ADR-148 — Cost-Optimal Model Router Artifact Lifecycle (wiring @metaharness/router with optional @ruvector/tiny-dancer FastGRNN backend)

Status: Proposed Date: 2026-06-15 Related: ADR-026 (3-tier model routing), ADR-074 (observable-not-inferred), ADR-086 (graceful-degradation), ADR-124 (optional native dependencies), ADR-142 (per-task bandit priors), ADR-143 (Tier-1 deterministic codemods), #2329 (closed, Option A), #2334 (open, Option B) Supersedes: nothing — this is the artifact-lifecycle decision the ADR-026 router was missing

Context

v3/@claude-flow/cli/src/ruvector/model-router.ts shipped a lexical complexity heuristic + Thompson-sampling Beta-Bernoulli bandit even though its file header and ADR-026 both described a @ruvector/tiny-dancer FastGRNN neural router. #2329 closed the documentation–implementation gap via Option A in #2330 (docs + labels updated, no behavior change). #2334 kept the open question of whether to actually wire the neural path — Option B in the original triage.

When #2334 was raised, three blockers stalled the work:

1. Safetensors layout undocumented. @ruvector/tiny-dancer's RouterConfig.modelPath required a trained FastGRNN safetensors artifact but the tensor names/shapes/dtype the loader expected were not documented in the npm package; reloadModel() failed opaquely without that knowledge.
2. No training pipeline. There was no exported function to produce an artifact — distilling from .swarm/model-router-state.json was not viable (it stores aggregates only, no per-decision rows).
3. Candidate modeling. Router.route({queryEmbedding, candidates}) required a Candidate.embedding for every candidate. With three Claude tiers it was unclear what per-tier embeddings should mean.

On 2026-06-15 two complementary upstream packages landed:

@ruvector/[email protected] (FastGRNN native backend)

Four releases between 12:47 and 16:07 UTC. New exports:

@metaharness/[email protected] (DRACO router, productized)

Published 2026-06-15 16:46 UTC, ~40 min after tiny-dancer 0.1.22. Described as "the productized DRACO Phase-2 finding" — a cost-optimal model router using k-NN over labelled embeddings, with three backends sharing one dataset shape:

resolveRouterBackend('auto') selects native when @ruvector/tiny-dancer is installed, else the pure-TS path. All three backends consume the same {embedding: number[], scores: Record<modelId, quality>} row shape — so the trajectory-collection format is one decision that serves every backend.

This is the strictly-better integration target than direct tiny-dancer:

• Cost-optimal semantics built in. qualityBar selects the cheapest candidate predicted to clear the bar, not just a binary cheap/strong flag.
• No native dep in the default path. The k-NN/KRR backends are pure TS. ADR-124 graceful degradation becomes "pure-TS always works; native is the acceleration."
• One dataset shape, three backends. Trajectory rows seed any of the three with no reshaping.

This ADR records the lifecycle for that artifact — how it's trained, stored, distributed, loaded, refreshed, and retired — the lifecycle missing from ADR-026.

Decision

Wire @metaharness/router@^0.3.2 into @claude-flow/cli's model routing path, with @ruvector/tiny-dancer@^0.1.22 as an optional peer for native acceleration. Default behavior remains byte-identical to the shipped heuristic + bandit until a model is intentionally adopted. Six phases:

1. Dependencies (ADR-124)

• @claude-flow/cli/package.json optionalDependencies:
  • @metaharness/router: ^0.3.2 (pure TS, no native)
  • @ruvector/tiny-dancer: ^0.1.22 (optional native acceleration)
• Both resolved via dynamic import() inside neural-router.ts. If either is absent, the missing-piece path silently falls back to bandit-fallback. The pure-TS router has no native binary requirement at all — graceful degradation applies only to the FastGRNN acceleration.

2. Backend selection

// At module init, once per process:
const backend = await m.resolveRouterBackend('auto');
//   'native' when tiny-dancer is installed and loadable
//   'js'     when only the pure-TS path is available

The result is captured in routedBy so callers know which backend produced each decision.

3. Inference: qualityBar cost-optimality

const router = m.Router.fromExamples(rows, prices, { qualityBar: 0.8 });
const pick = router.route(queryEmbedding);
// pick: { id: 'haiku'|'sonnet'|'opus', predictedQuality, costPerMTok, metBar }

The picked id is the cheapest candidate predicted to clear qualityBar. If no candidate clears it, the best-predicted is returned with metBar=false. That signal — "I had to pick the strongest model but I'm not confident it'll clear" — is exactly the uncertainty escalation ADR-142 / #2250 introduced in the bandit; the new router exposes it natively.

qualityBar starts at 0.80 (a defensible default for production routing) and is configurable via CLAUDE_FLOW_ROUTER_QUALITY_BAR.

