diff --git a/.changeset/sidebar-header-border-fix.md b/.changeset/sidebar-header-border-fix.md
new file mode 100644
index 000000000..dbdc8bfaf
--- /dev/null
+++ b/.changeset/sidebar-header-border-fix.md
@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-web": patch
+---
+
+Remove the white background override on the sidebar header in Modern theme so the right border extends seamlessly from top to bottom.
diff --git a/apps/kimi-web/src/style.css b/apps/kimi-web/src/style.css
index 86b3efdac..3fd11c83d 100644
--- a/apps/kimi-web/src/style.css
+++ b/apps/kimi-web/src/style.css
@@ -208,9 +208,10 @@ html[data-theme="modern"] .se.on {
   box-shadow: inset 0 0 0 1px var(--bd);
 }
 
-/* Tab bar + column header → clean white strips with a single hairline. */
-html[data-theme="modern"] .tabs,
-html[data-theme="modern"] .ch {
+/* Tab bar → clean white strip with a single hairline.
+   Sidebar header (.ch) keeps the sidebar body color so the right border
+   extends seamlessly from top to bottom. */
+html[data-theme="modern"] .tabs {
   background: #fff;
 }