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# Hybrid Retriever Weights — Guardrails and Fix Patterns
Use this page when BM25 plus dense retrieval performs worse than either one alone, when top k flips between runs, or when hybrid recall feels random after an index rebuild. The goal is to put both retrievers on a common score space, then set stable weights, add a single deterministic reranker, and verify with ΔS, coverage, and λ.
## Open these first
* Visual map and recovery: [rag-architecture-and-recovery.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/rag-architecture-and-recovery.md)
* End to end retrieval knobs: [retrieval-playbook.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/retrieval-playbook.md)
* Query split root cause: [patterns/pattern\_query\_parsing\_split.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/patterns/pattern_query_parsing_split.md)
* Reranking for order control: [rerankers.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/rerankers.md)
* Why this snippet and cite first: [retrieval-traceability.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/retrieval-traceability.md)
* Embedding vs meaning failures: [embedding-vs-semantic.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/embedding-vs-semantic.md)
* Vector store health: [vectorstore\_fragmentation.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/GlobalFixMap/Embeddings/vectorstore_fragmentation.md)
* Normalization pitfalls: [normalization\_and\_scaling.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/GlobalFixMap/Embeddings/normalization_and_scaling.md)
## When to use this page
* BM25 finds the right doc yet dense misses, or the reverse
* Hybrid returns unstable order while single retrievers look stable
* Raising k helps only one side while the other side adds noise
* A client upgrade changes analyzers or tokenization and hybrid breaks
## Acceptance targets
* ΔS(question, retrieved) ≤ 0.45
* Coverage of the target section ≥ 0.70
* λ remains convergent across three paraphrases and two seeds
* E\_resonance stays flat on long windows
---
## Core idea: put scores on the same ruler
Let `s_dense` be the dense similarity and `s_sparse` be the BM25 or keyword score. Raw values live in different spaces. Calibrate to a common space, then blend.
Common choices:
* Z score per retriever
`z = (s μ) / σ` computed on the candidate pool for the current query
* Min max per retriever
`m = (s min) / (max min)` on the candidate pool
* Platt style logistic calibration on a gold set
Fit `p = sigmoid(a s + b)` for each retriever
After calibration, blend with a single weight:
`S = α * z_dense + (1 α) * z_sparse`
Pick α by grid search on a gold set, then lock it.
Always add a single deterministic reranker on the union of candidates. See [rerankers.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/rerankers.md).
---
## 60 second fix checklist
1. Normalize both score streams per query. Prefer z score on the union pool.
2. Build a union candidate set. Choose `k_dense` and `k_sparse` so that the union size before rerank is about 20 to 50.
3. Grid search α in steps of 0.1 on your gold set. Pick α that maximizes coverage at k and stabilizes λ across seeds.
4. Add a single cross encoder reranker on top of the blended scores. Fix the seed.
5. Lock the header order and cite first to clamp λ while you tune. See [retrieval-traceability.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/retrieval-traceability.md).
6. Verify the targets on three paraphrases and two seeds.
---
## Symptom to likely cause
* Hybrid worse than BM25 alone
Likely cause. Dense scores unnormalized or wrong metric. Open [normalization\_and\_scaling.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/GlobalFixMap/Embeddings/normalization_and_scaling.md).
* Hybrid worse than dense alone
Likely cause. Analyzer mismatch or stopword removal on one path. Open [pattern\_query\_parsing\_split.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/patterns/pattern_query_parsing_split.md).
* Order flips between runs
Likely cause. Header reorder or union set not stable. Fix header order and use a stable tie break by `doc_id` then `section_id`. Open [retrieval-traceability.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/retrieval-traceability.md).
* Right doc appears only when k is very large
Likely cause. Fragmented stores or per tenant splits without union. Open [vectorstore\_fragmentation.md](https://github.com/onestardao/WFGY/blob/main/ProblemMap/GlobalFixMap/Embeddings/vectorstore_fragmentation.md).
---
## Minimal calibration recipe
**Gold set**
Ten to fifty questions with known anchor sections. Keep per domain if you support many domains.
**Union recall**
Collect `k_dense` and `k_sparse` candidates. Form the union. Store raw scores and features.
**Per query normalization**
Compute z scores on the union for each retriever. Keep the statistics for audits.
**Blend and rerank**
Score with `S = α * z_dense + (1 α) * z_sparse`.
Apply a single cross encoder rerank with a fixed seed.
Resolve ties by `doc_id` then `section_id`.
**Selection of α**
Grid search α from 0.0 to 1.0. Choose α that maximizes coverage at k and keeps ΔS ≤ 0.45 on the anchor.
**Freeze**
Record α, k values, normalization method, reranker id, and seed in your contract.
