diff --git a/apps/rag-pipeline/rag/__init__.py b/apps/rag-pipeline/rag/__init__.py
index 94b9ec9..2aa6d76 100644
--- a/apps/rag-pipeline/rag/__init__.py
+++ b/apps/rag-pipeline/rag/__init__.py
@@ -1,6 +1,7 @@
 """Pure Python RAG foundation utilities."""
 
 from .core import reciprocal_rank_fusion, split_text
+from .local_index import Document, LocalDocumentIndex
 from .pipeline import (
     INSUFFICIENT_INFORMATION,
     Citation,
@@ -14,7 +15,9 @@
 __all__ = [
     "INSUFFICIENT_INFORMATION",
     "Citation",
+    "Document",
     "Generator",
+    "LocalDocumentIndex",
     "RAGPipeline",
     "RAGResult",
     "RetrievalHit",
diff --git a/apps/rag-pipeline/rag/local_index.py b/apps/rag-pipeline/rag/local_index.py
new file mode 100644
index 0000000..2e94b28
--- /dev/null
+++ b/apps/rag-pipeline/rag/local_index.py
@@ -0,0 +1,66 @@
+"""Deterministic in-memory document chunking and lexical retrieval."""
+
+import re
+from collections.abc import Iterable, Mapping
+from dataclasses import dataclass, field
+
+from .core import split_text
+from .pipeline import RetrievalHit
+
+
+def _tokens(text: str) -> set[str]:
+    return set(re.findall(r"\b\w+\b", text.casefold()))
+
+
+@dataclass(frozen=True)
+class Document:
+    document_id: str
+    text: str
+    metadata: Mapping[str, object] = field(default_factory=dict)
+
+
+class LocalDocumentIndex:
+    """A rebuildable lexical index compatible with ``RAGPipeline`` retrievers."""
+
+    def __init__(
+        self,
+        *,
+        chunk_size: int = 1200,
+        chunk_overlap: int = 300,
+        min_chunk_chars: int = 40,
+    ) -> None:
+        self._chunk_options = {
+            "chunk_size": chunk_size,
+            "chunk_overlap": chunk_overlap,
+            "min_chunk_chars": min_chunk_chars,
+        }
+        self._chunks: list[tuple[RetrievalHit, set[str]]] = []
+
+    def rebuild(self, documents: Iterable[Document]) -> None:
+        """Replace the complete index with caller-supplied documents."""
+        rebuilt: list[tuple[RetrievalHit, set[str]]] = []
+        seen: set[str] = set()
+        for document in documents:
+            if document.document_id in seen:
+                raise ValueError(f"duplicate document id: {document.document_id}")
+            seen.add(document.document_id)
+            metadata = dict(document.metadata)
+            metadata["document_id"] = document.document_id
+            for index, chunk in enumerate(split_text(document.text, **self._chunk_options)):
+                chunk_id = f"{len(document.document_id)}:{document.document_id}:{index}"
+                rebuilt.append((RetrievalHit(chunk_id, chunk, metadata), _tokens(chunk)))
+        self._chunks = rebuilt
+
+    def __call__(self, query: str, limit: int) -> list[RetrievalHit]:
+        if limit <= 0:
+            return []
+        query_tokens = _tokens(query)
+        if not query_tokens:
+            return []
+        ranked = [
+            (len(query_tokens & tokens), hit)
+            for hit, tokens in self._chunks
+            if query_tokens & tokens
+        ]
+        ranked.sort(key=lambda item: (-item[0], item[1].document_id))
+        return [hit for _score, hit in ranked[:limit]]
diff --git a/apps/rag-pipeline/tests/test_local_index.py b/apps/rag-pipeline/tests/test_local_index.py
new file mode 100644
index 0000000..2bb4f3a
--- /dev/null
+++ b/apps/rag-pipeline/tests/test_local_index.py
@@ -0,0 +1,62 @@
+import unittest
+
+from rag import Document, LocalDocumentIndex
+
+
+class LocalDocumentIndexTests(unittest.TestCase):
+    def test_indexes_caller_documents_without_title_or_filename_collisions(self) -> None:
+        index = LocalDocumentIndex(chunk_size=30, chunk_overlap=0, min_chunk_chars=1)
+        index.rebuild(
+            [
+                Document("doc/a", "shared title alpha unique", {"source": "first", "title": "Same"}),
+                Document("doc:b", "shared title beta unique", {"source": "second", "title": "Same"}),
+            ]
+        )
+
+        alpha = index("alpha", 5)
+        beta = index("beta", 5)
+
+        self.assertEqual(len(alpha), 1)
+        self.assertEqual(len(beta), 1)
+        self.assertNotEqual(alpha[0].document_id, beta[0].document_id)
+        self.assertEqual(alpha[0].metadata, {"source": "first", "title": "Same", "document_id": "doc/a"})
+        self.assertEqual(beta[0].metadata["document_id"], "doc:b")
+
+    def test_rebuild_replaces_previous_contents_and_is_idempotent(self) -> None:
+        index = LocalDocumentIndex(min_chunk_chars=1)
+        documents = [Document("stable", "repeatable needle", {"source": "caller"})]
+
+        index.rebuild(documents)
+        first = index("needle", 10)
+        index.rebuild(documents)
+        second = index("needle", 10)
+
+        self.assertEqual(first, second)
+        index.rebuild([Document("replacement", "different token", {})])
+        self.assertEqual(index("needle", 10), [])
+
+    def test_ranks_overlap_deterministically_and_respects_limit(self) -> None:
+        index = LocalDocumentIndex(min_chunk_chars=1)
+        index.rebuild(
+            [
+                Document("z", "red blue", {}),
+                Document("a", "red blue green", {}),
+                Document("m", "red only", {}),
+            ]
+        )
+
+        hits = index("RED, blue green", 2)
+
+        self.assertEqual([hit.metadata["document_id"] for hit in hits], ["a", "z"])
+        self.assertEqual(index("absent", 3), [])
+        self.assertEqual(index("red", 0), [])
+
+    def test_rejects_duplicate_caller_ids(self) -> None:
+        index = LocalDocumentIndex(min_chunk_chars=1)
+
+        with self.assertRaisesRegex(ValueError, "duplicate document id"):
+            index.rebuild([Document("same", "one", {}), Document("same", "two", {})])
+
+
+if __name__ == "__main__":
+    unittest.main()