diff --git a/apps/web/src/components/chat/ComposerSurface.tsx b/apps/web/src/components/chat/ComposerSurface.tsx
index bcb994085eb0..5e3015f22f89 100644
--- a/apps/web/src/components/chat/ComposerSurface.tsx
+++ b/apps/web/src/components/chat/ComposerSurface.tsx
@@ -43,6 +43,7 @@ const outlineClasses =
 const contextSeamClasses =
   "group-data-with-context/composer-surface:after:[clip-path:polygon(0_0,100%_0,100%_100%,calc(100%-22px)_100%,calc(100%-22px)_calc(100%-2px),22px_calc(100%-2px),22px_100%,0_100%)]";
 
+/** Frames the input while leaving the shared edge open for the context strip's outline. */
 function Host({ className, ...props }: ComponentProps<"div">) {
   return (
     <div
@@ -59,6 +60,7 @@ function Host({ className, ...props }: ComponentProps<"div">) {
   );
 }
 
+/** Covers the input surface when an attached banner replaces the outer glass backdrop. */
 function Main({ className, ...props }: ComponentProps<"div">) {
   return (
     <div
@@ -79,13 +81,14 @@ function Main({ className, ...props }: ComponentProps<"div">) {
   );
 }
 
+/** Extends the composer outline below the input, masking its overlap up to one pixel above the shared edge so no seam shows. */
 function ContextStrip({ className, ...props }: ComponentProps<"div">) {
   return (
     <div
       data-slot="composer-context-strip"
       className={cn(
         "group/composer-context relative isolate mx-auto -mt-4 flex w-[calc(100%-2*var(--chat-composer-drawer-inset))] items-center gap-2 overflow-x-clip overflow-y-visible ps-1 pe-2 pt-5 pb-1",
-        "before:absolute before:inset-0 before:-z-1 before:rounded-b-2xl before:border before:border-(--chat-composer-outline) before:mask-b-from-transparent before:mask-b-from-4 before:mask-b-to-black before:mask-b-to-4 before:shadow-composer",
+        "before:absolute before:inset-0 before:-z-1 before:rounded-b-2xl before:border before:border-(--chat-composer-outline) before:mask-b-from-transparent before:mask-b-from-(length:--spacing-composer-seam) before:mask-b-to-black before:mask-b-to-(length:--spacing-composer-seam) before:shadow-composer",
         "dark:before:border-white/7 dark:before:bg-composer-seam-below dark:before:shadow-composer-dark",
         "group-has-data-[composer-banner-surface=attached]/composer-surface:before:bg-(--chat-composer-glass-surface)/(--glass-opacity) group-has-data-[composer-banner-surface=attached]/composer-surface:before:backdrop-blur-(--glass-blur) group-has-data-[composer-banner-surface=attached]/composer-surface:before:backdrop-saturate-(--glass-saturation)",
         "not-supports-[clip-path:shape(from_0_0,line_to_1px_1px)]:before:bg-(--chat-composer-glass-surface)/(--glass-opacity) not-supports-[clip-path:shape(from_0_0,line_to_1px_1px)]:before:backdrop-blur-(--glass-blur) not-supports-[clip-path:shape(from_0_0,line_to_1px_1px)]:before:backdrop-saturate-(--glass-saturation)",
diff --git a/apps/web/src/index.css b/apps/web/src/index.css
index 1039f23b19b8..ec92864b4dc8 100644
--- a/apps/web/src/index.css
+++ b/apps/web/src/index.css
@@ -179,6 +179,8 @@ html[data-mobile-composer-route-transition="true"]::view-transition-old(t3-mobil
      by the composer, its context strip and attached banners. */
   --shadow-composer: 0 12px 28px -18px rgb(0 0 0 / 40%);
   --shadow-composer-dark: 0 14px 32px -18px rgb(0 0 0 / 75%);
+  /* Overlap the lower toolbar backdrop by one pixel to cover the composer seam. */
+  --spacing-composer-seam: calc(var(--spacing) * 4 - 1px);
   /* Dark-mode shade where an attached surface meets the composer: 10px of
      shadow fading away from the seam. Inline, so the offsets resolve on the
      element. below: the context strip under the composer (1rem seam, faint