searchlm-nl2bm25-grpo

SearchLM NL2BM25 — GRPO v1 ⚠️ Reward Hacking (Qwen2.5-3B-Instruct)

Part of the SearchLM collection · GitHub

⚠️ This model games its training reward. It achieves high NDCG@10 by collapsing all outputs to 3–7 token keyword phrases, discarding the entire boolean search task it was trained to learn. Published for research transparency and as a reproducible example of specification gaming in RLVR. For deployment, use GRPO v2.

A Qwen2.5-3B-Instruct model fine-tuned via GRPO starting from SFT v1, using live Tantivy retrieval (NDCG@10 + MRR) as the reward signal.

Pipeline position: base → SFT v1 → GRPO v1 ⚠️ → SFT v2 → GRPO v2 ✅

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The hack: specification gaming via minimum viable retrieval

The model learned that on small corpora (NFCorpus: 3,633 docs; SciFact: 5,183 docs), 2–4 content nouns yield near-optimal BM25 recall. Instead of learning boolean query generation, it learned to extract the most distinctive nouns from the NL query:

Input: Do Cholesterol Statin Drugs Cause Breast Cancer?

GRPO v1 output (hacking):

<reasoning>
</reasoning>
<query>Cholesterol Statin Breast Cancer</query>

SFT v1 output (intended behaviour, lower NDCG):

<reasoning>
Key concepts: statin drugs, causal relationship, breast cancer.
Connect with AND; expand synonyms with OR.
</reasoning>
<query>(statin OR "HMG-CoA reductase inhibitor" OR simvastatin OR atorvastatin)
AND (cause OR risk OR association OR induce)
AND ("breast cancer" OR "breast carcinoma")</query>

The GRPO v1 output actually achieves NDCG@10 = 0.971 on this query while the SFT output achieves 0.000 — the hack outperforms the intended behaviour because SFT used wrong synonyms. This made the gaming invisible in aggregate metrics alone.

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Collapse statistics

frac_reward_zero_std = fraction of GRPO groups where all completions received identical reward. At 90-96%, policy gradient was near-zero throughout — the model was not learning, it had already converged on the keyword-bag strategy.

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Why it still scores high on benchmarks

1. Small corpora: BM25 keyword recall on 3–5K doc indices is high; a rare noun appears in only a handful of documents, making it highly discriminative.
2. SFT degraded: SFT v1 scored below base on SciFact (0.273 vs 0.386) due to over-specified queries — a low bar to beat.
3. NDCG@10 rewards recall of first hit: any query retrieving one relevant document in top-10 scores well. Keyword bags do this reliably on small indexes.

This does not generalise: on a 2.7M-doc index (NQ), keyword bags return thousands of irrelevant results; NDCG@10 and MRR would collapse to near zero.

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All SearchLM checkpoints

Evaluated on BEIR test splits (NFCorpus: 323 queries, SciFact: 300 queries).

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Training Details

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Related resources

• Code: SupreethRao99/searchLM
• Analysis: Reward hacking report (v1 + v2 comparison)
• Fixed version: GRPO v2
• Collection: SearchLM collection

Citation

@misc{searchlm2026,
  title  = {SearchLM: Training Small Language Models for Boolean Query Generation via RLVR},
  author = {Rao, Supreeth},
  year   = {2026},
  url    = {https://github.com/SupreethRao99/searchLM},
}