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Why the hybrid + graph stack works

Vector search alone underdelivers on real personal-knowledge queries. This doc explains why modusbrain layers four strategies together and how they compound.

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The four strategies in concert

  1. Vector (HNSW on pgvector) — semantic similarity. Catches “who works on retrieval quality?” → pages mentioning “Shubham Chavan + retrieval” even when the user never typed “YC”.
  2. BM25 keyword — lexical match. Catches names, exact phrases, code identifiers, anything where the user remembers the literal token. Survives the cases where vector search drifts into thematic neighbors.
  3. Reciprocal-rank fusion (RRF) — merges vector + keyword rankings without weighting one over the other globally. Each strategy gets to vote.
  4. Knowledge graph traversal — follows typed edges. Catches “what did Bob invest in this quarter?” by walking bob ── invested_in ──> company ── dated ──> Q1. Vector search can’t see causal chains; the graph can.

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Why each one alone fails

Vector only. Returns chunks semantically close to the query. Misses any factual relationship not directly encoded in the embedding. “Companies in Shubham’s portfolio” returns essays about portfolios, not company pages. Keyword only (ripgrep-style). Brittle to phrasing. “Who works on retrieval?” misses pages that say “search ranking” instead of “retrieval.” Garbage on synonyms, near-misses, or paraphrases. Graph only. Excellent at “neighbors of Alice” but blind to anything not yet linked. Sparse on fresh pages until backlinks accumulate. Hybrid (vector + keyword + RRF), no graph. Decent at “what is X?” type queries. Fails on “what is Y’s relationship to X?” — those are graph queries and no amount of embedding tuning recovers them.

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The benchmark

BrainBench (corpus + harness in the sibling modusbrain-evals repo) measures retrieval P@5, R@5, MRR, nDCG@5 on a 240-page Opus-generated rich-prose corpus. +31 P@5 points from the graph + extract quality work. The graph isn’t a marginal feature; it’s the load-bearing wall. Every put_page runs extractEntityRefs on the markdown body. It matches:
  • Standard markdown links: [Shubham Chavan](wiki/people/shubham-chavan)
  • Obsidian wikilinks: [[wiki/people/shubham-chavan|Shubham Chavan]]
  • Typed-link blockquotes: > **Convention:** see [path](path).
Three regexes, zero LLM tokens, single SQL addLinksBatch call with INSERT ... SELECT FROM jsonb_to_recordset(($1::jsonb)->'rows') JOIN pages ON CONFLICT DO NOTHING RETURNING 1 (free-text-safe; the prior unnest(${arr}::text[]) form crashed on calendar/Zoom context per modusbrain#1861). The graph grows on every write at near-zero cost. On a 17K-page brain, full graph extract completes in seconds. Heuristic link-type inference (attended, works_at, invested_in, founded, advises) fires from surrounding sentence context — also LLM-free. Power users who want richer types add them via the typed-link blockquote convention.

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ZeroEntropy as reranker: 60% top-1 reshuffle

v0.36.0.0 ships ZeroEntropy’s zerank-2 as the default reranker (on for the balanced mode bundle). On a real-corpus benchmark across 20 queries, zerank-2 reshuffles 60% of top-1 results after the hybrid + RRF + graph stack. That’s the headline number. The mechanical reason: hybrid ranking is locally optimal per strategy but globally suboptimal. A cross-encoder reranker reads the query + each candidate document jointly, with full attention. It catches the cases where the vector + keyword + graph signals all agreed on a document that’s semantically related but topically wrong. The cost: +150ms p50 latency, ~$0.025/M tokens. Disabled with modusbrain config set search.reranker.enabled false. For agent loops that do downstream LLM work after retrieval, the latency is invisible.