---
## Copy paste probes
```
Probe A — alpha sweep
For α in {0.0..1.0 step 0.1}:
compute coverage@k and median ΔS on the gold set
Pick α with highest coverage. Break ties by lower ΔS and lower variance.
Probe B — normalization sanity
Toggle {z, minmax, logistic} on the same union pool.
If results change wildly, per query statistics are unstable or candidate pools differ.
Probe C — union size
Sweep k_dense and k_sparse so union size ∈ [20, 50].
If coverage improves up to a point then drops, prune duplicates and near duplicates before rerank.
Probe D — header clamp
Swap harmless header lines. If top k flips, clamp header order and apply cite first. Re test α after clamp.
```
---
## Contract fields to add
```json
{
"retrievers": {
"dense": {
"model": "model-id",
"metric": "cosine",
"normalize": "zscore",
"k": 40
},
"sparse": {
"analyzer_rev": "bm25-v3",
"normalize": "zscore",
"k": 200
}
},
"hybrid": {
"alpha": 0.6,
"blend": "alpha_sum",
"tie_break": ["doc_id", "section_id"]
},
"reranker": {
"name": "cross-encoder-id",
"seed": 7,
"top_k": 20
}
}
```
---
## Verification checklist
* Coverage ≥ 0.70 on the gold anchors
* ΔS(question, retrieved) ≤ 0.45
* λ convergent across two seeds and three paraphrases
* Top k overlap across seeds ≥ 0.8 after rerank
* Stable results after harmless header reorders
---
## Copy paste prompt for the LLM step
```
TXT OS and WFGY Problem Map are loaded.
My issue: hybrid BM25 + dense is worse than single retrievers.
Traces:
- zscore_dense=..., zscore_sparse=..., alpha=...
- union_size=..., reranker=name, seed=...
- ΔS(question,retrieved)=..., coverage=..., λ across 3 paraphrases
Tell me:
1) the failing layer and why,
2) the exact WFGY page to open next,
3) a minimal calibration plan for hybrid weights and normalization,
4) a verification plan to reach coverage ≥ 0.70 and ΔS ≤ 0.45.
Use BBMC, BBCR, BBPF, BBAM when relevant.
```
---
### 🔗 Quick-Start Downloads (60 sec)
| Tool | Link | 3-Step Setup |
| -------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------ | -------------------------------------------------------------------------------------- |
| **WFGY 1.0 PDF** | [Engine Paper](https://github.com/onestardao/WFGY/blob/main/I_am_not_lizardman/WFGY_All_Principles_Return_to_One_v1.0_PSBigBig_Public.pdf) | 1⃣ Download · 2⃣ Upload to your LLM · 3⃣ Ask “Answer using WFGY + \<your question>” |
| **TXT OS (plain-text OS)** | [TXTOS.txt](https://github.com/onestardao/WFGY/blob/main/OS/TXTOS.txt) | 1⃣ Download · 2⃣ Paste into any LLM chat · 3⃣ Type “hello world” to boot |
---
### 🧭 Explore More
| Module | Description | Link |
| --------------------- | ------------------------------------------------------- | -------------------------------------------------------------------------------------------------- |
| WFGY Core | WFGY 2.0 engine. full symbolic reasoning and math stack | [View →](https://github.com/onestardao/WFGY/tree/main/core/README.md) |
| Problem Map 1.0 | Initial 16 mode diagnostic and symbolic fixes | [View →](https://github.com/onestardao/WFGY/tree/main/ProblemMap/README.md) |
| Problem Map 2.0 | RAG focused failure tree and pipelines | [View →](https://github.com/onestardao/WFGY/blob/main/ProblemMap/rag-architecture-and-recovery.md) |
| Semantic Clinic Index | Prompt injection. memory bugs. logic drift | [View →](https://github.com/onestardao/WFGY/blob/main/ProblemMap/SemanticClinicIndex.md) |
| Semantic Blueprint | Layer based symbolic reasoning. semantic modulations | [View →](https://github.com/onestardao/WFGY/tree/main/SemanticBlueprint/README.md) |
| Benchmark vs GPT-5 | Stress test with full WFGY suite | [View →](https://github.com/onestardao/WFGY/tree/main/benchmarks/benchmark-vs-gpt5/README.md) |
| Starter Village | A guided first run | [Start →](https://github.com/onestardao/WFGY/blob/main/StarterVillage/README.md) |
---
> 👑 Early Stargazers. [See the Hall of Fame](https://github.com/onestardao/WFGY/tree/main/stargazers)
> Engineers, hackers, and open source builders who supported WFGY from day one.
> <img src="https://img.shields.io/github/stars/onestardao/WFGY?style=social" alt="GitHub stars"> ⭐ [WFGY Engine 2.0](https://github.com/onestardao/WFGY/blob/main/core/README.md) is live. ⭐ Star the repo to help others discover it and unlock more on the Unlock Board.
<div align="center">
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