diff --git a/CHANGELOG.md b/CHANGELOG.md
index 60d0be6..8ce4bab 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+- **Documentation**: Added warning about JavaScript runtime (deno) requirement when using cookie authentication
+  - YouTube authenticated API endpoints require JS challenge solving
+  - Without deno, downloads will fail with "n challenge solving failed" error
+
 ---
 
 ## [0.8.3] - 2025-12-25
diff --git a/README.md b/README.md
index 35bcf9b..2b9975c 100644
--- a/README.md
+++ b/README.md
@@ -408,6 +408,10 @@ YTDLP_MAX_TRANSCRIPT_LENGTH=50000
 
 To access private videos, age-restricted content, or avoid rate limits, configure cookies:
 
+> ⚠️ **Important**: Cookie authentication requires a JavaScript runtime (deno) to be installed. When using cookies, YouTube uses authenticated API endpoints that require JavaScript challenge solving. Without deno, downloads will fail with "n challenge solving failed" error.
+>
+> Install deno: https://docs.deno.com/runtime/getting_started/installation/
+
 ```bash
 # Extract cookies from browser (recommended)
 YTDLP_COOKIES_FROM_BROWSER=chrome