diff --git a/api/server-server/definitions/unsigned_pdu.yaml b/api/server-server/definitions/unsigned_pdu.yaml
index ab281224..64991d22 100644
--- a/api/server-server/definitions/unsigned_pdu.yaml
+++ b/api/server-server/definitions/unsigned_pdu.yaml
@@ -78,7 +78,10 @@ properties:
           required: ['sha256']
   depth:
     type: integer
-    description: The maximum depth of the ``prev_events``, plus one.
+    description: |-
+      The maximum depth of the ``prev_events``, plus one. Must be less than the
+      maximum value for an integer (2^63 - 1). If the room's depth is already at
+      the limit, the depth must be set to the limit.
     example: 12
   auth_events:
     type: array