SuperBizAgent-java/openspec/changes/archive/2026-07-27-rag-chunk-evidence-identity-dedup/design.md

Design: RAG chunk evidence identity and dedup

Context

• Modular RAG pipeline already exists (KnowledgeQueryTransformer → retriever → post-processor → packer → assembler → projector).
• Delivery baseline: mvp/engineering/rag/Milvus-Hybrid接入清单.md §1.1 Delivery 1.
• Historical decision (modular RAG): L0 is hint-only; L1 is fact evidence. This change keeps that boundary.
• Legacy Milvus SDK search path will be abandoned later; this change must not thicken SDK-specific logic.

Goals / Non-Goals

Goals

• Preserve multiple relevant chunks from the same document in one lookup.
• Make evidence identity stable enough for future hybrid hits.
• Separate retrieve width from return width.
• Keep Agent tool name/input unchanged (lookup_knowledge(query)).

Non-Goals

• Hybrid / BM25 / sparse schema.
• Cross-encoder rerank.
• Deleting SDK mode.
• Session dedup tracker.

Decisions

D1. Evidence identity key

evidenceKey =
  if docId != null && chunkIndex != null:
    docId + "#chunk-" + chunkIndex
  else if vectorId != null:
    "vector:" + vectorId
  else:
    "rank:" + originalRank

Rationale: works with current metadata (docId, chunkIndex) and degrades safely for older rows.

D2. Dedup granularity

• Dedup key = evidenceKey only.
• True duplicates (same key) merge hitReasons; keep higher-ranked content (first after score sort).
• Do not merge different chunks of the same source into one content block.

D3. Per-document cap

• After score sort, accept at most rag.max-chunks-per-document (default 2) evidence blocks per docId.
• If docId missing, treat each evidenceKey as its own document bucket.

D4. retrieve-k / return-n

rag.retrieve-k=20   # vector recall width
rag.return-n=5      # max evidence blocks after post-process (before projector budget)
rag.max-chunks-per-document=2

Compatibility:

• If only legacy rag.top-k is set, use it as fallback for both until removed.
• Prefer explicit retrieve-k/return-n when present.

D5. Agent projection identity (behavior change)

• Projected document_id SHOULD be evidenceKey (chunk-scoped), not raw source.
• This is intentional so EvidenceGuard references remain 1:1 with returned excerpts.
• source remains human-readable document path/id and MAY repeat across chunks.
• Marked as observable behavior change for Agent consumers and report references.

D6. KnowledgeSearchPort (thin)

KnowledgeSearchPort.search(KnowledgeSearchRequest) -> List<KnowledgeSearchHit>

• Default adapter delegates to existing VectorSearchService.searchSimilarDocuments.
• Request carries query, topK, categoryFilter, mode placeholder (DENSE only in this change).
• Hit carries id, content, score fields, metadata map, and extracted identity fields when available.
• No hybrid implementation in this change.

Module flow

LookupKnowledge
  -> transform (L0 hints)
  -> KnowledgeSearchPort.search(retrieveK, filter)
  -> map to RetrievedEvidenceCandidate (+ identity)
  -> post-process:
       score/sort
       evidenceKey dedup
       maxChunksPerDocument
       return-n truncate
  -> pack + assemble
  -> RagResultProjector (dedupe by evidence document_id=evidenceKey)

Interface impact

Level	What
L2	Internal DTO fields on candidate/EvidenceBlock
L3	Agent-facing document_id becomes chunk-scoped evidence id

Migration/compat:

• In-repo guards/tests updated to accept chunk-scoped ids.
• External human readers still see source/title.

Risks / Trade-offs

Risk	Mitigation
Larger Agent payload	return-n + maxChunksPerDocument + existing projector budgets
Missing chunkIndex in old data	vector id fallback keeps chunks distinct
document_id semantic shift	documented; projector tests updated

Open questions

None remaining for Delivery 1. Hybrid belongs to next change.