differential diagnosis includes"
inputs = tok(prompt, return_tensors="pt").to(model.device)
out = model.generate(**inputs, max_new_tokens=128, do_sample=False)
print(tok.decode(out[0], skip_special_tokens=True))

Standard Qwen3ForCausalLM. This is a base (completion) model — no chat template. For instructions / tools use KOS-V4-Instruct. Pin RoPE θ = 25000 on export; add_bos_token=false; eos = <|endoftext|> (id 0).

Tokenizer

32,000-token byte-level BPE (BBPE), NFKC-normalized, full 256-byte alphabet (no out-of-vocabulary). The vocabulary is 32,000 tokens with 100 reserved slots (<|reserved_0|> … <|reserved_99|>) and a single special token <|endoftext|> at id 0, which serves as EOS = BOS = PAD = UNK and the document separator. There are no other special tokens — no chat / ChatML tokens; the base was not trained on any. add_bos_token = false. The 38 residual byte-fragment "sink" merges present at pretraining have been removed (corrected tokenizer); the vocabulary stays 32,000. BPB is per-byte, so this does not confound cross-model comparison.

Where the tokenizer does well. On raw byte-compression efficiency (tokens-per-byte 0.2591) it ranks 4th of 17, essentially tied with the most byte-efficient general tokenizers (Llama-3 0.2573) — fewer tokens per document, a multiplicative saving on both training and inference. It deliberately does not optimize for whole-word medical vocabulary, so its medical single-token rate (T1b STRR) is the lowest in the pool — but that tradeoff is vindicated: despite the lowest single-token rate, the model still ranks 1st of 17 on held-out medical BPB and leads the pool on medical entity extraction. A high single-token rate is neither necessary for nor sufficient for strong downstream medical modeling.

Pre-training

Trained from scratch, not distilled or continued. Pure next-token cross-entropy (no auxiliary losses), full-parameter on the LlamaFactory trainer, AdamW, peak LR 3.0e-4 cosine (warmup 1%), grad-clip 1.0, 1 epoch, seq 24,576 (whole-document neat-packing; any doc > 24,576 tokens is dropped, never split; 4-D block-diagonal mask so there is no cross-document attention; position_ids reset per doc; 99.8% fill), bf16 + FlashAttention-2 + Liger kernels, gradient checkpointing off. 305,613 steps / 180.3 B token-positions (batch 589,824 tokens/step). Final train loss 1.6637. 24× H200 (3 nodes × 8), pure DDP, 5.73 days, ~3,300 H200-GPU-hours.

Representation health is handled data-side, not with geometric regularizers: corpus cleaning that removes structural sink tokens, source-balanced neat-packing, and train-time shuffle. KOS-V4-Base is a direct test of whether a clean tokenizer + a clean corpus — without auxiliary losses — yields healthy representations (the attention / geometry diagnostics below measure this).

Pre-training datasets — 49 sources, ~180.3 B tokens, English-only

Cleaned (ftfy → NFKC → whitelist; PMC body-only; radiology → natural-language headers; PHI-run collapse), single-phase. Token counts are char-estimates against the packed cache.

Backbone — biomedical literature + general web (~97% of tokens):

source	tokens	source	tokens
PubMed Central (pmc)	76.97 B	mimic-iv discharge (PhysioNet)	0.83 B
FineWeb-Edu 350BT (FK≤10)	40.52 B	clinical_trials (ClinicalTrials.gov)	0.72 B
FineWeb-Edu 10BT	10.38 B	wikipedia (sci/med subset)	0.62 B
mMedC-en	6.33 B	mimic-iv radiology (PhysioNet)	0.45 B
BlueScrubs	4.55 B	biorxiv / medrxiv	0.41 B
Hindawi OA journals	2.93 B	s2orc	1.21 B
MeDAL (PubMed abstracts)	2.49 B

+ ~30 smaller sources: open clinical narratives, knowledge / guidelines (stackexchange-science 0.27 B, dailymed, cpg, gene_ontology, medlineplus, orphapacket, medmentions, trialgpt), pharmacovigilance / relational rendered to NL (ctd, faers, aeolus, onsides, sider, cdc_places, cbioportal, civic, ade_corpus_v2), and deliberate register-diversity (locus legal-code).

(A small number of additional sources with unresolved licenses are intentionally omitted from this list pending license verification; they will be added once resolved.)

