Second external baseline for BrainBench. Pure cosine-similarity ranking
using the SAME text-embedding-3-large model gbrain uses internally —
apples-to-apples on the embedding layer so any gbrain lead reflects the
graph + hybrid fusion, not a better embedder.
Files:
eval/runner/adapters/vector-only.ts ~130 LOC
eval/runner/adapters/vector-only.test.ts 6 unit tests (cosine math)
Design:
- One vector per page (title + compiled_truth + timeline, capped 8K chars).
- No chunking (intentional; chunked vector RAG would be EXT-2b later).
- No keyword fallback (that's EXT-3 hybrid-without-graph).
- Embeddings in batches of 50 via existing src/core/embedding.ts (retry+backoff).
- Cost on 240 pages: ~$0.02/run.
Three-adapter side-by-side on 240-page rich-prose corpus, 145 relational queries:
| Adapter | P@5 | R@5 | Correct top-5 |
|---------------|--------|--------|---------------|
| gbrain-after | 49.1% | 97.9% | 248/261 |
| ripgrep-bm25 | 17.1% | 62.4% | 124/261 |
| vector-only | 10.8% | 40.7% | 78/261 |
Interesting finding: vector-only scores WORSE than BM25 on relational queries
like "Who invested in X?" — exact entity match matters more than semantic
similarity for these templates. BM25 nails the entity-name term; vector-only
returns topically-similar-but-not-mentioning pages. This is the known failure
mode of pure-vector RAG on precise relational/identity queries. Real-world
vector RAG systems always add keyword fallback; EXT-3 (hybrid-without-graph)
will be that fairer comparator.
gbrain's lead widens in vector-only comparison: +38.4 pts P@5, +57.2 pts R@5.
The graph layer is doing the heavy lifting for relational traversal; pure
vector RAG can't express "traverse 'attended' edges from this meeting page."
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
