diff --git a/src/anthropic/lib/tools/_tool_dispatch.py b/src/anthropic/lib/tools/_tool_dispatch.py
index 80517eab2..fc45ca868 100644
--- a/src/anthropic/lib/tools/_tool_dispatch.py
+++ b/src/anthropic/lib/tools/_tool_dispatch.py
@@ -15,6 +15,8 @@
 from typing import Union, TypeVar, Iterable, Awaitable
 from typing_extensions import Protocol
 
+from anyio.to_thread import run_sync
+
 from ._beta_functions import ToolError, BetaFunctionToolResultType
 from ...types.beta.beta_message_param import BetaMessageParam
 from ...types.beta.beta_content_block_param import BetaContentBlockParam
@@ -132,13 +134,18 @@ def tool_error_content(exc: BaseException) -> BetaFunctionToolResultType:
 
 
 async def run_runnable_tool(tool: _CallableTool, input: dict[str, object]) -> BetaFunctionToolResultType:
-    """Call ``tool`` with ``input``, awaiting the result if the tool is async.
+    """Call ``tool`` without letting synchronous work block the event loop.
 
-    Bridges the sync (:class:`~anthropic.lib.tools.BetaFunctionTool`) and async
-    (:class:`~anthropic.lib.tools.BetaAsyncFunctionTool`) runnable-tool shapes
-    behind a single ``await``.
+    The sessions runner accepts both sync and async runnable tools. Native async
+    ``call`` methods execute on the event loop as usual; synchronous methods run
+    in AnyIO's worker thread. A synchronous wrapper that returns an awaitable is
+    still supported — the wrapper runs off-loop and its result is then awaited.
     """
-    result = tool.call(input)
+    if inspect.iscoroutinefunction(tool.call):
+        result = tool.call(input)
+    else:
+        result = await run_sync(tool.call, input)
+
     if inspect.isawaitable(result):
         return await result
     return result
diff --git a/tests/lib/tools/test_tool_dispatch_async_execution.py b/tests/lib/tools/test_tool_dispatch_async_execution.py
new file mode 100644
index 000000000..d79daf77a
--- /dev/null
+++ b/tests/lib/tools/test_tool_dispatch_async_execution.py
@@ -0,0 +1,80 @@
+from __future__ import annotations
+
+import threading
+from typing import Any, cast
+
+import pytest
+
+from anthropic.lib.tools._tool_dispatch import run_runnable_tool
+
+
+class _SyncTool:
+    def __init__(self) -> None:
+        self.call_thread: int | None = None
+
+    def call(self, input: object) -> str:
+        assert input == {"value": 1}
+        self.call_thread = threading.get_ident()
+        return "sync-result"
+
+
+class _AsyncTool:
+    def __init__(self) -> None:
+        self.call_thread: int | None = None
+
+    async def call(self, input: object) -> str:
+        assert input == {"value": 2}
+        self.call_thread = threading.get_ident()
+        return "async-result"
+
+
+class _SyncAwaitableTool:
+    def __init__(self) -> None:
+        self.call_thread: int | None = None
+        self.await_thread: int | None = None
+
+    def call(self, input: object) -> Any:
+        assert input == {"value": 3}
+        self.call_thread = threading.get_ident()
+
+        async def finish() -> str:
+            self.await_thread = threading.get_ident()
+            return "awaitable-result"
+
+        return finish()
+
+
+@pytest.mark.asyncio
+async def test_sync_tool_runs_in_worker_thread() -> None:
+    event_loop_thread = threading.get_ident()
+    tool = _SyncTool()
+
+    result = await run_runnable_tool(cast(Any, tool), {"value": 1})
+
+    assert result == "sync-result"
+    assert tool.call_thread is not None
+    assert tool.call_thread != event_loop_thread
+
+
+@pytest.mark.asyncio
+async def test_native_async_tool_stays_on_event_loop_thread() -> None:
+    event_loop_thread = threading.get_ident()
+    tool = _AsyncTool()
+
+    result = await run_runnable_tool(cast(Any, tool), {"value": 2})
+
+    assert result == "async-result"
+    assert tool.call_thread == event_loop_thread
+
+
+@pytest.mark.asyncio
+async def test_sync_wrapper_returning_awaitable_is_still_supported() -> None:
+    event_loop_thread = threading.get_ident()
+    tool = _SyncAwaitableTool()
+
+    result = await run_runnable_tool(cast(Any, tool), {"value": 3})
+
+    assert result == "awaitable-result"
+    assert tool.call_thread is not None
+    assert tool.call_thread != event_loop_thread
+    assert tool.await_thread == event_loop_thread