0081: Chroma-backed naive baseline

Context

ADR 0001 keeps naive_baseline as the side-by-side control for the agentic pipeline. That contract previously covered the retrieval algorithm but left the vector DB backend implicit through BIDMATE_INDEX_BACKEND, whose default was the local numpy-backed memory store.

Vector DB backend effects are a separate axis from embedding model effects. Issue #1580 tracks Chroma as the desired baseline vector-store candidate, while MiniLM/BGE-M3 and other semantic embedding comparisons remain separate work.

Decision

naive_baseline is now a Chroma-backed baseline: its pipeline config declares vector_store_backend: chroma, and the zero-env BIDMATE_INDEX_BACKEND default is chroma.

The retrieval algorithm remains dense-only, fixed top-k, no metadata-first, no rerank, no verifier retry. memory remains an explicit legacy/control backend, and qdrant remains an ops comparison backend. Private aggregate baseline refresh is a separate follow-up after the Chroma-backed contract lands.

Consequences

  • Chroma becomes a base dependency because the default local path must run without optional install steps.
  • Eval summaries and deltas must carry vector_store_backend separately from embedding backend/model provenance.
  • A single eval config cannot mix vector-store backends; backend comparisons must run as separate commands with comparable config/index provenance.
  • Chroma must match the memory backend’s top-k ranking on parity tests. Ranking drift is not accepted as a completed canonical switch.

Alternatives considered

  • Keep Chroma opt-in only. Rejected because the chosen baseline contract is canonical, not just a backend candidate.
  • Regenerate committed private baselines in the same PR. Rejected to keep the implementation contract separate from aggregate metric refresh and privacy review.
  • Fold vector-store backend into retrieval backend. Rejected because retrieval_backend=dense|hybrid|m3|random describes ranking strategy, not vector DB storage/query infrastructure.

Verification