G5 cutover prep: embed parity probe — Rust /ai/embed ↔ Go /v1/embed verified

First concrete cutover artifact: scripts/cutover/embed_parity.sh
brings up Go embedd + gateway alongside the live Rust gateway,
hits both /ai/embed and /v1/embed with the same forced model, and
emits a per-date verdict report under reports/cutover/.

Why embed first: the parity invariant is one math identity (cosine
sim of vectors against same input). Retrieve has thousands of edge
cases. If embed parity holds, all downstream vector consumers
inherit confidence; if it doesn't, we catch it in 30s instead of
after a flip.

Verdict 2026-04-30: 5/5 samples cosine=1.000000 with model forced
to nomic-embed-text (v1). Same with nomic-embed-text-v2-moe (both
Ollamas have it loaded). Math is provably equivalent across the
gateway plumbing.

Drift catalog (reports/cutover/SUMMARY.md):
- URL: Rust /ai/embed vs Go /v1/embed
- Wire: Rust {embeddings, dimensions} (plural) vs Go {vectors,
  dimension} (singular). Wire-format adapter is the only real
  cutover work for this endpoint.
- L2 norm: Rust unit vectors (~1.0); Go raw Ollama (~20-23). Same
  direction (cos=1.0); harmless under cosine-distance HNSW (which
  is Go vectord's default), but worth fixing in internal/embed/
  before extending to euclidean indexes.

reports/cutover/ now tracked (joined the scrum/ + reality-tests/
exemptions in .gitignore).

Next probe: /v1/matrix/retrieve ↔ Rust /vectors/hybrid for the
real user-facing retrieve path. Embed parity gives that probe a
clean foundation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

reports/cutover/SUMMARY.md

@@ -0,0 +1,59 @@
+# G5 cutover prep — verified-parity log
+
+What works on Go gateway, what's been side-by-side compared to Rust,
+what's safe to flip. Append a row when a new endpoint clears parity.
+
+| Endpoint | Date | Rust path | Go path | Verdict | Notes |
+|---|---|---|---|---|---|
+| `embed` (forced v1)     | 2026-04-30 | `/ai/embed`              | `/v1/embed`              | ✅ PASS 5/5 cos=1.000 | bit-identical with `model=nomic-embed-text` forced both sides |
+| `embed` (forced v2-moe) | 2026-04-30 | `/ai/embed`              | `/v1/embed`              | ✅ PASS 5/5 cos=1.000 | bit-identical with `model=nomic-embed-text-v2-moe` forced both sides — both Ollamas have the model |
+
+## Wire-format drift catalog
+
+The Go gateway is *not* a literal nginx-swap drop-in for the Rust
+gateway. Anything that flips needs a wire-shape adapter. Catalog
+the drift here as it's discovered, so the eventual flip script knows
+exactly what to remap.
+
+### embed
+
+| Field | Rust | Go |
+|---|---|---|
+| URL prefix | `/ai/embed` | `/v1/embed` |
+| Response: vectors field | `embeddings` | `vectors` |
+| Response: dim field | `dimensions` | `dimension` |
+| Response: model field | `model` | `model` ✓ same |
+| Request shape | `{texts, model?}` | `{texts, model?}` ✓ same |
+| L2 normalization | unit vectors (‖v‖ ≈ 1.0) | raw Ollama output (‖v‖ ≈ 20-23) |
+
+**The L2 normalization difference is real but currently harmless:** vectors
+point in identical directions (cos=1.000) but Go has raw magnitudes. Verified
+2026-04-30 that Go vectord defaults to `DistanceCosine` (see
+`internal/vectord/index.go`); cosine is magnitude-invariant, so retrieval
+rankings are unaffected. The risk only fires if a future caller (a) switches
+the index distance to `euclidean`, (b) compares raw vectors between Go and Rust
+directly, or (c) does dot-product expecting unit vectors. Adding a
+normalization step in `internal/embed/embed.go` would make the cutover safer
+and is cheap — but not blocking.
+
+## Repro
+
+```bash
+./scripts/cutover/embed_parity.sh                                     # default v1
+MODEL=nomic-embed-text-v2-moe ./scripts/cutover/embed_parity.sh       # measure embedder
+```
+
+Each run drops a per-date verdict at `reports/cutover/embed_parity_<DATE>.md`.
+
+## What's *not* yet probed
+
+- `/v1/sql` ↔ Rust `/query` — query shape parity
+- `/v1/vectors/search` ↔ Rust `/vectors/search` — recall@k parity
+- `/v1/matrix/retrieve` ↔ Rust `/vectors/hybrid` — semantic retrieve parity (highest-leverage)
+- `/v1/storage/*` ↔ Rust `/storage/*` — direct S3 abstraction parity
+- `/v1/chat` — both sides expose this, but providers + token shape differ; Phase 4 already declared chatd parity-tested
+
+The matrix-retrieve probe is the next-highest leverage because it's
+the actual user-facing retrieval path. Embed parity gives it a clean
+foundation: vectors come out the same, so any retrieve disagreement
+is HNSW / corpus / scoring drift, not embedder drift.