diff --git a/surfsense_backend/tests/unit/agents/multi_agent_chat/shared/citations/test_registry.py b/surfsense_backend/tests/unit/agents/multi_agent_chat/shared/citations/test_registry.py
new file mode 100644
index 000000000..ff90c445b
--- /dev/null
+++ b/surfsense_backend/tests/unit/agents/multi_agent_chat/shared/citations/test_registry.py
@@ -0,0 +1,104 @@
+"""Unit tests for the citation registry spine (ADR 0001 §3)."""
+
+from __future__ import annotations
+
+from app.agents.chat.multi_agent_chat.shared.citations import (
+    CitationRegistry,
+    CitationSourceType,
+    make_key,
+)
+
+
+def test_register_assigns_monotonic_labels() -> None:
+    registry = CitationRegistry()
+
+    first = registry.register(
+        CitationSourceType.KB_CHUNK, {"document_id": 42, "chunk_id": 880}
+    )
+    second = registry.register(
+        CitationSourceType.KB_CHUNK, {"document_id": 42, "chunk_id": 881}
+    )
+
+    assert (first, second) == (1, 2)
+    assert registry.next_n == 3
+
+
+def test_register_is_find_or_create_for_same_unit() -> None:
+    registry = CitationRegistry()
+    locator = {"document_id": 42, "chunk_id": 880}
+
+    first = registry.register(CitationSourceType.KB_CHUNK, locator)
+    again = registry.register(CitationSourceType.KB_CHUNK, locator)
+
+    assert first == again == 1
+    assert len(registry.by_n) == 1
+    assert registry.next_n == 2
+
+
+def test_dedup_is_insensitive_to_locator_key_order() -> None:
+    registry = CitationRegistry()
+
+    first = registry.register(
+        CitationSourceType.KB_CHUNK, {"document_id": 42, "chunk_id": 880}
+    )
+    reordered = registry.register(
+        CitationSourceType.KB_CHUNK, {"chunk_id": 880, "document_id": 42}
+    )
+
+    assert first == reordered
+
+
+def test_same_locator_values_across_types_do_not_collide() -> None:
+    registry = CitationRegistry()
+
+    chunk = registry.register(CitationSourceType.KB_CHUNK, {"id": 7})
+    chat = registry.register(CitationSourceType.CHAT_TURN, {"id": 7})
+
+    assert chunk != chat
+
+
+def test_resolve_returns_entry_with_locator_and_display() -> None:
+    registry = CitationRegistry()
+    n = registry.register(
+        CitationSourceType.WEB_RESULT,
+        {"url": "https://example.com"},
+        {"title": "Example"},
+    )
+
+    entry = registry.resolve(n)
+
+    assert entry is not None
+    assert entry.n == n
+    assert entry.source_type is CitationSourceType.WEB_RESULT
+    assert entry.locator == {"url": "https://example.com"}
+    assert entry.display == {"title": "Example"}
+
+
+def test_resolve_unknown_label_returns_none() -> None:
+    registry = CitationRegistry()
+
+    assert registry.resolve(999) is None
+
+
+def test_registry_round_trips_through_serialization() -> None:
+    registry = CitationRegistry()
+    registry.register(
+        CitationSourceType.KB_CHUNK,
+        {"document_id": 42, "chunk_id": 880},
+        {"title": "Q3 Launch Notes"},
+    )
+
+    restored = CitationRegistry.model_validate(registry.model_dump())
+
+    entry = restored.resolve(1)
+    assert entry is not None
+    assert entry.source_type is CitationSourceType.KB_CHUNK
+    assert restored.next_n == registry.next_n
+
+
+def test_make_key_is_stable_and_type_prefixed() -> None:
+    key_a = make_key(CitationSourceType.KB_CHUNK, {"document_id": 42, "chunk_id": 880})
+    key_b = make_key(CitationSourceType.KB_CHUNK, {"chunk_id": 880, "document_id": 42})
+
+    assert key_a == key_b
+    assert key_a.startswith("kb_chunk|")