diff --git a/packages/ai/src/chatTransport.ts b/packages/ai/src/chatTransport.ts
index 7c667f991..7e96d1e96 100644
--- a/packages/ai/src/chatTransport.ts
+++ b/packages/ai/src/chatTransport.ts
@@ -457,6 +457,8 @@ const BASE_URL_VALIDATION_ERRORS = {
   credentials: "baseURL must not include username or password credentials",
 } as const;
 
+// Includes standard whitespace plus common invisible separator/control marks
+// that can make URLs look valid while behaving unexpectedly.
 const INTERNAL_WHITESPACE_REGEX = /[\s\u200B\u200C\u200D\u2060\uFEFF]/u;
 
 function resolvePayloadMapper<