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+# Implementation Plan: Retriever Rewrite
+
+**Branch**: `plan/retriever-rewrite` | **Date**: 2026-09-08 | **Spec**: [spec.md](./spec.md)
+
+**Input**: Feature specification from `/specs/001-retriever-rewrite/spec.md`
+
+## Summary
+
+Replace `HybridRetriever`, which subclasses `MultiQueryRetriever` and reaches into
+LangChain internals in five places, with a plain `BaseRetriever` that owns its own
+fusion. Swap `SelfQueryRetriever` for plain semantic search (D1), keep BM25, and
+make the context budget an argument rather than a module constant (D2, D3).
+
+The work is deliberately staged so that a behaviour change and a structural change
+are never in the same commit. Stage 1 is a pure refactor that must produce
+byte-identical retrieval; only Stage 2 changes what the model sees.
+
+## Technical Context
+
+**Language/Version**: Python 3.12
+
+**Primary Dependencies**: `langchain-core` (BaseRetriever, Document), `langchain-chroma`,
+`langchain-community` (BM25Retriever), `rank-bm25`, `nltk`. The point of the rewrite
+is to depend on `langchain-core` contracts only.
+
+**Storage**: Chroma collections on disk, from an installed embeddings bundle;
+sibling `csv_files/*.csv` for BM25.
+
+**Testing**: pytest. `tests/retrievers/test_hybrid_retriever.py` already pins the
+fusion behaviour, including the tie-break and the `[1/n]*n` weighting that cannot
+affect ordering. `bin/retrieval_baseline` for before/after measurement.
+
+**Target Platform**: Linux server (container), and developer machines.
+
+**Project Type**: Library within a single application repository.
+
+**Performance Goals**: Retrieval makes exactly one LLM call per message, down
+from 21 (FR-009). No wall-clock target; the LLM answer dominates.
+
+**Constraints**: Sync and async must agree (FR-007). Behaviour must be measurable
+with the committed question set before and after (FR-008).
+
+**Scale/Scope**: 4 Reactome collections, ~121k documents total; `userguide` has its
+own retriever and is out of scope.
+
+## Constitution Check
+
+*GATE: must pass before implementation, re-checked after.*
+
+| Article | How this plan satisfies it |
+|---|---|
+| I — verify the user path | Every stage ends with the full RAG chain answering a real question through **both** `invoke` and `ainvoke`, not just the retriever in isolation. This is the failure mode that recurred most. |
+| II — measure, don't argue | `bin/retrieval_baseline capture` before Stage 1 and after each stage; Stage 1 must show **zero** difference. |
+| III — characterization tests pin behaviour | The existing retriever tests must pass unchanged through Stage 1. Any test that must change in Stage 2 changes together with the code and says why. |
+| IV — fail loudly | Missing bundle raises with an actionable message rather than passing `None` into a `Path` (FR-006), removing the `B008` suppression and four mypy baseline entries. |
+| V — derive from the source of truth | Budget and over-fetch stay single named parameters; the harness keeps importing them from the module rather than re-declaring. |
+| VI — bias to doing over filing | Stages are small enough to land individually; nothing here is deferred to an issue that could be done now. |
+
+**No violations.** The one judgement call is Stage 3 (budget as an argument), which
+could be deferred — it is kept because the interface should be settled while the
+file is open rather than bolted on later, per spec User Story 2.
+
+## Project Structure
+
+### Documentation (this feature)
+
+```text
+specs/001-retriever-rewrite/
+├── spec.md      # what and why, with D1–D4 settled
+└── plan.md      # this file
+```
+
+No `research.md`, `data-model.md` or `contracts/` are generated. There are no
+unresolved unknowns — D1–D4 are decided and the evidence is in the spec — and the
+feature introduces no new data model or external contract. Generating empty
+scaffolding would be ceremony.
+
+### Source Code
+
+```text
+src/retrievers/
+├── csv_chroma.py           # HybridRetriever -> plain BaseRetriever; owns RRF
+├── reactome/rag.py         # bundle becomes an explicit argument
+├── uniprot/rag.py          # same
+├── plantreactome/rag.py    # same
+└── userguide/             # already a plain BaseRetriever; the model to follow
+
+tests/retrievers/
+└── test_hybrid_retriever.py  # extended, not rewritten
+
+bin/retrieval_baseline        # already mirrors the pipeline
+```
+
+## Implementation Stages
+
+### Stage 1 — Structural only, zero behaviour change
+
+Replace the `MultiQueryRetriever` subclass with a `BaseRetriever` that implements
+`_get_relevant_documents` / `_aget_relevant_documents`, and vendor Reciprocal Rank
+Fusion into this repository (FR-001, FR-002).
+
+Query expansion stays (D4) and `SelfQueryRetriever` stays for now — this stage
+changes *how the code is arranged*, not what it returns.
+
+RRF is copied from `EnsembleRetriever.weighted_reciprocal_rank` as it behaves
+today: `weight / (rank + 60)` with rank counted from 1, de-duplicated on
+`page_content`, ties resolved by first appearance. Those properties are already
+pinned by tests.
+
+**Exit criteria**: existing tests pass unchanged; `retrieval_baseline compare`
+shows **zero** differences; the full chain answers a real question through both
+`invoke` and `ainvoke`.
+
+### Stage 2 — D1: plain semantic search replaces SelfQuery
+
+Swap `SelfQueryRetriever` for `vectordb.as_retriever(...)` (FR-009). Retrieval
+drops to one LLM call per message.
+
+`metadata_info.py` stays: `evaluator.py` and `bin/retrieval_baseline` still
+construct SelfQuery for comparison, so deleting it would break the tool that
+measures this change.
+
+**Exit criteria**: `retrieval_baseline compare` shows differences confined to the
+vector side; a test asserts retrieval makes exactly one LLM call; the chain
+answers real questions in both paths.
+
+### Stage 3 — D2/D3: the caller supplies the budget
+
+`MAX_DOCUMENTS_PER_COLLECTION` becomes a constructor argument with today's value
+as the default (FR-005). The bundle becomes an explicit argument, removing the
+`B008` suppression and the four `retrievers.*.rag` mypy baseline entries (FR-006,
+SC-006).
+
+**Exit criteria**: two callers with different budgets in one process get different
+amounts of context; the four baseline entries are deleted; gates green.
+
+## Complexity Tracking
+
+| Decision | Simpler alternative rejected | Why |
+|---|---|---|
+| Vendor RRF (~15 lines) | Keep calling `EnsembleRetriever.weighted_reciprocal_rank` | The method is public, but reaching it requires `EnsembleRetriever(retrievers=[])` — an empty retriever built solely to borrow a method. Ranking is the product's core quality; a library upgrade should not be able to reorder results silently. |
+| Three stages | One commit | A behaviour change and a structural change in the same diff cannot be attributed when results move. Same reason the rewrite precedes the LangChain upgrade. |
+| Keep `metadata_info.py` | Delete 339 dead lines | It is not dead: the evaluator and the baseline harness still construct SelfQuery. Deleting it would break the tool that measures Stage 2. |
+| Keep multi-query expansion | Remove it too | D4. Removing two things at once makes any quality change unattributable. Measured separately afterwards. |
+
+## Out of Scope
+
+- The LangChain upgrade itself. This unblocks it; it does not perform it.
+- Answer quality evaluation (ragas). This plan changes what reaches the model;
+  whether answers improve is measured separately.
+- `userguide` retriever — already a plain `BaseRetriever`.
+- Token-based budgeting (D3 caveat) and removing expansion (D4 follow-up).