diff --git a/docs/developer/api-reference.md b/docs/developer/api-reference.md
index 58808f6..1024686 100644
--- a/docs/developer/api-reference.md
+++ b/docs/developer/api-reference.md
@@ -1,5 +1,16 @@
 # Bookhoard API Reference
 
+> **⚠️ Legacy Document**: This is a monolithic API reference (1,600+ lines).
+> For updated, split endpoint documentation with interactive API explorer, see [API Documentation Portal](api/INDEX.md).
+>
+> **Use the split docs for:**
+> - Easier navigation by category
+> - Interactive API explorer
+> - Endpoint-specific examples
+> - Latest updates
+
+---
+
 Complete API documentation for Bookhoard v1.0 with Universal Cross-Platform Sync support.
 
 ## Table of Contents