4. Observability: routedBy, not inferred (ADR-074, ADR-086)

Every routing result carries:

routedBy: 'metaharness-js' | 'metaharness-knn' | 'metaharness-krr' | 'fastgrnn' | 'bandit-fallback' | 'heuristic'

The two metaharness-* variants distinguish the k-NN (no model) and KRR (trained, JSON-serialised) cases; fastgrnn is the native-accelerated KRR/k-NN result via tiny-dancer. Callers must never infer the active path from "did the import resolve?" — both can resolve while the artifact load silently failed.

5. Training: DRACO-shaped trajectories, opt-in

RouterTrajectoryRecorder writes one JSONL row per decision to .swarm/model-router-trajectories.jsonl when CLAUDE_FLOW_ROUTER_TRAJECTORY=1. Default: off (rows carry full task text + raw embeddings).

Row schema (versioned "v": 1):

{
  "v": 1, "ts": "ISO-8601", "task_hash": "fnv1a-32", "task": "≤500ch",
  "embedding": [384 floats], "complexity": 0.0..1.0,
  "model": "haiku|sonnet|opus", "confidence": 0..1, "uncertainty": 0..1,
  "routed_by": "metaharness-js|fastgrnn|bandit-fallback|heuristic"
}

Outcome rows (same file, "type": "outcome") join on task_hash and reconstruct the DRACO scores map. This shape feeds all three backends without reshaping.

6. Artifact: tiered distribution

• Default (everywhere): a bundled pre-trained KRR JSON (assets/model-router/seed-router.krr.json, ~96 kB) trained from assets/model-router/seed-rows.json (64 deterministic rows). Loaded via TrainedRouter.fromJSON() — pure TS, no native deps, no I/O after the one-time read. The pure-TS k-NN over the raw seed corpus stays as a fallback when the KRR artifact is missing or fails to parse.
• Trained (optional): a TrainedRouter JSON written via toJSON() from a larger corpus, distributed via IPFS using the existing hooks transfer channel. CLAUDE_FLOW_ROUTER_MODEL_PATH can point at a local path or an ipfs:// URI.
• Native-accelerated (optional): a .safetensors written by trainNativeRouter. Loaded only when tiny-dancer is installed and the artifact path is set.

Hot-reload via Router.reloadModel() is supported but only via an explicit CLI command — never per-call.

7. Retirement

Each artifact records trainedAt. If now − trainedAt > 90d and trajectory-collected accuracy on the most recent 10k rows is >5 pp below the artifact's reported looQuality/valAccuracy, the loader emits a one-time warning and continues using the artifact. Automatic invalidation is the ADR-086 footgun.

Consequences

Positive

• Closes #2334. The cost-optimal router lifecycle is recorded and the integration target is one package, not "tiny-dancer + custom wrapper".
• Zero-native-dep default. Pure-TS k-NN is the floor; native FastGRNN is the ceiling. ADR-124 graceful degradation becomes the gradient, not a binary "works/doesn't".
• The default behavior in every installation is identical to today's shipped behavior until both env-gates are set. Hard to regress.
• ADR-026's advertised architecture and the implementation finally agree.
• Single dataset shape feeds all three backends — one training pipeline, three deploy paths.

Negative

• Two new optional dependencies instead of one. Both are tiny: @metaharness/ router is pure TS with no transitive deps, tiny-dancer adds the native binary. The marginal install-size cost is ~40 kB of JS + ~6 kB of model.
• Storing trajectory rows on disk introduces a PII/retention surface that the bandit (aggregates-only) did not have. Mitigation: CLAUDE_FLOW_ROUTER_TRAJECTORY is opt-in, rotation policy documented, rows are local and never uploaded by default.
• The benchmark below uses a synthetic corpus with strong signal-to-noise ratio. Real-world tier-accuracy is expected to be lower; the +43–45 pp delta is a ceiling demonstration, not a forecast.

Neutral

• The Thompson bandit stays. It is the cold-start path, the bandit-fallback path, and the online-learning signal source for the next artifact retraining. ADR-142's per-task bandit priors are unaffected.

Measured numbers (before/after)

Benchmark: scripts/benchmark-router.mjs (374 lines, reproducible from this repo + npm install @metaharness/[email protected] @ruvector/[email protected]). darwin-arm64, node v22.22.1, N=400 (280 train / 120 test), dim=32, hidden=12, epochs=40. Heuristic+bandit exercised cold (no learned state — same condition as a fresh installation).

Cross-seed accuracy + latency (consistent across both runs)

Training/build cost:

Backend resolution:

• isNativeRouterAvailable() = true (tiny-dancer 2.2.3 installed)
• resolveRouterBackend('auto') = 'native'

Agreement (binary cheap/strong, fraction of test set):

• baseline vs k-NN: 55.0%   baseline vs KRR: 55.0%   baseline vs FastGRNN: 55.0%
• k-NN vs FastGRNN: 100.0% (the two trained backends agree on every test query — empirical confirmation that the dataset shape and the cost-optimal semantics produce equivalent routing decisions across backends)

What the numbers mean

• The cold bandit sits at the majority-class baseline (~53–55%). It needs time on real outcomes to converge; this is its install-day floor, not its asymptote.
• k-NN matches FastGRNN's accuracy with zero training time and zero artifact bytes. That is the strongest argument for @metaharness/router as the primary integration target.
• KRR is the fastest inference (0.020 ms p95 0.023 ms) but trades 83 s of training time and 440 kB JSON for that 1.8× latency improvement vs FastGRNN. Not worth it on the current corpus size; revisit when N > 10k.
• FastGRNN is the best balance: 100% accuracy, 26 ms train, 6 kB artifact, 0.036 ms inference. The right pick when tiny-dancer is installed; the cost of installing it is one native binary download (~1.5 MB on darwin-arm64).

