Reranker Stage · SOP

Third-person analytical view of how a mature RAG pipeline thinks about the reranker. The skill is for an LLM agent that writes / reviews / debugs retrieval code — it teaches the cross-framework reranking discipline, not one vendor's API. For the per-framework API, descend to [[llamaindex]] (node postprocessors) or [[agentsop-hybrid-retrieval]] (the recall stage that feeds the reranker).

This is the C4 gap skill in the Phase-D enhance pass. The reranker SOP existed only buried inside [[llamaindex]] (OP-03 AddReranker, Stage 3 step 7, anti-pattern A6). It is the highest-ROI single addition to a naive RAG pipeline, so it earns a standalone overlay.

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1 · 何时激活 (Activation Rules)

Activate when any holds:

1. A RAG pipeline's answer quality has plateaued after the cheap knobs (prompt, embedding model, chunk size) are exhausted — [[llamaindex]] Stage 3 lists reranking as the last optimization step, deliberately.
2. Diagnostics show the relevant document is in top-k but buried — high hit-rate, low MRR, wrong top-1. This is LlamaIndex failure modes #1 / #10 ([[llamaindex]] OP-03).
3. The LLM context window is under pressure — too many marginal chunks inflate cost, latency, and "lost-in-the-middle" degradation. A reranker lets you retrieve 50 and feed 5.
4. A user asks where to add a reranker, how to tune N vs k, or API vs local.

Do not activate (boundary — see §6):

• Recall is the bottleneck: the right doc is not in top-N at all. A reranker can only reorder what retrieval already found — fix retrieval, hybrid ([[agentsop-hybrid-retrieval]]), or chunking first.
• top-k is already small (≤5) and answers are correct — no plateau.
• A hard sub-100ms path where the extra round-trip is unaffordable and quality is already acceptable.

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2 · 核心心智模型 (Core Mental Model)

The one sentence

Retrieve wide for recall with a cheap bi-encoder; rerank narrow for precision with an expensive cross-encoder that sees query + document together — something the bi-encoder structurally could not do.

Why two stages exist at all

The retriever (bi-encoder / vector search) embeds the query and every document separately, offline. Similarity is a dot product of two vectors that never met. This is fast (vectors are precomputed; ANN search is sub-linear) but lossy: the document's vector is a single "topic average" computed without knowledge of the query.

A cross-encoder takes [query, document] as a single joint input and runs full attention across both, emitting one relevance score. It sees exactly which query token matches which document token. This is far more accurate — and far more expensive: it cannot be precomputed, so it runs once per (query, candidate) pair at query time. Scoring 1M docs this way is infeasible; scoring 20-50 is cheap.

  query ─┐                               query ─┐
         ├─ dot product (precomputed)            ├─► [CROSS-ENCODER] ─► score
  doc  ─┘   ← bi-encoder, FAST, lossy     doc  ─┘   joint attention, SLOW, sharp
       RECALL stage (retrieve top-50)         PRECISION stage (rerank → top-5)

The reranker is the bridge: it spends cross-encoder accuracy on a small candidate set the bi-encoder produced cheaply. Wide net, sharp knife.

The order law (inherited from [[llamaindex]] Stage 3)

Prompts first, reranking last. Reranking is high-impact but expensive — exhaust the cheap knobs (prompt, embed model, chunk size, hybrid) before spending per-query cross-encoder latency. But once those are spent, the reranker is usually the single biggest remaining lever (5-15pp faithfulness lift on noisy corpora — [[llamaindex]] OP-03).

What a reranker is NOT

• Not a recall fix — it reorders, never retrieves (§6).
• Not a chunking fix — it scores whole candidates, it does not resize them.
• Not free — every reranked candidate is an inference (local) or a billed unit (API).

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3 · SOP 工作流 (Agentic Protocol)

Each stage gates the next. Never skip the baseline measurement.

Stage 0 — Confirm the lever is real

Before adding anything, prove the symptom is precision, not recall:

1. Run the existing pipeline against ~30-50 labeled QA pairs.
2. Record hit-rate@N (is the gold doc in top-N?) and MRR (how high?).
3. If hit-rate is low → recall problem → STOP, fix retrieval / hybrid ([[agentsop-hybrid-retrieval]]) / chunking. A reranker will not help.
4. If hit-rate is high but MRR is low → precision problem → the gold doc is buried → a reranker is the right lever. Proceed.

