diff --git a/CHANGELOG.md b/CHANGELOG.md
index e141cb4c..ac005d6f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- Clarify that event `default` visibility is Google-only
+
 ## [8.4.0] - 2026-06-24
 
 ### Added
diff --git a/src/models/events.ts b/src/models/events.ts
index 955a84b5..5ef0c244 100644
--- a/src/models/events.ts
+++ b/src/models/events.ts
@@ -415,6 +415,10 @@ type RsvpStatus = 'yes' | 'no' | 'maybe';
 
 /**
  * Enum representing the visibility of an event.
+ *
+ * 'default' is only valid for Google events, where it defers to the calendar's own sharing
+ * settings. Microsoft and EWS events only support 'public' and 'private'; sending 'default'
+ * for these providers returns a 400 error.
  */
 type Visibility = 'default' | 'public' | 'private';