ADR-152 — Genome Similarity Search

Status: Accepted (spike landed iter 35 — both invariants pass) Date: 2026-06-16 (revised same-day with spike result) Parent: ADR-151 (Phase 3 scope shell — Harness Intelligence Layer) Inherits: ADR-150's four architectural constraints (removable / optional / graceful / CI-gate) Spike: plugins/ruflo-metaharness/scripts/_spike-similarity.mjs (iter-35 commit)

Spike result (iter 35 — measured)

similarity(LEGAL, LEGAL).overall = 1.0000    ✓ Invariant 1 (self-match) — exact
similarity(LEGAL, SUPPORT).overall = 0.8296
similarity(LEGAL, DEVOPS).overall  = 0.5840  ✓ Invariant 2 (vertical affinity) — support > devops

Per-component (LEGAL vs SUPPORT):  cosine=0.9987  categorical=0.75   jaccard=0.2857
Per-component (LEGAL vs DEVOPS):   cosine=0.9734  categorical=0      jaccard=0

Notable findings from the spike:

• Numerical cosine alone is too coarse — LEGAL vs DEVOPS scored cosine=0.9734 despite being unrelated verticals. The numerics are clustered around similar scorecard ranges. Categorical + jaccard pulled the composite correctly to 0.58 vs 0.83.
• The 0.6/0.25/0.15 weighting from the §Decision section reproduces the intended ordering on the synthetic LEGAL/SUPPORT/DEVOPS fixtures.
• categorical: 0 for LEGAL/DEVOPS correctly fires because they share no enum field (different archetypes, different templates, different recommendedMode). This is a strong feature of the design — categorical disagreement is a clean kill-switch.

Context

ADR-151 Phase-3 §3.1 calls for a similarity function over MetaHarness genome + scorecard JSON. It is the critical-path dependency for §3.2 (Recommendation Engine), §3.3 (Fleet Drift), and §3.5 (Plugin Compatibility) — three of the four other Phase-3 sub-capabilities consume similarity scores.

What we have already (post-ADR-150 implementation):

• harness genome <repo> emits a 7-section JSON blob: repo_type, agent_topology[], risk_score, mcp_surface, test_confidence, publish_readiness.
• harness score <repo> emits a 5-dimension JSON blob: harnessFit, compileConfidence, taskCoverage, toolSafety, memoryUsefulness, plus archetype and template.
• harness threat-model <repo> emits a worst severity + a list of categorized findings.
• The iter-7 oia-audit records bundle all three above per timestamp in metaharness-audit memory namespace.

What we need: a function similarity(genomeA, genomeB) → number ∈ [0,1] plus a per-dimension breakdown explaining where the two harnesses agree or differ.

Decision

Implement genome similarity as a pure-TS function in the existing ruflo-metaharness plugin with three components:

1. Cosine similarity over a numerical feature vector

Project the structured genome + scorecard into a fixed-length numerical vector:

Cosine similarity over these 9 dims gives a [0, 1] score where 1 = identical and 0 = orthogonal. Cheap, deterministic, byte-identical for identical inputs.

2. Categorical agreement over enum fields

Three fields are categorical:

Each contributes 1 if matching, 0 if not. Sum / 4 → categorical-agreement score.

3. Set agreement over agent_topology[]

The agent_topology field is an array (e.g. ["maintainer", "tester", "security", "release"]). Jaccard similarity = |A ∩ B| / |A ∪ B|.

Composite

overallSimilarity = 0.6 · cosine + 0.25 · categorical + 0.15 · jaccard

Weights chosen so that:

• Numerical similarity dominates (most signal density).
• Categorical agreement is the next tier (high-signal but low-cardinality).
• Topology agreement is the tie-breaker (interpretable but coarse).

Return shape:

interface SimilarityResult {
  overall: number;                                // [0,1]
  components: {
    cosine: number;                               // [0,1]
    categorical: number;                          // [0,1]
    jaccard: number;                              // [0,1]
  };
  perDimension: Record<string, {
    a: number | string | string[];
    b: number | string | string[];
    contribution: number;                         // signed [-w, +w]
  }>;
  generatedAt: string;
}

The perDimension breakdown lets consumers explain why two harnesses scored as they did — critical for the §3.2 Recommendation Engine's confidence calculation and the §3.3 Drift Detection's alert reason.

