diff --git a/docs/api/index.rst b/docs/api/index.rst
index 6313b7a5..cb7014f1 100644
--- a/docs/api/index.rst
+++ b/docs/api/index.rst
@@ -1,3 +1,5 @@
+.. _api-ref:
+
 *************
 API reference
 *************
diff --git a/docs/extensiondev.rst b/docs/extensiondev.rst
index 5530d2f2..34b26e57 100644
--- a/docs/extensiondev.rst
+++ b/docs/extensiondev.rst
@@ -329,3 +329,11 @@ all your custom GStreamer elements.
 
 For examples of custom GStreamer elements implemented in Python, see
 :mod:`mopidy.audio.mixers`.
+
+
+Use of Mopidy APIs
+==================
+
+When writing an extension, you should only use APIs documented at
+:ref:`api-ref`. Other parts of Mopidy, like :mod:`mopidy.utils` may change at
+any time, and is not something extensions should rely on being stable.