diff --git a/src/retrievers/csv_chroma.py b/src/retrievers/csv_chroma.py
index e114d34..f833885 100644
--- a/src/retrievers/csv_chroma.py
+++ b/src/retrievers/csv_chroma.py
@@ -1,11 +1,11 @@
 import asyncio
+import csv
 from collections.abc import Coroutine
 from pathlib import Path
 from typing import Any, TypedDict
 
 import chromadb.config
 from langchain_chroma.vectorstores import Chroma
-from langchain_community.document_loaders.csv_loader import CSVLoader
 from langchain_community.retrievers import BM25Retriever
 from langchain_core.callbacks import (
     AsyncCallbackManagerForRetrieverRun,
@@ -21,6 +21,8 @@
 from nltk.tokenize import word_tokenize
 from pydantic import ConfigDict
 
+from data_generation.metadata_csv_loader import MetaDataCSVLoader
+
 
 def chroma_settings() -> chromadb.config.Settings:
     """A *fresh* Settings object for every Chroma store.
@@ -220,6 +222,13 @@ def list_chroma_subdirectories(directory: Path) -> list[str]:
     ]
 
 
+def _csv_column_names(csv_path: Path) -> list[str]:
+    """Every column in the file, so BM25 metadata matches what was embedded."""
+    with csv_path.open(newline="", encoding="utf-8") as handle:
+        header = csv.reader(handle)
+        return next(header, [])
+
+
 def create_bm25_chroma_ensemble_retriever(
     llm: BaseChatModel,
     embedding: Embeddings,
@@ -282,7 +291,28 @@ def from_subdirectory(
             # set up BM25 retriever
             csv_file_name = subdirectory + ".csv"
             reactome_csvs_dir: Path = embeddings_directory / "csv_files"
-            loader = CSVLoader(file_path=reactome_csvs_dir / csv_file_name)
+            csv_path = reactome_csvs_dir / csv_file_name
+            # MetaDataCSVLoader, not CSVLoader: the plain loader gives every
+            # document metadata of only {row, source}, so the BM25 half of this
+            # ensemble returned documents with no `st_id` while the Chroma half
+            # -- built from the same CSV, by this same loader at embedding time
+            # -- had one.
+            #
+            # Two things followed. Those documents could not be cited, so for one
+            # measured question 50 of 62 retrieved documents were unattributable
+            # while their stable id sat in the text as "st_id: R-HSA-...". And
+            # `unique_documents` keys on `st_id or page_content`, so a BM25 copy
+            # and its Chroma twin deduplicated to different keys and both
+            # survived -- 11 of those 50 were exact duplicates of a citable
+            # document, taking context space and contributing twice to the RRF
+            # fusion.
+            #
+            # Every column is promoted, from the header, so this cannot drift
+            # from whatever the CSV holds. Content is unchanged: the loader only
+            # trims content when `content_columns` is given, which it is not.
+            loader = MetaDataCSVLoader(
+                file_path=str(csv_path), metadata_columns=_csv_column_names(csv_path)
+            )
             data = loader.load()
             bm25_retriever = BM25Retriever.from_documents(
                 data,
diff --git a/tests/retrievers/test_bm25_metadata.py b/tests/retrievers/test_bm25_metadata.py
new file mode 100644
index 0000000..b0cc030
--- /dev/null
+++ b/tests/retrievers/test_bm25_metadata.py
@@ -0,0 +1,70 @@
+"""BM25 documents must carry the same metadata the embedded ones do.
+
+The ensemble retriever loads each CSV twice over: Chroma holds documents embedded
+with `MetaDataCSVLoader`, and BM25 is built by loading the same CSV at startup.
+When the second used a plain `CSVLoader`, its documents had metadata of only
+{row, source} -- so a document retrieved by keyword could not be cited, even
+though its stable id was sitting in the page content where nothing could reach
+it. Measured on one question: 50 of 62 documents the citation path walked were
+unattributable.
+"""
+
+from pathlib import Path
+
+import pytest
+
+from data_generation.metadata_csv_loader import MetaDataCSVLoader
+from retrievers.csv_chroma import _csv_column_names
+
+HEADER = "st_id,display_name,pathway_name,url"
+ROW = "R-HSA-8863013,CDK5 binds p25,Neuronal System,https://reactome.org/x"
+
+
+@pytest.fixture
+def csv_file(tmp_path: Path) -> Path:
+    path = tmp_path / "reactions.csv"
+    path.write_text(f"{HEADER}\n{ROW}\n")
+    return path
+
+
+def test_every_column_is_read_from_the_header(csv_file: Path) -> None:
+    assert _csv_column_names(csv_file) == [
+        "st_id",
+        "display_name",
+        "pathway_name",
+        "url",
+    ]
+
+
+def test_a_missing_or_empty_file_asks_for_no_metadata(tmp_path: Path) -> None:
+    """An empty list is falsy, so the loader behaves as it did before."""
+    empty = tmp_path / "empty.csv"
+    empty.write_text("")
+    assert _csv_column_names(empty) == []
+
+
+def test_loaded_documents_carry_the_stable_id(csv_file: Path) -> None:
+    """The property the citation path depends on."""
+    documents = MetaDataCSVLoader(
+        file_path=str(csv_file), metadata_columns=_csv_column_names(csv_file)
+    ).load()
+
+    assert len(documents) == 1
+    assert documents[0].metadata["st_id"] == "R-HSA-8863013"
+    assert documents[0].metadata["display_name"] == "CDK5 binds p25"
+
+
+def test_the_content_is_not_trimmed_by_promoting_columns(csv_file: Path) -> None:
+    """BM25 scores page_content, so changing it would change retrieval.
+
+    `MetaDataCSVLoader` only narrows content when `content_columns` is given, and
+    the retriever does not give it. This pins that: every column stays in the
+    text, which is why retrieval measured identical before and after the change.
+    """
+    documents = MetaDataCSVLoader(
+        file_path=str(csv_file), metadata_columns=_csv_column_names(csv_file)
+    ).load()
+
+    content = documents[0].page_content
+    for column in ("st_id", "display_name", "pathway_name", "url"):
+        assert f"{column}:" in content, f"{column} vanished from the indexed text"