# ADR-0024: Boundary vs Corpus Embedding Model roles

## Status

Implemented. Supersedes ADR-0019’s single global Active Embedding Model for new work.

## Context

Embedding happens in three moments: (1) Semantic Boundary Detection for `semantic` / `semantic_parent_child`, (2) finished-chunk vectors into Qdrant, (3) query vectors for search. ADR-0019 forced one Active Embedding Model for all three so Strategy comparisons stayed fair. Operators now want to vary the model used for cuts independently from the model used for retrieval — but storage and query must remain the same vector space or RAG breaks.

## Decision

- Split into two roles from the Embedding Model Registry:
  - **Boundary Embedding Model** — Cosine cuts only; ignored by Strategies without boundary detection. Uses that model’s `semantic_threshold`.
  - **Corpus Embedding Model** — Finished-chunk storage **and** query embedding (always paired). Selects the Model Corpus (`{strategy}__{corpus_model_id}_collection`).
- **Not** three independent knobs: Query may not diverge from Corpus.
- Admin stores **Default Boundary** and **Default Corpus** (migrate legacy `active_embedding_model_id` → both defaults).
- Per-operation overrides (Decision B): Process may set Boundary + Corpus; Query/Experiment set Corpus only (Boundary irrelevant at query time). Snapshot for the operation.
- Process UI: show Boundary picker only when a semantic Strategy is selected.
- Provenance: always record Corpus on Query/Experiment; record Boundary when semantic Strategies are in play. Compare warns when Corpus, Boundary (if present), or Neighbors differ.
- Query/Benchmark Corpus choice should prefer models that have a usable Model Corpus for the target doc/strategy and warn when empty.

## Consequences

- ADR-0019’s single Active switcher is replaced; call sites that `snapshot_active_model()` must become Corpus (and Boundary where needed).
- Cross-model science is allowed for boundary vs corpus; silent unfair Strategy×model mixes remain out of scope as an Experiment axis.
- Implementation is live: Admin defaults, Process Boundary+Corpus overrides, Query/Experiment Corpus override, dual provenance, Compare warnings.