diff --git a/apps/web/src/components/ComposerPromptEditorTiptap.tsx b/apps/web/src/components/ComposerPromptEditorTiptap.tsx
index 5126246dd05c7e5e74602f57bbf251dd233d851a..c108207304c43ac69e250b1664ba2099dd7a601f 100644
--- a/apps/web/src/components/ComposerPromptEditorTiptap.tsx
+++ b/apps/web/src/components/ComposerPromptEditorTiptap.tsx
@@ -725,6 +725,19 @@ function ComposerPromptEditorTiptapInner(props: ComposerPromptEditorProps) {
     );
   }, []);
 
+  const editorAttributes = useMemo(
+    () => ({
+      class: cn(
+        "composer-tiptap block max-h-50 min-h-17.5 w-full overflow-y-auto whitespace-pre-wrap wrap-break-word bg-transparent leading-relaxed text-foreground focus:outline-none",
+        className,
+      ),
+      "data-testid": "composer-editor",
+      "data-composer-rich-text": richText ? "true" : "false",
+      "aria-placeholder": placeholder,
+    }),
+    [className, placeholder, richText],
+  );
+
   const editor = useEditor(
     {
       extensions: [
@@ -787,15 +800,7 @@ function ComposerPromptEditorTiptapInner(props: ComposerPromptEditorProps) {
       ),
       editable: !disabled,
       editorProps: {
-        attributes: {
-          class: cn(
-            "composer-tiptap block max-h-50 min-h-17.5 w-full overflow-y-auto whitespace-pre-wrap wrap-break-word bg-transparent leading-relaxed text-foreground focus:outline-none",
-            className,
-          ),
-          "data-testid": "composer-editor",
-          "data-composer-rich-text": richText ? "true" : "false",
-          "aria-placeholder": placeholder,
-        },
+        attributes: editorAttributes,
         handleKeyDown: (view, event) => {
           if (
             isMacPlatform(navigator.platform) &&
@@ -995,6 +1000,17 @@ function ComposerPromptEditorTiptapInner(props: ComposerPromptEditorProps) {
     editorHolder.current = editor;
   }, [editor]);
 
+  // Tiptap forwards option changes to the view from a passive effect, so a
+  // class change here would reach the ProseMirror element one tick after
+  // React commits. The chat composer measures its resting and expanded
+  // geometry in layout effects that run first, and it clamps the prompt
+  // through `className`, so the attributes are pushed to the view here for
+  // those measurements to see the layout they are about to reserve for.
+  useLayoutEffect(() => {
+    if (!editor?.isInitialized) return;
+    editor.view.setProps({ attributes: editorAttributes });
+  }, [editor, editorAttributes]);
+
   const readSnapshot = useCallback(() => {
     const snapshot = snapshotRef.current;
     if (!editor) return snapshot;