kotraj-qwen3.5-2B

A 2B parameter model fine-tuned for Korean agentic tool use — multi-turn conversations where the model calls tools, reads their responses, and completes a user's task.

Korean agentic tool use improves by +23.2pp; general (English) tool calling is preserved.

	benchmark	base	this model
Korean, agentic	kotraj-bench pass@1	0.514	0.746 (+23.2pp)
English, general	BFCL v3 AST	0.818	0.819 (+0.1pp)

Results

Evaluated on kotraj-bench (290 held-out Korean tool-use tasks, 3 runs each = 870 rollouts). Both models ran against an identical frozen environment and user simulator.

Numbers revised 2026-08-30. An evaluation harness defect was fixed and both models were re-measured on the corrected harness. Earlier figures understated both.

Metric	Qwen3.5-2B (base)	kotraj-qwen3.5-2B	Δ
pass@1	0.514	0.746	+23.2pp
pass^3 (correct on all 3 runs)	0.307	0.597	+29.0pp
pass_any (correct at least once)	0.707	0.869	+16.2pp
tool precision	0.916	0.941	+2.5pp
format violation rate	0.017	0.007	−1.0pp
avg turns to completion	4.94	4.61	−0.33

McNemar's test on paired tasks: for pass^3, 102 tasks flipped to success versus 18 that regressed — p = 3.5e-14. For pass_any, 59 versus 12 — p = 4.8e-08. Both gains are statistically significant.

By task type

Task type	base	this model
single call	0.586	0.782
multi-turn	0.409	0.706
sequential calls	0.496	0.767
parallel calls	0.600	0.842
no-call (should refuse/answer directly)	0.733	0.656

By domain

Domain	base	this model
calendar	0.512	0.694
commerce	0.407	0.667
travel	0.613	0.870

General tool use is preserved

Fine-tuning on Korean data risks eroding the base model's general tool-calling ability. It did not.

Evaluated on BFCL v3 non-live categories — 1,240 English tasks, AST matching against reference answers, no execution and no LLM judge. Greedy decoding, single turn.

Category	n	base	this model	LFM2.5-1.2B	gemma-4-E2B-it
simple	400	0.863	0.865	0.895	0.752
multiple	200	0.865	0.865	0.885	0.715
parallel	200	0.760	0.760	0.750	0.700
parallel_multiple	200	0.740	0.740	0.645	0.575
irrelevance	240	0.654	0.650	0.812	0.842
AST overall	1000	0.818	0.819	0.814	0.699

base 0.818 → this model 0.819. Of 1,240 items, one flipped to correct and one flipped to incorrect. The Korean fine-tune left English tool calling untouched.

Taken together with the Korean results above: +23.2pp on Korean agentic tool use, −0.0pp on general tool calling.

Two caveats worth stating plainly:

• irrelevance is this model's weakest BFCL category (0.650, lowest of the four). It measures the same behaviour as no_call on kotraj-bench, where the model also regressed (0.733 → 0.656). Two independent benchmarks point at the same weakness: the model calls tools when it should not.
• The parser must handle each model family's native call format, including dotted function names such as spotify.play — 53% of BFCL items use them. A parser that silently drops those scores gemma-4-E2B-it at 0.350 instead of 0.699.

Usage

import torch
from transformers import AutoModelForCausalLM, AutoTokenizer

model_id = "drlee1/kotraj-qwen3.5-2B"
tok = AutoTokenizer.from_pretrained(model_id)
model = AutoModelForCausalLM.from_pretrained(
    model_id, dtype=torch.bfloat16, device_map="auto", attn_implementation="sdpa")

tools = [{
    "type": "function",
    "function": {
        "name": "track_delivery",
        "description": "주문의 배송 현황을 조회한다.",
        "parameters": {
            "type": "object",
            "properties": {"order_id": {"type": "string"}},
            "required": ["order_id"],
        },
    },
}]

messages = [
    {"role": "system", "content": "너는 한국어 개인 비서 에이전트다. 제공된 도구만 사용해 사용자의 요청을 끝까지 수행한다."},
    {"role": "user", "content": "ORD0001 배송 지금 어디까지 왔는지 추적 좀 해줘."},
]

text = tok.apply_chat_template(
    messages, tools=tools, tokenize=False,
    add_generation_prompt=True, enable_thinking=False)
inputs = tok(text, return_tensors="pt").to(model.device)

out = model.generate(**inputs, max_new_tokens=512, do_sample=False,
                     eos_token_id=tok.eos_token_id)
print(tok.decode(out[0][inputs.input_ids.shape[1]:], skip_special_tokens=False))

