diff --git a/app/services/bm25_index.py b/app/services/bm25_index.py
new file mode 100644
index 0000000..c3f03c6
--- /dev/null
+++ b/app/services/bm25_index.py
@@ -0,0 +1,96 @@
+"""
+BM25 keyword search over the page_chunks corpus.
+
+MIT — no tier gate. Available to all users with no Ollama required.
+"""
+
+from __future__ import annotations
+
+import logging
+import sqlite3
+from dataclasses import dataclass
+
+from rank_bm25 import BM25Okapi
+
+logger = logging.getLogger(__name__)
+
+
+@dataclass(frozen=True)
+class BM25Result:
+    """A single BM25 search result."""
+
+    chunk_id: str
+    doc_id: str
+    page_number: int
+    text: str
+    score: float
+
+
+class BM25Index:
+    """
+    In-memory BM25 index over page_chunks. Rebuilt lazily on demand.
+
+    Thread-safety note: rebuilt synchronously in the request thread. For
+    single-user local deployments this is acceptable.
+    """
+
+    def __init__(self) -> None:
+        self._index: BM25Okapi | None = None
+        self._chunks: list[dict] = []
+        self._dirty: bool = True
+
+    def mark_dirty(self) -> None:
+        """Signal that the index needs rebuilding (call after any ingest completes)."""
+        self._dirty = True
+
+    def ensure_fresh(self, db_path: str) -> None:
+        """Rebuild from SQLite if dirty."""
+        if not self._dirty:
+            return
+        conn = sqlite3.connect(db_path)
+        conn.row_factory = sqlite3.Row
+        rows = conn.execute(
+            "SELECT id, doc_id, page_number, text FROM page_chunks ORDER BY doc_id, page_number"
+        ).fetchall()
+        conn.close()
+        self._load_chunks([dict(r) for r in rows])
+        self._dirty = False
+        logger.info("BM25 index rebuilt: %d chunks", len(self._chunks))
+
+    def _load_chunks(self, chunks: list[dict]) -> None:
+        self._chunks = chunks
+        tokenized = [c["text"].lower().split() for c in chunks]
+        self._index = BM25Okapi(tokenized) if tokenized else None
+
+    def query(
+        self,
+        query_text: str,
+        top_k: int = 10,
+        doc_ids: list[str] | None = None,
+    ) -> list[BM25Result]:
+        """Search the corpus. Returns results sorted by descending BM25 score."""
+        if not self._index or not self._chunks:
+            return []
+
+        scores = self._index.get_scores(query_text.lower().split())
+        ranked = sorted(enumerate(scores), key=lambda x: x[1], reverse=True)
+
+        results: list[BM25Result] = []
+        for i, score in ranked:
+            if score <= 0:
+                continue
+            c = self._chunks[i]
+            if doc_ids is not None and c["doc_id"] not in doc_ids:
+                continue
+            results.append(
+                BM25Result(
+                    chunk_id=c["id"],
+                    doc_id=c["doc_id"],
+                    page_number=c["page_number"],
+                    text=c["text"],
+                    score=float(score),
+                )
+            )
+            if len(results) >= top_k:
+                break
+        return results
diff --git a/tests/test_bm25_index.py b/tests/test_bm25_index.py
new file mode 100644
index 0000000..221cea2
--- /dev/null
+++ b/tests/test_bm25_index.py
@@ -0,0 +1,95 @@
+"""Tests for app.services.bm25_index."""
+
+from __future__ import annotations
+
+import pytest
+
+from app.services.bm25_index import BM25Index, BM25Result
+
+
+def _seeded_index() -> BM25Index:
+    idx = BM25Index()
+    idx._load_chunks(
+        [
+            {
+                "id": "c1",
+                "doc_id": "book-a",
+                "page_number": 1,
+                "text": "Fireball deals 8d6 fire damage on a failed Dexterity saving throw.",
+            },
+            {
+                "id": "c2",
+                "doc_id": "book-a",
+                "page_number": 2,
+                "text": "A wizard can cast one spell per turn unless they have Action Surge.",
+            },
+            {
+                "id": "c3",
+                "doc_id": "book-b",
+                "page_number": 5,
+                "text": "Grapple rules apply when the attacker uses the Attack action to grab a target.",
+            },
+        ]
+    )
+    return idx
+
+
+def test_query_returns_relevant_result():
+    idx = _seeded_index()
+    results = idx.query("fireball fire damage")
+    assert len(results) >= 1
+    assert results[0].chunk_id == "c1"
+    assert results[0].score > 0
+
+
+def test_query_respects_top_k():
+    idx = _seeded_index()
+    results = idx.query("rules", top_k=2)
+    assert len(results) <= 2
+
+
+def test_query_filters_by_doc_id():
+    idx = _seeded_index()
+    results = idx.query("rules", doc_ids=["book-b"])
+    assert all(r.doc_id == "book-b" for r in results)
+
+
+def test_query_empty_corpus_returns_empty():
+    idx = BM25Index()
+    idx._load_chunks([])
+    results = idx.query("anything")
+    assert results == []
+
+
+def test_mark_dirty_triggers_rebuild(tmp_path):
+    import sqlite3
+
+    db_path = str(tmp_path / "test.db")
+    conn = sqlite3.connect(db_path)
+    conn.execute(
+        "CREATE TABLE page_chunks(id TEXT, doc_id TEXT, page_number INT, text TEXT)"
+    )
+    conn.execute(
+        "INSERT INTO page_chunks VALUES ('x1','doc-1',1,'Ranger favored enemy favored terrain terrain bonuses bonuses action attack')"
+    )
+    conn.execute(
+        "INSERT INTO page_chunks VALUES ('x2','doc-1',2,'Wizard can cast spells and perform actions')"
+    )
+    conn.execute(
+        "INSERT INTO page_chunks VALUES ('x3','doc-1',3,'Fighter attacks and deals damage with weapon')"
+    )
+    conn.commit()
+    conn.close()
+
+    idx = BM25Index()
+    idx.mark_dirty()
+    idx.ensure_fresh(db_path)
+    results = idx.query("ranger terrain")
+    assert len(results) >= 1
+    assert results[0].chunk_id == "x1"
+
+
+def test_bm25_result_is_frozen():
+    r = BM25Result(chunk_id="x", doc_id="d", page_number=1, text="hello", score=0.5)
+    with pytest.raises(Exception):
+        r.score = 1.0  # type: ignore[misc]