fix(prompts): await awaitable result returned by sync prompt callback (#3709)

Closes #3709.

Author: nikolas-sapa

src/mcp/server/mcpserver/prompts/base.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import functools
+import inspect
 from collections.abc import Awaitable, Callable, Sequence
 from typing import TYPE_CHECKING, Annotated, Any, Literal
 
@@ -188,6 +189,9 @@ async def render(
             else:
                 result = await anyio.to_thread.run_sync(functools.partial(self.fn, **call_args))
 
+            if inspect.isawaitable(result):
+                result = await result
+
             if isinstance(result, InputRequiredResult):
                 return result
 

tests/server/mcpserver/prompts/test_base.py

@@ -1,4 +1,5 @@
 import threading
+from collections.abc import Awaitable
 from pathlib import Path
 from typing import Any
 
@@ -332,3 +333,20 @@ def fn() -> Image:
 
     with pytest.raises(ValueError):
         await Prompt.from_function(fn).render(None, Context())
+
+
+@pytest.mark.anyio
+async def test_prompt_render_awaits_awaitable_returned_by_sync_callback():
+    async def resolve() -> list[str]:
+        return []
+
+    coroutine = resolve()
+
+    def prompt() -> Awaitable[list[str]]:
+        return coroutine
+
+    try:
+        messages = await Prompt.from_function(prompt).render(None, Context())
+    finally:
+        coroutine.close()
+    assert messages == []