1. Overview

A Korean text-embedding model for the BC Card domain, built by LoRA fine-tuning Qwen/Qwen3-Embedding-4B on BC Card in-domain data (personal / merchant / corporate / VIP). It is intended as the retriever (bi-encoder) stage of a BC Card RAG pipeline.

This is the 4B-scale sibling of BCCard/MoAI-Embedding-0.6B — a larger-capacity variant for higher retrieval quality at the cost of compute/latency.

On a held-out in-domain test set it improves NDCG@10 by +6.1% and Accuracy@1 by +8.9% over the base Qwen3-Embedding-4B (full metrics in §2.3).

1.1. TL;DR

• Base model: Qwen/Qwen3-Embedding-4B — 36 layers, hidden 2560, last-token pooling, instruction-aware
• Domain / Language: Finance (BC Card — personal / merchant / corporate / VIP) / Korean
• Task: Query-document retrieval (QA search, document similarity), RAG retriever
• Method: PEFT (LoRA) + Multiple Negatives Ranking (contrastive)
• Format: merged standalone (LoRA fused into base; loads with sentence-transformers, no peft)
• Embedding dimension: 2560 · Max sequence length: 1024 · Similarity: cosine (outputs are L2-normalized)
• Intended use
  • In-house BC Card-domain RAG retriever (Top-K candidate retrieval)
  • QA search, document-similarity scoring

1.2. Usage

The model was trained with an instruction prefix on the query side only (documents get no instruction). Inject the same instruction at inference so query/document encoding matches training.

from sentence_transformers import SentenceTransformer

model = SentenceTransformer("BCCard/MoAI-Embedding-4B")

# Query-side instruction (identical to training) - prepend to every query at inference time
QUERY_INSTRUCTION = "Instruct: Given a web search query, retrieve relevant passages that answer the query\nQuery: "

queries = ["BC카드 연회비는 어떻게 되나요?"]
documents = [
    "BC카드 연회비는 카드 종류와 혜택 구성에 따라 다르게 책정됩니다 ...",
    "바로카드 연회비는 국내 전용과 해외 겸용 여부에 따라 차등 부과됩니다 ...",
    "전월 실적 등 조건을 충족하면 다음 해 연회비가 면제되는 카드도 있습니다 ...",
    "카드 분실 신고는 고객센터 또는 앱에서 즉시 가능합니다 ...",
    ...
]

# Queries: inject the instruction · Documents: no instruction
q_emb = model.encode(queries, prompt=QUERY_INSTRUCTION)
d_emb = model.encode(documents)

scores = model.similarity(q_emb, d_emb)   # cosine; rank documents by score
print(scores)

The instruction is also stored in the model config, so model.encode(queries, prompt_name="query") is equivalent to passing prompt=QUERY_INSTRUCTION explicitly. Documents use no prompt (prompt_name="document" is an empty string).

• Query prompt (instruction): Instruct: Given a web search query, retrieve relevant passages that answer the query\nQuery:
• Document prompt: none

1.3. Training Data

• Sources: BC Card financial QA (BCAI) + website crawl + synthetic data (chunking + multi-query generation)
• Triplets are constructed via hard-negative mining over the unified corpus.

1.4. Training Procedure

2. Evaluation

2.1. Setup

• Queries: 1,000 (held-out test split) · Corpus: 36,281 unique chunks
• Protocol: binary-relevance information retrieval; the same evaluator used during training
• Metrics: NDCG@10 (primary), MRR@10, Recall@{1,10}, Accuracy@1, MAP@10
• Models compared: base (Qwen3-Embedding-4B, no fine-tuning) vs. v4 (r64 / lr5e-5 / 3ep, released)

2.2. Training

Trained for 3 epochs (early-stopped) with a cosine schedule; training loss decreases steadily while validation NDCG@10 climbs early and plateaus (peak ≈ 0.695 around epoch ~1.4), and the best checkpoint is selected at the peak. Curves (loss / learning rate / validation NDCG@10) are logged to Weights & Biases.

2.3. In-domain Retrieval Benchmark

v4 is the released model. Fine-tuning lifts in-domain retrieval by roughly +7% over the base Qwen3-Embedding-4B, with the largest gains on top-rank precision (Accuracy@1, Recall@1). It also surpasses the 0.6B sibling (test NDCG@10 0.6695) by +0.021 (+3.2%) — a modest scale gain at ~7× the parameters, so the 0.6B remains the better pick for latency-sensitive serving.

Comparison with other encoders

On the same in-domain test set, untuned encoders — our own Qwen3-Embedding base (0.6B / 4B) and public multilingual SOTA models (each run with its own native prompt format) — all fall well below this model: domain fine-tuning beats general-purpose scale:

This model improves over its own Qwen3-Embedding-4B base by +0.040 NDCG@10 (+6.1%) and leads the best general-purpose baseline (e5-large) by +0.043 NDCG@10. Notably, the untuned 4B base (0.6508) trails the fine-tuned 0.6B sibling (0.6695) — fine-tuning outweighs scale. Caveat: these baselines are not tuned on BC Card data — the comparison illustrates the value of domain adaptation, not a defect in the baselines.

2.4. Limitations

• Domain-specific — tuned for BC Card Korean financial text; out-of-domain or non-Korean performance is not guaranteed.
• Compute cost — at 4B, this model is markedly heavier (memory / latency) than the 0.6B sibling; for latency- or throughput-sensitive serving, consider the 0.6B variant.
• Re-ranking recommended — as a bi-encoder it favors recall over fine-grained precision.
  • Recommended pipeline: Bi-Encoder (this model) Top-K → Cross-Encoder re-ranking
• Sequence length — inputs are truncated at 1,024 tokens; content past that limit is not encoded, so very long documents should be chunked before indexing.
• Exact-value matching — fine-grained numeric/tabular facts (fees, rates, dates, terms) are not reliably distinguished by dense similarity alone; pair with lexical (BM25) retrieval or a re-ranker when exactness matters.
• Retrieval only — this is an embedding model, not a generator; it ranks passages and does not produce answers.
• Synthetic data influence — part of the training set is LLM-synthesized (chunking + multi-query), which may carry the generator's stylistic/coverage biases.

3. Future Work

• Data quality improvement & re-training
  • Human-annotation labeling
  • More rigorous hard-negative mining (iterative, mined with this model)
  • Broader/higher-quality data (incl. general financial corpora)
• System-level
  • Cross-Encoder re-ranker for precision
  • HyDE / dynamic instruction injection at query time

4. Meta Info

4.1. Citation

@misc{bccard2026moaiembedding4b,
  title        = {MoAI-Embedding-4B: A BC Card-Domain Korean Text Embedding Model},
  author       = {BC Card AX Team},
  year         = {2026},
  howpublished = {https://huggingface.co/BCCard/MoAI-Embedding-4B},
  note         = {LoRA fine-tune of Qwen3-Embedding-4B for BC Card-domain Korean retrieval}
}

4.2. See Also

• 0.6B sibling model: BCCard/MoAI-Embedding-0.6B
• Training dataset: BCCard/BCAI-Finance-Kor-Embedding-Triplet
• Corpus dataset: BCCard/BCAI-Finance-Kor-Embedding-Pair