Disclosed issue: ~35% of tokens are duplicates (a FineWeb-Edu sharding build bug + PMC repetition); a deduped corpus (v4_dedup, 79.4 M docs, 0% dup) is ready but was not trained — this release is the original single-epoch corpus.

Evaluation — 19-test suite, 75 metrics, vs a 16-model fleet

KOS-V4-Base is benchmarked as a from-scratch, single-epoch base against 16 external models trained on 1.7–200× more data (0.3–36 T tokens). Pool of 17 models, 95 ranked metrics. BPB (bits-per-byte) is tokenizer-agnostic and included as a ranked measure.

Tally: 20 outright rank-1 wins · 6 best-tied (frontier parity) · 9 top-3 · 60 trailing. Wins concentrate on held-out medical BPB (rank 1/17 mean), attention health, internal representation / spectral geometry, medical entity extraction, and reasoning discrimination.

Where it wins (rank-1 of 17): held-out medical BPB (T1 rad / cxr / clin / 5-domain mean 0.4309), long-context BPB (T10 L/2, L/4), attention health (T2 collapsed-head frac 0.0506, entropy-min, BOS-sink), medical entity extraction (RadGraph DR.1/2/3 macro-F1 0.7665 / 0.7990 / 0.8150; BLURB T7 BIOSSES / HoC), reasoning discrimination (T6b MedThink rank-1 0.9450), calibration (T9 mean ECE 0.1366). Notably, the clean-tokenizer + clean-corpus recipe produces healthy internals with no geometric regularizer: logit-lens decodability rises through depth (T3 final-lens accuracy 0.5715 vs BioMedLM 0.2903 — a top-3 placement) and the weight-matrix spectra are mature (T5 WeightWatcher median power-law α wins; safe / under-trained layer ratios place top-3). Beyond the 20 outright wins, KOS-V4-Base reaches frontier parity (best-tied) on 6 metrics (near-zero dead-neuron rate; saturated needle retrieval at several depths) and top-3 on 9 more.

Radiology and clinical text is its home turf — and there it beats every trillion-token model in the pool. On per-domain held-out BPB (lower = better) it ranks 1st of 17 on radiology reports (0.4761 vs Qwen3-4B/36 T 0.7995, Llama-3-8B 0.7444, Gemma-2-9B 0.7760), chest-X-ray reports (0.5887), and clinical narratives (0.3221); and 1st of 17 on radiology entity/relation extraction (RadGraph DR.1/2/3 macro-F1 0.7665 / 0.7990 / 0.8150). It trails on the general axes the trillion-token models saturate — biomedical literature (rank 17/17) and textbooks (rank 12/17) — but the clinical/radiology wins are decisive enough to carry the 5-domain BPB mean to rank 1/17. A 3 B model on 180 B tokens out-modeling 4–9 B models trained on 8–36 T tokens, specifically on the clinical text it was built for, is the payoff of a from-scratch clinical corpus.

Where it is weak:

Closed-book medical MCQ (T6 — near-chance): as a 180 B-token base it does not reliably recall parametric medical facts; it is worst on the hardest professional/college splits (mmlu_professional_medicine 0.2096, mmlu_college_medicine 0.2312, both below BioMedLM). Token-volume + instruction-tuning bound.
Long-context needle retrieval (T10): pool-relative trailing except at the saturated depths (it does beat BioMedLM on the 3-depth mean, 0.8667 vs 0.3667). Token-volume bound.
Hallucination discipline (Med-HALT T8 FCT 0.0400 / NOTA 0.6600 / FQT 0.3868): all three trail the specialist BioMedLM (0.162 / 0.842 / 0.705) — a real-PubMed-format, instruction-format gap the SFT/RL line addresses.
Effective-rank geometry (T4 RankMe 170.8 vs pool leaders' ~187) and the byte-level tokenizer single-token rate (T1b 0.0448 vs 0.0572): the model trades a little representational rank / single-token coverage for a smaller, byte-efficient vocabulary. (Anisotropy / isotropy also run high — a known BBPE base characteristic.)
Demographic bias (T11 CrowS-Pairs |disparity| 1.0791 — rank 9/17, mid-pack, and notably more biased than the biomedical specialists BioMedLM 0.3705 / MedGemma 0.2978): the clean-corpus recipe did not remove social-stereotype bias. Treat generations as unaudited for fairness.

