[March-7]

Scope

• Small change that can be reviewed in one short sitting
• Substantial change approved by a maintainer (link below)

Related issues and approval

No existing issue found for this behavior. This is a small, self-contained fix under the direct-PR path in CONTRIBUTING.md.

Problem and resulting behavior

With query decomposition and rag.mmr.diversity: 0, MMR re-sorts round-robin retrieval results by their original per-query scores. Those scores are not comparable: a later sub-query hit can replace the first result even though zero diversity promises plain top-k.

Return the incoming leading hits unchanged at zero diversity. Positive-diversity scoring and selection remain unchanged.

Approach and review guidance

Add the zero-diversity fast path and regressions for non-monotonic scores, the DuckDB retrieval endpoint with decomposition, full-pool behavior, and positive-diversity relevance/novelty controls.

AI assistance was used to investigate, implement, and validate this change. The endpoint tests use synthetic vectors and deterministic model responses; database queries and retrieval/ranking code run normally. No real model-quality improvement is claimed.

Verification

• Identical new regression selection: base 4 failed / 4 passed; patched 8 passed (29 deselected in both).
• python -m pytest tests/test_rag_quality.py tests/test_documents.py tests/test_hybrid.py -q: 100 passed on the patch.
• Independent local HTTP/SDK + DuckDB replay: both affected decomposition cases recover the expected order; two ordinary-query controls also pass.
• python -m ruff check serviette tests: passed with Ruff 0.16.10, within the project's dev pin.
• git diff --check: passed.

A broader baseline test selection had 112 passes and 3 import failures because LiteLLM is not installed. Full package, engine-backed indexing, external-model, Docker/backend integration, and multi-version tests were not run.

Checklist

• I have read CONTRIBUTING.md.
• For substantial work, I linked maintainer approval and explained the approach.
• I followed the project's code style and reported relevant verification results.
• I updated tests and documentation where needed, or explained why no updates are needed.
• I described any breaking changes and migration steps, if applicable.

No configuration or API change; the existing zero-diversity contract is restored. No documentation update is needed for the behavior already documented in MmrConfig.

[CLAassistant]

All committers have signed the CLA.