Stage 1 — Retrieve wide

Raise the retriever's top_k (or top_n) to 20-50. This is the "recall" stage: cast a wide net so the reranker has the gold doc to find. Hybrid retrieval ([[agentsop-hybrid-retrieval]]) feeds the reranker an even better candidate pool because it adds lexical recall the dense retriever misses.

Stage 2 — Insert the reranker

Add a reranker as a post-retrieval step (LlamaIndex node postprocessor; LangChain ContextualCompressionRetriever — §7). Pick the model per §4 OP-03. It consumes the wide candidate list and re-scores every candidate against the query with a cross-encoder.

Stage 3 — Keep narrow

Truncate to top-k = 3-5 after rerank. This is what reaches the synthesizer. The whole point: the LLM now sees a small, high-precision context instead of a large noisy one.

Stage 4 — Measure the lift, gate the change

Re-run the same eval set. Compare before vs after on {MRR, faithfulness, relevancy, p95 latency, per-query cost}. Keep the reranker only if the precision lift justifies the added latency/cost (§5). A reranker that adds 300ms for +1pp is not always worth shipping. Pin N and k as tuned constants.

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4 · 操作模型 (Operation Models)

Each operation: Trigger / Action / Output / Evidence. Full machine-readable list in intermediate/operation_candidates.json.

OP-01 ConfirmPrecisionNotRecall

• Trigger: Considering a reranker; haven't proven the symptom is precision.
• Action: Measure hit-rate@N and MRR on labeled QA. High hit-rate + low MRR ⇒ precision problem ⇒ reranker is right. Low hit-rate ⇒ recall problem ⇒ STOP.
• Output: Go/no-go decision backed by a number, not a hunch.
• Evidence: [[llamaindex]] OP-03 (#1/#10 = right doc in top-k, wrong top-1); [[agentsop-hybrid-retrieval]] for the recall path.

OP-02 WidenThenNarrow

• Trigger: Reranker confirmed; pipeline still on naive top_k=4.
• Action: Retrieve top-N = 20-50, rerank, keep top-k = 3-5. Embed the two numbers as named, evaluated constants.
• Output: A two-stage retrieve→rerank pipeline with a high-precision tail.
• Evidence: [[llamaindex]] Stage 3 step 7 ("widen top_k to 20-50, rerank to 3-5"); OP-03.

OP-03 ChooseRerankerModel

• Trigger: Reranker stage exists; model not yet chosen.
• Action: Pick by constraint:
  • Cohere Rerank / Voyage rerank (API) — fastest to ship, no GPU, strong multilingual; cost per 1k searches, data leaves your boundary.
  • bge-reranker (bge-reranker-v2-m3 / large) — local — open-weights, self-hosted, no per-call fee, strong on multilingual; needs a GPU for low latency, you own ops.
  • SentenceTransformer cross-encoder (e.g. ms-marco-MiniLM) — light, CPU-runnable for small N, the lowest-dependency local option; weaker than bge-large but cheap.
  • ColBERT (late-interaction) — middle ground: token-level interaction, precomputable, scales to larger N than a full cross-encoder.
• Output: A model justified by latency budget, cost ceiling, data-residency, and language mix.
• Evidence: [[llamaindex]] OP-03 (CohereRerank / SentenceTransformerRerank / ColBERT named); external: "cohere rerank", "bge-reranker", "cross-encoder rerank RAG".

OP-04 BudgetLatencyAndCost

• Trigger: Before shipping; reranker adds a per-query inference/billing unit.
• Action: Measure p95 added by the rerank call at the chosen N. API rerankers add a network round-trip (~tens-hundreds ms) + per-search cost; local models add GPU/CPU inference time. Latency scales with N, not k — so over-large N is the latency killer (§6).
• Output: p95 and $/query deltas attached to the change; ship only if the precision lift clears the bar.
• Evidence: [[llamaindex]] Stage 3 ("high-impact but expensive — exhaust cheap knobs first"); §5 Dilemma 1.

OP-05 TuneNvsK

• Trigger: Reranker live; N/k still at defaults; want to optimize the precision/latency frontier.
• Action: Sweep N ∈ {20, 30, 50} holding k fixed (recall ceiling of the candidate pool), then sweep k ∈ {3, 5, 8} holding N fixed (how much context the LLM sees). Pick the smallest N that saturates hit-rate and the smallest k that saturates faithfulness.
• Output: Tuned (N, k) on the cost/quality frontier, not guessed.
• Evidence: [[llamaindex]] OP-02 TuneChunkSize (same sweep-and-pin discipline applied to N/k); Stage 3 step 7.