Tool call format

The model emits Qwen3.5's native XML tool-call syntax:

<tool_call>
<function=track_delivery>
<parameter=order_id>
ORD0001
</parameter>
</function>
</tool_call>

Feed results back as a tool role message and call apply_chat_template again to continue.

Two notes that will save you time

• Set eos_token_id explicitly. The base model ships <|endoftext|> as its generation EOS, but chat turns end with <|im_end|>. Omit it and generation runs to max_new_tokens, hallucinating user turns and tool responses. This repo's generation_config.json already sets the correct value, but pass it explicitly if you override the config.
• Set enable_thinking=False. Training data contains no reasoning traces, so enabling thinking mode creates a train/inference mismatch.

Training

Base	Qwen3.5-2B
Method	LoRA SFT (r=16, α=32, dropout 0.05), merged into base weights
LoRA targets	q/k/v/o_proj, in_proj_qkv, out_proj, gate/up/down_proj
Sequence length	8192 (samples exceeding it were dropped, not truncated)
Precision	bf16
Hardware	1× RTX 5080 (16GB)

Training ran in two rounds so that data synthesis and GPU training could proceed in parallel:

Round	Data	Init	Epochs	LR
0	6,967 samples	fresh LoRA	3	2e-4
1	8,583 samples (round 0 data + new)	round 0 adapter	1	1e-4

Training data

Training data: drlee1/kotraj. Synthetic Korean tool-use trajectories generated with an Upstage-family model as teacher, across three domains (commerce, travel, calendar) with 30 mock tools.

Every trajectory passed a deterministic rule gate before training: JSON Schema validation of call arguments, call/response ID consistency, replay against the mock environment for determinism, required-call satisfaction, and terminal-state verification. An LLM judge scored goal completion, consistency, and naturalness on top of that gate. Roughly 60% of the rule gate's rejections came from unmet required calls — the gate, not the judge, is the quality backbone.

Limitations

• no_call regressed (base 0.733 → 0.656), and BFCL irrelevance shows the same thing (0.654 → 0.650, the model's weakest BFCL category). Cases where the correct behavior is to not call a tool — out-of-scope requests, questions answerable directly — are underrepresented in the training mix (~11%). The model has become more eager to call tools, and that eagerness costs it on both benchmarks. If your application depends on refusal behavior, evaluate this specifically.
• Single seed. All results come from one training run (seed 0). Task-level variance is controlled by the paired test, but training randomness is not.
• Three domains only. Generalization to unseen tool schemas is untested.
• Roughly a quarter of rollouts still fail (pass@1 0.746). Of those failures, 35% are missing or wrongly-argued required calls, 11% are ordering violations, and 25% are trajectories that satisfied every criterion but hit the harness's per-turn tool-round cap of 6 and were terminated. The last group is a harness limit, not a model limit.
• pass@1 0.746 vs pass^3 0.597. A 15-point gap between single-run and all-three-runs success means the model is not yet consistent; run-to-run variance remains the largest headroom.

Citation

@misc{kotraj2026,
  title  = {kotraj: Korean Agentic Tool-Use Trajectories},
  author = {DONGRYEOL LEE},
  year   = {2026},
  url    = {https://huggingface.co/drlee1/kotraj-qwen3.5-2B}
}

Related

• drlee1/kotraj — the training dataset (11,357 trajectories)
• drlee1/kotraj-bench — the Korean benchmark used for the results above
• BFCL v3 — the English benchmark used for the general tool-use check