A cross-encoder reranker for RAG retrieval: BAAI/bge-reranker-base fine-tuned on hard-negative groups mined from the Natural Questions train split. It was produced as Stage 8 of a controlled ablation study of RAG chunking, where the diagnosis was explicit — at the best chunk size the answer was already inside the BGE top-20 pool for ~96% of questions while Recall@1 sat at ~0.63, so ranking, not pool recall, was the bottleneck.

Training


Base model	`BAAI/bge-reranker-base`
Data	2034 groups (1 positive + 7 hard negatives) mined from 2161 NQ train questions
Mining	deployment-matched chunking (fixed 15 sentences / overlap 0), BGE top-20 pool (`BAAI/bge-base-en-v1.5`)
Objective	listwise softmax cross-entropy over each group
Schedule	2 epochs, lr 2e-5, warmup 10%, weight decay 0.01, 4 groups/step, fp16, seed 42
Hardware	~17 min on a single T4
Loss	0.904 → 0.513

Every evaluation below uses the NQ validation split, disjoint from training.

Results

In-domain (NQ, 1000 docs / 1032 questions). Δ vs the off-the-shelf reranker, with all arms reranking one identical BGE top-20 pool (2 SE = 0.030):

chunking config	dense R@1	off-the-shelf R@1	this model R@1	Δ vs off-the-shelf
fixed 6/0	0.509	0.545	0.650	+0.106
fixed 15/0	0.628	0.629	0.736	+0.107
fixed 15/1	0.626	0.627	0.720	+0.093
bilstm 15/0	0.630	0.627	0.714	+0.087
transformer 15/0	0.637	0.625	0.732	+0.107

At fixed 15/0 this closes about a third of the gap between dense R@1 (0.628) and the pool ceiling (0.964). R@3 and R@5 rise too; no metric trades down.

Cross-dataset (TriviaQA rc.wikipedia, 472 docs / 300 questions, 2 SE = 0.054). Reported because it is the honest limit of the result:

	Δ vs off-the-shelf R@1	Δ vs no reranking (dense)
in-domain (NQ), fixed 15/0	+0.107	+0.107
cross-dataset (TriviaQA), fixed 15/0	+0.057	+0.003

Out of domain the off-the-shelf reranker actively hurts (−0.03…−0.07 R@1 vs plain dense retrieval). This model recovers to parity with the dense baseline — so its cross-dataset win over the off-the-shelf reranker is largely damage control, not net lift. The direction is consistent (positive at 5/5 configs, 4/5 above 2 SE), but the magnitude is about half the in-domain gain.

Use it in-domain, or expect parity rather than improvement.

Usage

python

from sentence_transformers import CrossEncoder

model = CrossEncoder("sfczaa/bge-reranker-base-nq-ft", max_length=512)
scores = model.predict([(question, passage) for passage in candidates])
ranked = [c for _, c in sorted(zip(scores, candidates), reverse=True)]

Drop-in replacement for BAAI/bge-reranker-base — same architecture, tokenizer and 512-token truncation. Reranking costs ~0.56 s/question at depth 20 on a T4, unchanged by fine-tuning.

Caveats

Trained and validated on Natural Questions; questions are split-disjoint, but popular Wikipedia pages can appear in both splits' corpora (standard for NQ).
The cross-dataset evaluation uses distant-supervised TriviaQA gold (answer string match on entity pages), weaker than NQ's annotated gold; absolute recall is not comparable between the two benchmarks.
Evaluated only on English Wikipedia-style passages at 6–15 sentence chunks.

License

MIT, matching the BAAI/bge-reranker-base base model.