diff --git a/src/attune_rag/expander.py b/src/attune_rag/expander.py
index fe61316..84140e7 100644
--- a/src/attune_rag/expander.py
+++ b/src/attune_rag/expander.py
@@ -2,10 +2,15 @@
 
 from __future__ import annotations
 
+import asyncio
 import json
 import logging
 
 logger = logging.getLogger(__name__)
+# Touch reference so import remains after auto-format passes; the
+# real consumer is ``QueryExpander.expand_async`` below.
+_ASYNCIO_TO_THREAD = asyncio.to_thread
+__all__ = ["QueryExpander"]
 
 _SYSTEM = """\
 You expand developer queries for a documentation retrieval system.
@@ -88,3 +93,18 @@ def expand(self, query: str) -> list[str]:
         if self._cache is not None:
             self._cache[query] = expansions
         return expansions
+
+    async def expand_async(self, query: str) -> list[str]:
+        """Async variant of :meth:`expand` for use from async event loops.
+
+        Wraps the synchronous Anthropic call in :func:`asyncio.to_thread`
+        so callers like FastAPI route handlers don't block the event
+        loop. The cache is shared with :meth:`expand` so a hit on either
+        path serves a hit on the other.
+
+        Returns the same shape and same fail-soft empty-list semantics
+        as :meth:`expand`.
+        """
+        if self._cache is not None and query in self._cache:
+            return self._cache[query]
+        return await _ASYNCIO_TO_THREAD(self.expand, query)
diff --git a/tests/unit/test_expander_reranker.py b/tests/unit/test_expander_reranker.py
index cce14cc..3ea0b7a 100644
--- a/tests/unit/test_expander_reranker.py
+++ b/tests/unit/test_expander_reranker.py
@@ -339,3 +339,63 @@ def test_expander_and_reranker_compose(self, corpus: FakeCorpus):
         exp_client.messages.create.assert_called_once()
         rer_client.messages.create.assert_called_once()
         assert not result.fallback_used
+
+    def test_expand_async_returns_same_as_expand(self) -> None:
+        """Async variant must produce the same shape as the sync method
+        for the same input + mock response.
+        """
+        import asyncio
+
+        expander = QueryExpander(cache=False)
+        mock_client = MagicMock()
+        mock_client.messages.create.return_value = _fake_response('["a", "b"]')
+        expander._client = mock_client
+        result = asyncio.run(expander.expand_async("q"))
+        assert result == ["a", "b"]
+
+    def test_expand_async_uses_cache_without_to_thread(self) -> None:
+        """When the cache already has the answer, ``expand_async`` returns
+        synchronously without dispatching to a thread.
+        """
+        import asyncio
+
+        expander = QueryExpander(cache=True)
+        mock_client = MagicMock()
+        mock_client.messages.create.return_value = _fake_response('["from-cache"]')
+        expander._client = mock_client
+
+        # Prime the cache via the sync method
+        first = expander.expand("k")
+        assert first == ["from-cache"]
+        assert mock_client.messages.create.call_count == 1
+
+        # Async hit should pull from the same cache, no second API call
+        second = asyncio.run(expander.expand_async("k"))
+        assert second == ["from-cache"]
+        assert mock_client.messages.create.call_count == 1
+
+    def test_expand_async_does_not_block_event_loop(self) -> None:
+        """``expand_async`` must dispatch the blocking call via a thread
+        so the event loop stays responsive while it runs.
+        """
+        import asyncio
+        import threading
+
+        expander = QueryExpander(cache=False)
+        mock_client = MagicMock()
+        api_thread: dict[str, int] = {}
+
+        def slow_create(**_kwargs):
+            api_thread["tid"] = threading.get_ident()
+            return _fake_response('["slow"]')
+
+        mock_client.messages.create.side_effect = slow_create
+        expander._client = mock_client
+
+        async def runner() -> None:
+            main_tid = threading.get_ident()
+            await expander.expand_async("q")
+            # The Anthropic call must not have run on the event-loop thread
+            assert api_thread["tid"] != main_tid
+
+        asyncio.run(runner())