diff --git a/surfsense_backend/tests/unit/middleware/test_numbered_document.py b/surfsense_backend/tests/unit/middleware/test_numbered_document.py
new file mode 100644
index 000000000..955c619b5
--- /dev/null
+++ b/surfsense_backend/tests/unit/middleware/test_numbered_document.py
@@ -0,0 +1,92 @@
+"""Unit tests for the numbered-document read preamble."""
+
+import pytest
+
+from app.agents.chat.multi_agent_chat.shared.middleware.filesystem.backends.numbered_document import (
+    build_read_preamble,
+    compute_matched_line_ranges,
+)
+
+pytestmark = pytest.mark.unit
+
+
+_BODY = "alpha\nbravo\ncharlie\ndelta"
+
+
+class TestComputeMatchedLineRanges:
+    def test_maps_matched_chunk_spans_to_line_ranges(self):
+        chunks = [(1, 0, 12), (2, 12, len(_BODY))]
+        ranges = compute_matched_line_ranges(_BODY, chunks, {2})
+        assert ranges == [(3, 4)]
+
+    def test_includes_only_matched_chunks(self):
+        chunks = [(1, 0, 5), (2, 6, 11)]
+        ranges = compute_matched_line_ranges(_BODY, chunks, {1})
+        assert ranges == [(1, 1)]
+
+    def test_skips_chunks_without_spans(self):
+        chunks = [(1, None, None)]
+        ranges = compute_matched_line_ranges(_BODY, chunks, {1})
+        assert ranges == []
+
+    def test_sorted_and_deduplicated(self):
+        chunks = [(1, 12, len(_BODY)), (2, 0, 5), (3, 0, 5)]
+        ranges = compute_matched_line_ranges(_BODY, chunks, {1, 2, 3})
+        assert ranges == [(1, 1), (3, 4)]
+
+
+class TestBuildReadPreamble:
+    def test_contains_document_metadata(self):
+        preamble = build_read_preamble(
+            document_id=42,
+            document_type="FILE",
+            title="Test Doc",
+            url="https://example.com",
+            matched_line_ranges=[],
+        )
+        assert "<document_id>42</document_id>" in preamble
+        assert "<document_type>FILE</document_type>" in preamble
+        assert "Test Doc" in preamble
+        assert "https://example.com" in preamble
+
+    def test_citation_hint_uses_document_id(self):
+        preamble = build_read_preamble(
+            document_id=42,
+            document_type="FILE",
+            title="Test Doc",
+            url="",
+            matched_line_ranges=[],
+        )
+        assert "[citation:d42#L" in preamble
+
+    def test_lists_matched_line_ranges(self):
+        preamble = build_read_preamble(
+            document_id=7,
+            document_type="NOTE",
+            title="Notes",
+            url="",
+            matched_line_ranges=[(12, 18), (40, 40)],
+        )
+        assert "<matched_lines>" in preamble
+        assert "12-18" in preamble
+        assert "40" in preamble
+
+    def test_omits_matched_lines_block_when_empty(self):
+        preamble = build_read_preamble(
+            document_id=7,
+            document_type="NOTE",
+            title="Notes",
+            url="",
+            matched_line_ranges=[],
+        )
+        assert "<matched_lines>" not in preamble
+
+    def test_ends_with_trailing_newline_so_body_follows_cleanly(self):
+        preamble = build_read_preamble(
+            document_id=1,
+            document_type="FILE",
+            title="t",
+            url="",
+            matched_line_ranges=[],
+        )
+        assert preamble.endswith("\n")