Agentsop Multiscale Chunking

Multi-scale Chunking · C5 Enhancement Overlay

Overlay on top of [[llamaindex]]. The base skill teaches the 5-layer RAG pipeline and lists DecoupleChunkScope as one optimization knob among many. This overlay zooms in on that single knob and turns it into a standalone recipe: how to resolve the chunk paradox when one chunk size is provably not enough. Third-person analytical view for an agent writing / reviewing RAG ingestion code — not an end-user tutorial.

1. 何时激活 (Activation Rules)

Activate this overlay when all three RAG preconditions hold and the chunk paradox has actually surfaced:

1. The corpus is long documents — prose manuals, financial filings, legal contracts, research papers, codebases — where a single answer-bearing fact sits inside a larger context that the LLM needs to interpret it.
2. A chunk-size sweep has stalled: small chunks (128–256) win retrieval precision but the LLM answers from fragments; large chunks (1024–2048) give rich context but recall on specific queries drops because the embedding becomes a "topic average". The official failure-mode checklist documents both poles as separate failures — #2 (wrong chunk from too-small) and #6 (context overflow / dilution from too-large) (cited in [[llamaindex]] R3).
3. Faithfulness or relevancy is plateauing below target and bumping chunk_size only moves the failure from one pole to the other.

Concrete triggers:

• "Answers are technically retrieved but the model lacks context to explain them."
• "I keep retuning chunk_size and it never wins on both faithfulness and recall."
• A reviewer sees SentenceSplitter(chunk_size=4096) shipped as the fix for "incomplete answers" (this is anti-pattern A1 in [[llamaindex]]).

Do not activate when:

• The corpus is short/static (<100k tokens) — prompt-stuff with caching; multi-scale chunking is over-engineering (§6).
• The chunk-size sweep did converge on a single winner (e.g. 1024 for prose) — pin it and stop.
• Retrieval quality is fine and the bottleneck is orchestration or synthesis.

2. 核心心智模型 (Core Mental Model)

Decouple the embed-unit from the return-unit. Embed small for retrieval precision; return large for generation context.

The naive assumption is that the unit you index is the unit you feed the LLM. That single identity is the source of the paradox: it forces one chunk size to serve two opposing jobs.

NAIVE (one unit, two jobs)          MULTI-SCALE (two units, one job each)
─────────────────────────          ─────────────────────────────────────
       [ chunk ]                    embed unit  →  small  (precision job)
      /         \                          │
 embed it     feed it                   match
 (wants       (wants                       │
  small)       large)                 return unit →  large  (context job)
   ↓             ↓                          ▲
  CONFLICT — pick one,                   expand from
  lose the other                        match → parent / window

Three load-bearing sub-principles:

1. A Node is a graph node, not a chunk. In LlamaIndex a Node carries relationships (PREV/NEXT/PARENT/CHILD). Those links are exactly what let you store a small node for matching and resolve it to a larger node for return ([[llamaindex]] Principle 2). Multi-scale chunking is "build a chunk-graph", not "split into chunks".

2. Two geometries of expansion. Once embed ≠ return, you must choose how the small match expands into the large return:

   • Horizontal — return N adjacent sentences around the matched sentence (sentence-window). The expansion is positional.
   • Vertical — return the parent chunk when enough sibling children match (auto-merging / parent-child). The expansion is hierarchical.

3. Match the geometry to the document, not to taste. Flat narrative prose → horizontal. Documents with real structure (headings, sections, tables of contents) → vertical. This is the central dilemma case (§5.1).

The overlay's promise: this strictly dominates a compromise chunk size when the sweep frontier is non-flat — you no longer average two bad sizes.

3. SOP 工作流 (Decision Protocol)

A three-gate protocol. Do not skip Gate 0 — multi-scale chunking is only justified once a single chunk size has been proven insufficient.

Gate 0 — Pick the base chunk size first (and try to stop here)

Run the canonical sweep from [[llamaindex]] OP-02 before reaching for any multi-scale machinery:

from llama_index.core.evaluation import (
    FaithfulnessEvaluator, RelevancyEvaluator,
)
# 1. ~20 eval QA pairs via DatasetGenerator.from_documents(docs)
# 2. sweep:
for cs in (128, 256, 512, 1024, 2048):           # overlap = 0.1–0.2 × cs
    idx = build_index(docs, chunk_size=cs, overlap=int(0.15 * cs))
    record(cs, faithfulness=eval_f(idx), relevancy=eval_r(idx), p95=latency(idx))

• If a single chunk size dominates both faithfulness and relevancy → pin it and stop. LlamaIndex's own Uber 10-K study peaked at 1024 for prose; code lands at 80–160 tokens (§5.2).
• If the frontier is non-flat (small wins precision, large wins context, no single winner) → proceed to Gate 1. Do not compromise on a middle size.

Gate 1 — Choose the expansion strategy by document structure

Set the embed-unit small (single sentence, or 128–256-token leaf) and the return-unit large (the window, or the parent/root chunk).

