Don't block the event loop on sync resource and prompt functions (#2380)

tests/server/mcpserver/resources/test_resource_template.py

@@ -1,4 +1,5 @@
 import json
+import threading
 from typing import Any
 
 import pytest
@@ -310,3 +311,22 @@ class TestResourceTemplateMetadata:
         assert resource.meta == metadata
         assert resource.meta["category"] == "inventory"
         assert resource.meta["cacheable"] is True
+
+
+@pytest.mark.anyio
+async def test_sync_fn_runs_in_worker_thread():
+    """Sync template functions must run in a worker thread, not the event loop."""
+
+    main_thread = threading.get_ident()
+    fn_thread: list[int] = []
+
+    def blocking_fn(name: str) -> str:
+        fn_thread.append(threading.get_ident())
+        return f"hello {name}"
+
+    template = ResourceTemplate.from_function(fn=blocking_fn, uri_template="test://{name}")
+    resource = await template.create_resource("test://world", {"name": "world"}, Context())
+
+    assert isinstance(resource, FunctionResource)
+    assert await resource.read() == "hello world"
+    assert fn_thread[0] != main_thread