Implementation surface

• One pure-TS function in plugins/ruflo-metaharness/scripts/_similarity.mjs (shared module convention — see iter-1 _harness.mjs and iter-73 _sessions.mjs).
• One CLI skill harness-similarity invoked as npx ruflo metaharness similarity --a <genomeA.json> --b <genomeB.json> (or --a-key/--b-key for memory-namespace lookup, mirroring iter-15 audit-trend).
• One MCP tool mcp__claude-flow__metaharness_similarity so agents can call it during conversation.

NO new dependency on @metaharness/* — the function operates on JSON shapes that the existing CLI already emits. Genuinely zero blast radius on ADR-150's four constraints:

• Removable ✓ (pure-TS, reads JSON the CLI already produces)
• Optional ✓ (no new dep)
• Graceful ✓ (gracefully reports degraded if inputs are malformed)
• CI-gate ✓ (standalone unit-testable; no npx required)

Smallest demonstrable spike (the implementation gate)

Before ADR-152 is marked Accepted, ship a 30-LOC proof that:

1. Reads two harness genome + harness score JSON files from disk.
2. Computes cosine over the 9 numeric dims.
3. Returns a single number.
4. Verifies: similarity(X, X) === 1 (exact self-match).
5. Verifies: similarity(legal_harness, devops_harness) < similarity(legal_harness, support_harness) using the 13 known metaharness verticals as fixtures.

The spike script: plugins/ruflo-metaharness/scripts/_spike-similarity.mjs. Lives in the plugin to avoid polluting global scripts; deleted after ADR-152 graduates.

Consequences

Positive

• Unblocks §3.2 / §3.3 / §3.5 — three of four Phase-3 consumers can begin work in parallel once similarity ships.
• No new dependencies, no runtime cost, pure-function semantics. Failure modes are limited to "garbage in → low confidence out" — never a crash.
• The 9-dim feature vector + 3-component weighted composite is a single source of truth for similarity semantics; downstream ADRs reference back to this one rather than reinventing.

Negative

• Choice of weights (0.6/0.25/0.15) is opinionated. A different weighting could change the recommender's rankings. Mitigation: expose weights as optional CLI flags; defaults are documented; ship the fixtures from the spike as a regression-suite so weight changes are visible.
• Numerical vector excludes threat-model worst because it's categorical and would inflate the categorical component too far. If worst regression is what matters for drift detection (§3.3), drift consumers must check worst separately.
• No semantic similarity over agent prompts / SKILL.md text. ADR-151's §3.4 (capability graph) calls for that; this ADR explicitly defers it to keep §3.1 small.

Neutral

• The function is invocable from any layer (CLI / MCP tool / direct Node import / future browser-side) without changes — the JSON contract is the only coupling.

Alternatives Considered

Alternative A: Use sentence embeddings over the README + agent prompt text.

• Pros: captures semantic similarity humans care about.
• Cons: requires embeddings model + 384-dim vectors + similarity index. Adds a heavy dep, breaks ADR-150's removability rule for harnesses that disable optional deps.
• Verdict: Defer to §3.4 (capability graph) where it's actually needed.

Alternative B: Skip cosine, use only categorical + jaccard.

• Pros: simpler, no float math.
• Cons: loses signal from the 5 scorecard + 3 genome numeric dimensions. harnessFit: 82 vs harnessFit: 45 should clearly contribute to similarity, and categorical can't see that.
• Verdict: cosine over the 9 numerics is the right primitive.

Alternative C: Train a similarity model on real harness pairs.

• Pros: would learn actual user-perceived similarity.
• Cons: requires labeled pairs that don't exist; circular (the recommender's job is to PRODUCE those pairs). Premature ML.
• Verdict: defer until §3.2 has shipped enough data to bootstrap.

Open Questions

• Should weights be a configurable field in mcp-policy.json? Lean no — the 0.6/0.25/0.15 split should be a single global default. Per-org tuning is out of scope for §3.1.
• Should the function ALSO emit a Mermaid diagram visualizing the breakdown? Probably yes via an optional --format mermaid. Cheap to add post-spike.
• How does similarity interact with the iter-15 audit-trend script? audit-trend already diffs two timestamps of the SAME harness; this ADR adds a separate primitive for diffing two DIFFERENT harnesses. Both should live as siblings, not subtypes.

References

• ADR-151 — Phase 3 scope shell (parent)
• ADR-150 — Phase 1+2 implementation, architectural constraints
• iter-15 audit-trend — plugins/ruflo-metaharness/scripts/audit-trend.mjs (sibling drift-detection primitive)
• iter-1 _harness.mjs — shared-module convention reference
• Upstream harness genome / score / threat-model outputs documented in ruvnet/agent-harness-generator source