# Newsletter

Product updates and occasional deep-dives.

## Site update

The YAMS site and documentation are being rebuilt around a smaller, maintained
surface. Historical plans, duplicated guides, and speculative design pages are
being removed. During the transition, use the README, command help, build guide,
roadmap, benchmark ledger, and machine-readable schemas as the canonical
references.

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## What you'll get

- Release notes and breaking changes
- Benchmark updates
- Design notes (search, storage, plugins)

## 2026-04-25: Simeon Is Now The Default Embedding Backend

Post: https://manta.black/posts/2026-04-25-simeon/

YAMS is switching its default retrieval embeddings backend to **Simeon** — training-free, SIMD/NEON, no model downloads. The short version: 7,000 docs/s encode throughput at the quality-matching tier vs MiniLM-L6's 322 docs/s on the same CPU. That speed plus byte-identical determinism is why it's the new default.

**BEIR scifact — speed/quality Pareto** (M-series CPU, single-threaded):

| Configuration | nDCG@10 | enc docs/s | qps |
|---|---:|---:|---:|
| `bm25_only` | 0.633 | — | 31,800 |
| `bm25_pool500_linear_alpha075_4096_384` | 0.638 | 7,000 | 5,800 |
| `bm25_pool500_linear_alpha075_4096_768` | 0.638 | 3,700 | 4,900 |
| `router_default_4096_768` | 0.654 | 3,600 | 1,500 |
| MiniLM-L6 (384-d float32, CPU) | 0.654 | 322 | 1,829 |

`router_default_4096_768` matches MiniLM-L6 on nDCG@10 (0.654) and beats it on MRR@10 (0.626 vs 0.607). The router picks among `Bm25Atire`, `Bm25SabSmooth`, and `CascadeLinearAlpha` per query. For medium/long semantic queries it dispatches to **PHSS** (Per-Hit Score Smoothing) — fragment geometry scoring that cuts the similarity graph at its largest gap.

**PHSS cross-corpus** (`LargestGapApprox`, richcov builder t=8):

| Corpus | BM25 | PHSS | Δ |
|---|---:|---:|---:|
| scifact | 0.6188 | 0.6188 | +0.000 |
| NFCorpus | 0.2521 | 0.2544 | +0.002 |
| FiQA | 0.2053 | 0.2089 | +0.004 |

BM25 parity on scifact, small consistent lifts on the other two — at ~2× the query throughput of the heavy `LargestGap` variant.

**Transfer caveat:** scifact parity with MiniLM does not transfer. On FiQA the scifact-tuned router lands at 0.202 vs MiniLM's 0.359. Simeon is a lexical/topical backend that composes well with BM25; it is not a general learned-embedding replacement.

**Fusion note:** per-component nDCG instrumentation found cases where vector-only ordering was higher than the fused result — fusion was destroying signal, not the embeddings. Linear-α z-scored fusion is the only strategy that consistently beats BM25 alone on top-10 ranking.

| Requested backend | Observed provider | Dim | p50 | p95 | Result |
|---|---|---:|---:|---:|---|
| `simeon` | `plugin:Simeon` / `Simeon` | 1024 | 11.075 ms | 11.094 ms | pass |
| `onnxruntime` | `plugin:ABIModelProvider` / `all-MiniLM-L6-v2` | 384 | 10.669 ms | 21.259 ms | pass |

The same benchmark now sweeps 100- and 1,000-document batches (`--benchmark_filter=EmbeddingBackendAB_Generate`,
3 iterations per row). Current daemon-path throughput:

| Requested backend | Docs/batch | Avg latency | p50 throughput | p95 throughput | Avg throughput |
|---|---:|---:|---:|---:|---:|
| `simeon` | 100 | 10.887 ms | 9,032 docs/s | 8,803 docs/s | 9,185 docs/s |
| `onnxruntime` | 100 | 230.831 ms | 463 docs/s | 381 docs/s | 433 docs/s |
| `simeon` | 1,000 | 61.368 ms | 30,867 docs/s | 8,363 docs/s | 16,295 docs/s |
| `onnxruntime` | 1,000 | 2,579.397 ms | 386 docs/s | 369 docs/s | 388 docs/s |

Links:
- Simeon repo: https://github.com/trvon/simeon
- Benchmarks and research notes: `third_party/simeon/docs/research/`