Bench JSON: docs/benchmarks/runs/router-4way-seed42-2026-06-15T*.txt and sibling .json files.

Alternatives considered

1. Direct tiny-dancer score() only (the original #2334 plan). Rejected in favor of @metaharness/router because (a) the latter wraps the same FastGRNN with cleaner cost-optimal semantics, (b) provides a zero-native-dep fallback at no accuracy cost on the bench, (c) ships KRR alongside k-NN at no extra integration cost.
2. Multi-candidate Router.route({queryEmbedding, candidates}). Rejected because the per-tier candidate embedding is not a well-defined quantity for three Claude models. Defer to a future ADR if per-task-family routing becomes a product requirement.
3. Keep heuristic + bandit only; do nothing. Rejected because (a) ADR-026 /file-header mismatch keeps recurring as an audit finding and (b) the cold bandit measures at ~54% binary accuracy on the bench — barely above majority class.
4. Distillation from .swarm/model-router-state.json. Rejected per rcraw's #2334 finding: aggregates-only.
5. Hot-reload artifacts on every routing call / auto-invalidate stale artifacts. Rejected as ADR-086 footguns.
6. KRR as the default trained backend. Rejected: 83 s training time and 440 kB artifact for negligible accuracy gain over k-NN on the bench. KRR stays as an optional path for larger corpora.

Open questions

1. Seed corpus source. Trajectory collection produces data over time, but the first bundled seed corpus needs an origin. Candidates: (a) hand-curated ~50 queries with rcraw's review, (b) replay historical bandit outcomes from representative installations, (c) bootstrap from a public agent benchmark. Resolves in the follow-up PR.
2. Acceptance bar for "default neural path on". Proposed: held-out tier accuracy ≥ heuristic+bandit baseline + 5 pp and cost-adjusted reward no worse, on ≥500 labelled trajectories. Confirm with rcraw.
3. Platform coverage. tiny-dancer 0.1.22 ships 8 platform/libc combos. The pure-TS path moots this for the default install — Decision needed only for the recommended configuration in docs.

Implementation plan

Sequenced for the smallest credible PR first:

PR 1 — Phase 1 (smallest, default behavior byte-identical)

• Add @metaharness/router ^0.3.2 and @ruvector/tiny-dancer ^0.1.22 to @claude-flow/cli/package.json optionalDependencies.
• Add v3/@claude-flow/cli/src/ruvector/neural-router.ts exporting one function: tryCostOptimalRoute(embedding) → Promise<{model, predictedQuality, metBar, routedBy} | null>. Returns null unless CLAUDE_FLOW_ROUTER_NEURAL=1 is set, a seed corpus or trained artifact resolves, and the backend selects.
• Bundle assets/model-router/seed-rows.json (~50 queries) for the k-NN cold path so the gate-on result is non-empty out of the box.
• Thread the embedding through ModelRouter.route(task, embedding?) for the inference call only.
• Add RouterTrajectoryRecorder (gated by CLAUDE_FLOW_ROUTER_TRAJECTORY=1) writing DRACO-shaped JSONL.
• Add routedBy to every result.
• Tests: graceful-degradation (both optional deps missing → bandit-fallback), backend resolution (auto selects native ↔ js per env), gate open vs closed → byte-identical decisions on the bench corpus.

PR 2 — seed corpus + bundled artifact

• Land a real seed corpus and (optionally) the first trained FastGRNN .safetensors bundled under assets/model-router/.
• Decide open question 1.

PR 3 — flip default

• Once the acceptance bar is met on a real corpus, set CLAUDE_FLOW_ROUTER_NEURAL=1 as the package default and document the opt-out.

References

• Issues: #2329 (closed, Option A), #2334 (open, Option B), #2250 (closed)
• ADRs: ADR-026 (3-tier routing), ADR-074, ADR-086, ADR-124, ADR-142, ADR-143
• Code: v3/@claude-flow/cli/src/ruvector/model-router.ts, v3/@claude-flow/cli/src/ruvector/enhanced-model-router.ts
• Upstream:
  • @metaharness/[email protected] (2026-06-15, exports Router, TrainedRouter, trainRouter, NativeRouter, trainNativeRouter, resolveRouterBackend, isNativeRouterAvailable)
  • @ruvector/[email protected] (2026-06-15, exports trainRouter, score, Router)
  • ruvector commits: 5173ce7, 39fb398, 3c1f701
• Bench: scripts/benchmark-router.mjs, docs/benchmarks/runs/router-4way-seed42-2026-06-15T*.txt