diff --git a/granian/server/common.py b/granian/server/common.py
index 2337f964..e2715711 100644
--- a/granian/server/common.py
+++ b/granian/server/common.py
@@ -457,6 +457,10 @@ def serve(
                     'Number of workers will now fallback to 1.'
                 )
 
+        if self.interface != Interfaces.WSGI and self.blocking_threads > 1:
+            logger.error('Blocking threads > 1 is not supported on ASGI and RSGI')
+            raise ConfigurationError('blocking_threads')
+
         if self.websockets:
             if self.interface == Interfaces.WSGI:
                 logger.info('Websockets are not supported on WSGI, ignoring')
diff --git a/granian/server/mt.py b/granian/server/mt.py
index c1aa31c4..0526ac9b 100644
--- a/granian/server/mt.py
+++ b/granian/server/mt.py
@@ -302,6 +302,8 @@ def serve(
         target_loader: Optional[Callable[..., Callable[..., Any]]] = None,
         wrap_loader: bool = True,
     ):
+        logger.warning('free-threaded Python support is experimental!')
+
         if self.reload_on_changes:
             logger.error('The changes reloader is not supported on the free-threaded build')
             raise ConfigurationError('reload')
diff --git a/pyproject.toml b/pyproject.toml
index 88f39d34..16bfdfef 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -114,7 +114,7 @@ extend-ignore = [
     'S110', # except pass is fine
 ]
 flake8-quotes = { inline-quotes = 'single', multiline-quotes = 'double' }
-mccabe = { max-complexity = 15 }
+mccabe = { max-complexity = 16 }
 
 [tool.ruff.format]
 quote-style = 'single'