diff --git a/backend/indexer.py b/backend/indexer.py
index fd2b429..11bb168 100644
--- a/backend/indexer.py
+++ b/backend/indexer.py
@@ -13,6 +13,7 @@
 class CompassIndexer:
     def __init__(self, model: str, workspace: str):
         self.client = PageIndexClient(model=model, workspace=workspace)
+        self._structure_cache: dict[str, str] = {}
 
     def index_document(self, filepath: str) -> str | None:
         path = Path(filepath)
@@ -33,10 +34,13 @@ def index_document(self, filepath: str) -> str | None:
         logger.info(f"Indexing: {path.name}")
         doc_id = self.client.index(str(path))
         logger.info(f"Done: {path.name} → {doc_id}")
+        self._structure_cache.pop(doc_id, None)  # invalidate on re-index
         return doc_id
 
     def get_structure(self, doc_id: str) -> str:
-        return self.client.get_document_structure(doc_id)
+        if doc_id not in self._structure_cache:
+            self._structure_cache[doc_id] = self.client.get_document_structure(doc_id)
+        return self._structure_cache[doc_id]
 
     def get_page_content(self, doc_id: str, pages: str) -> str:
         return self.client.get_page_content(doc_id, pages)
diff --git a/tests/test_indexer.py b/tests/test_indexer.py
new file mode 100644
index 0000000..bde7f63
--- /dev/null
+++ b/tests/test_indexer.py
@@ -0,0 +1,87 @@
+"""
+Unit tests for CompassIndexer — focuses on caching and deduplication logic.
+PageIndexClient is mocked so no disk I/O or LLM calls are made.
+"""
+
+import pytest
+from unittest.mock import MagicMock, patch, call
+
+
+def _make_client(doc_name="doc", structure='[{"title": "T", "nodes": []}]'):
+    client = MagicMock()
+    client.documents = {
+        "doc-001": {"doc_name": doc_name, "type": "md", "page_count": None, "doc_description": ""}
+    }
+    client.get_document_structure.return_value = structure
+    client.index.return_value = "doc-002"
+    return client
+
+
+@pytest.fixture
+def indexer():
+    with patch("backend.indexer.PageIndexClient") as MockClient:
+        MockClient.return_value = _make_client()
+        from backend.indexer import CompassIndexer
+        idx = CompassIndexer(model="test-model", workspace="/tmp/ws")
+        yield idx
+
+
+class TestStructureCache:
+    def test_first_call_hits_client(self, indexer):
+        indexer.get_structure("doc-001")
+        indexer.client.get_document_structure.assert_called_once_with("doc-001")
+
+    def test_second_call_uses_cache(self, indexer):
+        indexer.get_structure("doc-001")
+        indexer.get_structure("doc-001")
+        # client should only be called once
+        indexer.client.get_document_structure.assert_called_once()
+
+    def test_cache_returns_same_value(self, indexer):
+        first = indexer.get_structure("doc-001")
+        second = indexer.get_structure("doc-001")
+        assert first == second
+
+    def test_cache_invalidated_on_reindex(self, indexer):
+        # Pre-populate cache
+        indexer._structure_cache["doc-001"] = "cached-structure"
+
+        # Simulate what index_document does after a successful index call
+        indexer._structure_cache.pop("doc-001", None)
+
+        assert "doc-001" not in indexer._structure_cache
+
+    def test_cache_pop_is_called_after_index(self, indexer):
+        indexer._structure_cache["doc-002"] = "stale"
+        indexer.client.index.return_value = "doc-002"
+        # doc-002 is not in existing documents, so it will be indexed
+        indexer.client.documents = {}
+
+        indexer.index_document("newfile.md")
+
+        assert "doc-002" not in indexer._structure_cache
+
+    def test_different_docs_cached_independently(self, indexer):
+        indexer.client.documents["doc-002"] = {
+            "doc_name": "other", "type": "md", "page_count": None, "doc_description": ""
+        }
+        indexer.client.get_document_structure.side_effect = lambda doc_id: f"structure-{doc_id}"
+
+        s1 = indexer.get_structure("doc-001")
+        s2 = indexer.get_structure("doc-002")
+
+        assert s1 == "structure-doc-001"
+        assert s2 == "structure-doc-002"
+        assert indexer.client.get_document_structure.call_count == 2
+
+
+class TestDeduplication:
+    def test_does_not_reindex_existing_doc(self, indexer):
+        result = indexer.index_document("doc.md")
+        assert result == "doc-001"
+        indexer.client.index.assert_not_called()
+
+    def test_indexes_new_doc(self, indexer):
+        result = indexer.index_document("new-file.md")
+        assert result == "doc-002"
+        indexer.client.index.assert_called_once()