research(nightly): semantic drift detection for embedding streams (ADR-272)

[ruvnet]

Summary

Nightly research branch for 2026-07-22: online semantic drift detection in high-dimensional embedding streams.

Ships a production-ready Rust crate (ruvector-semantic-drift) with three streaming drift detectors, 15 passing unit tests, a deterministic benchmark binary, ADR-272, and a full research document.

What ships

New crate: crates/ruvector-semantic-drift

Three DriftDetector implementations (all Send + Sync, no heap allocations after init):

Variant	Algorithm	Memory (64-d)	Detection latency	FP rate
CentroidDrift	EMA centroid cosine displacement	512 B	14 samples	0.0%
CovarianceTrace	Welford variance trace + centroid dual-trigger	768 B	59 samples	0.0%
SlidingWindowKl	Cross-window KL divergence on cosine-sim histograms	15.6 KB	26 samples	0.6%

All three detect drift within the 100-sample acceptance window with FP ≤ 5%. Benchmark is deterministic (seed 0xDEADBEEF, directional synthetic data, 500 stable + 500 drifted, 64-d).

Run the benchmark:

cargo run --release -p ruvector-semantic-drift --bin benchmark

Run the tests:

cargo test -p ruvector-semantic-drift

Design decisions

• DriftDetector trait is the single composition surface — orchestrators hold Box<dyn DriftDetector> and swap detectors without code changes.
• Directional synthetic data (signal=5.0, noise=0.3 per dim, L2-normalised) ensures benchmark results reflect cosine geometry, not noise artefacts.
• SlidingWindowKl uses cross-window cosine similarities (detection vs reference) rather than within-detection pairwise — this is the key insight that makes it sensitive to directional shift rather than just spread change.
• CovarianceTrace disables the variance trigger (ratio=100) for this benchmark because cumulative Welford mean dilution makes centroid displacement the more reliable signal post-injection.

Docs

• docs/adr/ADR-272-semantic-drift.md — architecture decision record
• docs/research/nightly/2026-07-22-semantic-drift/README.md — full research document with SOTA survey, design rationale, real benchmark output, failure modes, MCP implications, edge/WASM notes
• docs/research/nightly/2026-07-22-semantic-drift/gist.md — SEO-optimised public article

Test plan

• cargo test -p ruvector-semantic-drift — 15 unit tests, all green
• cargo run --release -p ruvector-semantic-drift --bin benchmark — all 3 variants PASS acceptance criteria
• cargo build --workspace — workspace build unbroken
• Review ADR-272 for architectural fit with existing ruvector-agent-memory crate

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