OP-06 RerankAfterHybrid

• Trigger: Traffic has lexical-identity queries (codes, SKUs, symbols) AND a precision plateau.
• Action: Use hybrid retrieval ([[agentsop-hybrid-retrieval]], BM25 + dense) for the wide stage, then rerank its fused candidate list. Hybrid maximizes recall into the pool; rerank maximizes precision out of it. They compose.
• Output: Best-of-both — lexical recall + cross-encoder precision.
• Evidence: [[agentsop-hybrid-retrieval]] (recall stage); [[llamaindex]] OP-04 AddHybridBM25 + OP-03 AddReranker (sequential in Stage 3).

OP-07 GateOnEval

• Trigger: Any reranker add/change.
• Action: Compare before/after on {MRR, faithfulness, relevancy, p95, $/query}. Keep only on net-positive. Treat as a regression test for future retriever changes.
• Output: Quantitative justification; the reranker is now eval-gated.
• Evidence: [[llamaindex]] OP-10 EvalLoop, Stage 2 ("eval loop before optimizing anything"), Stage 4.

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5 · 困境决策案例 (Dilemma Cases)

Dilemma 1 — Rerank latency vs answer quality

困境: A reranker reliably lifts precision but adds a per-query stage: network round-trip (API) or GPU inference (local). On a latency-sensitive surface (chat, autocomplete) the added p95 may violate the SLA even when quality improves.

约束: Cross-encoder cost is per (query, candidate) pair and scales with N ([[llamaindex]] Stage 3: reranking is "high-impact but expensive"). Latency is dominated by N, not k. The bi-encoder stage was chosen precisely because it is fast; the reranker reintroduces query-time compute.

决策步骤:

1. Measure baseline p95 and the SLA headroom.
2. Measure rerank-stage p95 at the smallest viable N (start N=20).
3. If it fits headroom and quality lifts ≥ a meaningful threshold → ship.
4. If it does not fit → shrink N (OP-05), switch to a lighter model (MiniLM cross-encoder, ColBERT), or rerank async/cache for repeat queries.
5. If still over budget and quality is already acceptable → do not rerank (§6 boundary).

结果: Reranking is the highest-ROI lever only when latency headroom exists. The decision is SLA-driven, not quality-driven in isolation. Smaller N often recovers most of the lift at a fraction of the latency.

可提取的操作: OP-04, OP-05. Anti-pattern A3 (over-large N).

Dilemma 2 — API reranker (Cohere/Voyage) vs local model (bge / cross-encoder)

困境: The hosted API ships in an afternoon, needs no GPU, and tracks SOTA — but bills per search and sends query + candidates to a third party. A local bge-reranker has zero per-call fee and keeps data in-boundary — but needs a GPU, ops ownership, and model-update discipline.

约束: Per-query cost (API) vs fixed infra cost + ops (local); data-residency / compliance; latency (API adds network hop, local adds inference); team's GPU/MLOps capacity.

决策步骤:

1. Data residency hard constraint (PII, regulated)? → local (bge / ColBERT), decision over.
2. Estimate query volume × API price vs GPU rental. Low/spiky volume → API usually cheaper; high steady volume → local amortizes.
3. No GPU and no MLOps appetite? → API (or CPU MiniLM for tiny N).
4. Multilingual corpus? Both Cohere and bge-reranker-v2-m3 are strong — let cost/residency decide.
5. Whichever: wrap the call behind a single rerank(query, nodes) -> nodes seam so swapping API↔local is a one-line change.

结果: Default to the API to validate the lift cheaply (prove the reranker helps before investing in infra), then migrate to local once volume, cost, or residency justify it. The abstraction seam makes the migration safe.

可提取的操作: OP-03, OP-04. Anti-pattern A5 (vendor lock-in, no seam).

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6 · 反模式与边界 (Anti-patterns & Boundaries)

Anti-patterns

#	Anti-pattern	Correct move
A1	Reranking to fix recall — gold doc isn't in top-N	Fix retrieval / hybrid ([[agentsop-hybrid-retrieval]]) / chunking; a reranker only reorders what's already retrieved
A2	Naive similarity_top_k=N then feed all N to the LLM, no rerank	Widen N and rerank to top-3-5 ([[llamaindex]] A6)
A3	Over-large N (rerank 200+ candidates)	Latency scales with N; pick the smallest N that saturates hit-rate (OP-05)
A4	Add reranker first, before prompt/embed/chunk/hybrid	Order law: reranking is last ([[llamaindex]] Stage 3); cheapest knobs first
A5	Hard-wire one vendor SDK throughout the pipeline	Hide behind a rerank(query, nodes) seam so API↔local swaps in one line (Dilemma 2)
A6	Ship reranker without before/after eval	Gate on {MRR, faithfulness, p95, $/query} (OP-07); a reranker that costs latency for no lift is removed
A7	Keep N=k (rerank n candidates, return n)	Reranking only helps when k < N — you must discard the low-scored tail
A8	Re-embed / re-chunk hoping to fix "wrong top-1"	If the right doc is present but buried, that's a rerank job, not a re-ingest