Gate 2 — Wire it and measure the lift

• Sentence-Window: SentenceWindowNodeParser must be paired with MetadataReplacementPostProcessor — otherwise the metadata-stuffed matched sentence (not the window) reaches the LLM, defeating the entire point (§6 A2).
• Auto-Merging: HierarchicalNodeParser builds leaf+parent nodes; store all leaves in the docstore and AutoMergingRetriever merges children → parent when ≥ threshold siblings match.
• Re-run the same eval set from Gate 0. Both patterns should beat naive top-k on faithfulness; if neither does, the bottleneck is elsewhere (revert).
• Pin the chosen parser + retriever config and the embedding-model version into index metadata, exactly as Gate 0's chunk size would have been pinned.

4. 操作模型 (Operation Models)

Each operation: Trigger / Action / Output / Evidence. These refine [[llamaindex]] OP-02 and OP-05 into executable sub-steps.

MSC-01 — ChunkSizeSweep

• Trigger: New long-doc corpus; chunk size unknown; before any multi-scale work.
• Action: Generate ~20 QA pairs; sweep chunk_size ∈ {128,256,512,1024,2048}, overlap = 10–20%; build one VectorStoreIndex per config; record faithfulness + relevancy + p95 latency.
• Output: Either a pinned single winner, OR a documented non-flat frontier that authorizes multi-scale chunking.
• Evidence: llamaindex.ai/blog/evaluating-the-ideal-chunk-size-for-a-rag-system-using-llamaindex-6207e5d3fec5; [[llamaindex]] OP-02.

MSC-02 — SentenceWindowSetup (horizontal expansion)

• Trigger: Flat prose; point-facts that need a surrounding paragraph; lowest setup cost wanted.
• Action: SentenceWindowNodeParser(window_size=3) to embed single sentences with N-neighbor windows in metadata; at query time apply MetadataReplacementPostProcessor(target_metadata_key="window") so the LLM receives the window, not the lone sentence.
• Output: Precise sentence-level matching + paragraph-level synthesis context.
• Evidence: developers.llamaindex.ai SentenceWindow / MetadataReplacement docs; [[llamaindex]] R3 Dilemma 5.

MSC-03 — AutoMergingHierarchy (vertical expansion)

• Trigger: Documents with clear hierarchy; bursty multi-chunk relevance.
• Action: HierarchicalNodeParser.from_defaults(chunk_sizes=[2048,512,128]) to build a leaf→parent→root tree; index leaf nodes in a VectorStoreIndex, keep all nodes in a docstore; retrieve with AutoMergingRetriever, which returns the parent once a configured fraction of its children are in the hit set.
• Output: Leaf-level precision that escalates to section-level context when warranted.
• Evidence: developers.llamaindex.ai/python/framework/integrations/retrievers/auto_merging_retriever/; [[llamaindex]] OP-05.

MSC-04 — WindowVsMergeChoice

• Trigger: Multi-scale authorized (MSC-01 non-flat) but strategy undecided.
• Action: Inspect document structure (Gate 1 table). Flat → MSC-02; structured/bursty → MSC-03; unknown → start MSC-02 (cheaper), escalate to MSC-03 if it underperforms on multi-chunk queries.
• Output: A strategy chosen by document shape, not by elegance.
• Evidence: [[llamaindex]] R3 Dilemma 5; medium.com/@harsh_77214/beyond-naive-rag-comparing-basic-sentence-window-and-auto-merging-retrieval-....

MSC-05 — EmbedReturnDecouple (the core flip)

• Trigger: Any time chunk-size tuning oscillates between precision and context.
• Action: Set embed-unit small, return-unit large; never let them be the same object once the sweep is non-flat. Verify by inspecting what text the retriever actually sends to the synthesizer (must be the large unit).
• Output: Pareto improvement on precision×context that no single size achieves.
• Evidence: [[llamaindex]] Principle 2 (Node relationships); R3 Dilemma 1 resolution.

MSC-06 — MetadataBudgetGuard

• Trigger: Small embed-units (≤256 tokens) with rich metadata propagated into payload.
• Action: Ensure metadata never occupies >50% of the embed-unit token budget; strip/shorten metadata before shrinking chunks (GitHub #12200, #13792).
• Output: Embed-units carry signal, not mostly boilerplate metadata.
• Evidence: github.com/run-llama/llama_index/issues/12200, #13792; [[llamaindex]] A7.

MSC-07 — MeasureOrRevert

• Trigger: Multi-scale config wired but lift unverified.
• Action: Re-run the Gate-0 eval set; require a measurable faithfulness lift over the best single chunk size; if none, revert to the pinned single size.
• Output: Evidence that the added complexity earns its keep — or its removal.
• Evidence: [[llamaindex]] Stage 2 (eval loop gates every change).

5. 困境决策案例 (Dilemma Cases)