diff --git a/aip/general/0121.md b/aip/general/0121.md
index d8c1a68ac2..9b4b36fb22 100644
--- a/aip/general/0121.md
+++ b/aip/general/0121.md
@@ -39,7 +39,7 @@ When designing an API, consider the following (roughly in logical order):
 
 A resource-oriented API **should** generally be modeled as a resource
 hierarchy, where each node is either a simple resource or a collection of
-resources.
+resources. Resource hierarchies **must not** be recursive.
 
 A _collection_ contains resources of _the same type_. For example, a publisher
 has the collection of books that it publishes. A resource usually has fields,
@@ -181,6 +181,7 @@ and in turn do not increase resource management complexity.
 
 ## Changelog
 
+- **2023-09-18**: Disallow recursive resource hierarchies.
 - **2023-08-24**: Added guidance on consistency guarantees of methods.
 - **2023-07-23**: Clarify stateless protocol definition.
 - **2023-01-21**: Explicitly require matching schema across standard methods.