The token- and format-bound axes (T6 knowledge, T10 needle, Med-HALT NOTA / FQT, IFEval) scale with raw pretraining-token volume and instruction tuning — a token-budget gap rather than an architecture flaw; the format-bound ones are the job of the SFT/RL line (KOS-V4-Instruct). Bias and effective-rank are not simply token-budget artifacts and remain open limitations.

Apples-to-apples vs BioMedLM (the only non-commercial, biomedical-only peer; ~300 B PubMed tokens)

KOS-V4-Base wins 28 of 38 comparable cells. Selected:

test	KOS-V4-Base	BioMedLM	Δ
T1 BPB 5-corpus mean (lower = better)	0.4309	0.9059	−0.475 ✅
T2 collapsed-head frac (lower = better)	0.0506	0.4844	−0.434 ✅
RadGraph-XL macro-F1 (DR.1)	0.7665	0.6196	+0.147 ✅
RadGraph CXR macro-F1 (DR.2)	0.7990	0.6304	+0.169 ✅
T6b MedThink rank-1	0.9450	0.8250	+0.120 ✅
T7 BLURB mean / 4	0.7465	0.6829	+0.064 ✅
pubmedqa	0.6980	0.4440	+0.254 ✅
T3 final-lens accuracy (higher = better)	0.5715	0.2903	+0.281 ✅
T10 needle 3-depth mean (higher = better)	0.8667	0.3667	+0.500 ✅
T9 mean ECE (lower = better)	0.1366	0.2231	−0.086 ✅
T8 FCT (Med-HALT)	0.0400	0.1620	−0.122 ✗
T8 NOTA (Med-HALT)	0.6600	0.8420	−0.182 ✗
T8 FQT (Med-HALT)	0.3868	0.7054	−0.319 ✗
T4 RankMe effective rank (higher = better)	170.8	186.8	−16.0 ✗
T11 CrowS-Pairs \|disparity\| (lower = better)	1.0791	0.3705	+0.709 ✗

KOS-V4-Base wins 28 of 38 comparable cells. Wins concentrate on attention pathology (T2), representation flow (T3), spectral health (T5), radiology entity extraction (Block C), and chemical/disease NER. Losses concentrate on corpus-volume- / instruction-format-bound axes (Med-HALT FCT / NOTA / FQT, long-form closed-book MCQ, BLURB linear probes at low token count) plus two that are not token-budget artifacts: lower effective rank (T4) and higher demographic-bias disparity (T11). BioMedLM is architecturally capped at 1024 context (GPT-2 learned positions); KOS-V4-Base (RoPE, 24,576) holds the long-context axis.

Compute footprint

Pre-training: 24× H200 (3 nodes, DDP), 5.73 days, ~3,300 H200-GPU-hours.

Deployment (inference)

Precision	Approx. VRAM	Notes
bfloat16	7 GB	native weights (6.03 GB) + activations; a single 16 GB GPU is comfortable

Fine-tunes built on this base

KOS-V4-Instruct — instruction following + tool / function calling (IFEval 61.6, official BFCL 72.75 / 73 / 60.5) + GGUF.

Intended use & limitations

Intended use: a base foundation for medical-NLP research and downstream fine-tuning. Strong on held-out medical BPB, attention / representation health, and medical entity extraction.
Not a knowledge-QA model. As a base LM it does not reliably recall parametric medical facts (closed-book MCQ is near-chance) — fine-tune and/or ground it with retrieval.
English only. Strong public-benchmark numbers are not validation on real clinical data.
Measured demographic bias. On CrowS-Pairs (T11) the model is mid-pack (rank 9/17) and more biased than the biomedical specialists (|disparity| 1.0791 vs BioMedLM 0.37 / MedGemma 0.30). The clean-corpus recipe removes structural sink tokens but does not remove social-stereotype bias — outputs are unaudited for fairness.
Measured hallucination weakness. Med-HALT (T8 FCT / NOTA / FQT) trails the specialist BioMedLM; the model may fabricate confidently.
Not otherwise safety-tested. Beyond CrowS-Pairs and Med-HALT above, this model has not been red-teamed or evaluated for toxicity or clinical safety. It may produce harmful, biased, or medically inaccurate content.