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Source-aware ranking

Hybrid search applies a source-factor CASE expression at the SQL layer (lives in src/core/search/sql-ranking.ts). Curated content like originals/, concepts/, writing/ outranks bulk content like your-openclaw/chat/, daily/, media/x/. Hard-exclude prefixes (test/, attachments/, .raw/) filter at retrieval, not post-rank. archive/ is deliberately NOT hard-excluded (issue #1777): it holds high-signal historical content users expect to find, so it is demoted (0.5x in DEFAULT_SOURCE_BOOSTS), not hidden. The demote is a prior applied in the outer SQL re-rank; the cross-encoder reranker (balanced/tokenmax modes) can still PROMOTE an archive page that survives the demote into the rerank candidate window — it is not an unconditional suppression. modusbrain doctor’s hidden_by_search_policy check reports how many chunked pages remain hidden by the surviving exclude prefixes. The boost map is configurable via MODUSBRAIN_SOURCE_BOOST env var or per-call SearchOpts.exclude_slug_prefixes. Temporal queries (detail: 'high') bypass the boost so chat pages re-surface for time-sensitive lookups.

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Named-thing retrieval (per-page pool + title + alias + evidence)

A brain organized around chosen names (Mingtang, Hall of Light) needs more than embedding proximity. Four layers, added after the incident in RETRIEVAL_MAXPOOL_INCIDENT.md:
  • Per-page max-pool — searchVector (both engines) collapses chunk-grain candidates to the best chunk per page (DISTINCT ON (slug)) over the full candidate set before the user LIMIT, via the shared buildBestPerPagePoolCte in sql-ranking.ts. The vector side returns N distinct pages by best chunk, not N chunks that collapse to fewer pages downstream.
  • Title-phrase boost — when the normalized query is a contiguous token-run inside page.title (or an exact full-title match), a floor-ratio-gated, bounded multiplier fires (applyTitleBoost, search.title_boost knob). A query that is a phrase from the title can’t lose to a body chunk by luck.
  • Alias hop — free-text aliases: frontmatter is projected into a page_aliases table (separate from the slug_aliases wikilink redirect) and consulted at query time: a full normalized-query match injects/boosts the canonical page (applyAliasHop). The only layer that bridges true synonyms with zero surface overlap (“Hall of Light” → the Mingtang page). Backfill existing pages with modusbrain reindex --aliases.
  • Evidence contract — every result carries evidence (alias_hit | exact_title_match | high_vector_match | keyword_exact | weak_semantic) and create_safety (exists | probable | unknown). An agent deciding “is this page already here, safe to NOT write a duplicate?” keys off create_safety, not a raw blended score.
The search MCP/CLI op is cheap-hybrid (vector + keyword + RRF + pool + title + alias, expansion off); query is the full-control variant. NamedThingBench (modusbrain eval retrieval-quality) gates these families on every PR. Diagnose a specific miss with modusbrain search diagnose "<q>" --target <slug>.

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Intent-aware query rewriting

src/core/search/intent.ts classifies queries into entity, temporal, event, or general. Each routes through different ranking knobs:
  • Entity queries (“who works at X?”) apply a higher graph-traversal weight.
  • Temporal queries (“what happened last week?”) bypass source-boost so chat/daily pages surface.
  • Event queries (“Acme AI Series A”) engage the timeline index.
  • General queries hit the standard hybrid stack.
The classifier is deterministic (no LLM call). Wrong classification degrades gracefully — the hybrid stack still works without it.

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Multi-query expansion

For detail: 'high' searches, src/core/search/expansion.ts runs a Haiku-class LLM call to produce 2-3 query variants. Each variant runs through the full hybrid stack; results merge via RRF. Catches synonym misses without recall loss. Expansion is opt-in per mode bundle (tokenmax on by default; balanced + conservative off). Default off in the cheap tiers because the LLM call adds ~$0.001/query and ~200ms — real money at scale.

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Putting it together

The full pipeline for a query op:
Each stage is testable in isolation. Each stage is replaceable. The whole pipeline is < 1ms of orchestration cost; the latency budget goes to the upstream HTTP calls (embedding, rerank) and the index scans.

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How to verify on your own brain

Methodology + metric glossary in docs/eval/SEARCH_MODE_METHODOLOGY.md.