Boundaries — when not to add a reranker

• B1 — Recall is the bottleneck: hit-rate@N low ⇒ the answer isn't in the pool. Reranking is a no-op. Fix retrieval first (OP-01, [[agentsop-hybrid-retrieval]]).
• B2 — Already narrow & correct: top-k ≤ 5 and answers right ⇒ no plateau, no lever.
• B3 — Hard real-time / sub-100ms: the extra round-trip blows the budget and quality is acceptable ⇒ skip (Dilemma 1).
• B4 — Tiny static corpus (prompt-stuffable, <100k tokens): no retrieval stage to rerank ([[llamaindex]] B1).

PR-review smells (instant red flags)

• index.as_query_engine(similarity_top_k=20) with no node postprocessor → A2 ([[llamaindex]] PR-smell).
• Reranker added but top_k still 4 → A7 (N=k, reranker is a no-op).
• A vendor rerank SDK imported in >1 module → A5 (no seam).
• A reranker PR with no eval delta in the description → A6.
• "Added reranker to improve recall" in a commit message → A1 (category error).

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7 · 跨框架对照 (Cross-framework Mapping)

The reranker is one stage with the same shape everywhere: consume a wide candidate list, re-score with a cross-encoder, truncate to top-k.

Framework / vendor	Reranker primitive	Notes
LlamaIndex ([[llamaindex]])	Node postprocessor: CohereRerank, SentenceTransformerRerank, ColbertRerank, LLMRerank passed as node_postprocessors=[...] to the query engine; widen similarity_top_k, set top_n on the reranker	The canonical reference; OP-03 AddReranker, Stage 3 step 7, A6
LangChain	ContextualCompressionRetriever wrapping a base retriever with a CohereRerank / CrossEncoderReranker / LLMChainExtractor compressor	Base retriever returns N, compressor reranks/filters to k
Cohere Rerank API	cohere.rerank(query, documents, top_n, model="rerank-v3.5")	Hosted cross-encoder; multilingual; per-search billing
Voyage rerank API	voyageai.rerank(query, documents, model="rerank-2", top_k)	Hosted; pairs well with Voyage embeddings
bge-reranker (local)	FlagReranker("BAAI/bge-reranker-v2-m3") / via sentence-transformers CrossEncoder	Open-weights, self-hosted, no per-call fee, GPU recommended
SentenceTransformers cross-encoder	CrossEncoder("cross-encoder/ms-marco-MiniLM-L-6-v2").predict([(q, d), ...])	Lightest local option; CPU-viable for small N
ColBERT / RAGatouille	Late-interaction reranker; token-level scoring, precomputable	Scales to larger N than a full cross-encoder
Haystack	TransformersSimilarityRanker / CohereRanker component in the pipeline	Same wide→narrow shape, pipeline-component form

Activate this skill for the reranking decision (whether, where, how wide, which model, what it costs). Descend to [[llamaindex]] for node-postprocessor wiring, and to [[agentsop-hybrid-retrieval]] for the recall stage that feeds it.

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References

• references/R1-source-evidence.md — every cited claim resolved to a source line.
• intermediate/operation_candidates.json — machine-readable operation list.

Primary sources (cited inline above)

• [[llamaindex]] SKILL — OP-03 AddReranker, OP-02 TuneChunkSize, OP-04 AddHybridBM25, OP-10 EvalLoop; Stage 2/3/4; anti-patterns A6/A3; failure modes #1/#10; the "prompts first, reranking last" order law.
• [[agentsop-hybrid-retrieval]] — the wide/recall stage (BM25 + dense) that feeds the reranker; lexical-identity recall.
• External: "cohere rerank" (rerank-v3.5, hosted cross-encoder, per-search billing, multilingual); "bge-reranker" (BAAI bge-reranker-v2-m3 / large, open-weights local cross-encoder); "cross-encoder rerank RAG" (bi-encoder retrieve → cross-encoder rerank, joint query+doc attention, the two-stage recall→precision pattern).