diff --git a/chat/templates/chat.html b/chat/templates/chat.html
index 4d9d24b..8833062 100644
--- a/chat/templates/chat.html
+++ b/chat/templates/chat.html
@@ -26,7 +26,7 @@
   </section>
 
   <form class="turn-input" method="post" action="/chats/{{ chat.id }}/turns">
-    <textarea name="prose" rows="3" placeholder="What do you say or do?"></textarea>
+    <textarea name="prose" rows="3" placeholder="What do you say or do?" required></textarea>
     <button type="submit">Send</button>
   </form>
 
diff --git a/chat/web/turns.py b/chat/web/turns.py
index 7a7db0e..260bb21 100644
--- a/chat/web/turns.py
+++ b/chat/web/turns.py
@@ -122,6 +122,9 @@ async def post_turn(
     conn=Depends(get_conn),
     client=Depends(get_llm_client),
 ):
+    if not prose.strip():
+        raise HTTPException(status_code=400, detail="prose cannot be empty")
+
     chat = get_chat(conn, chat_id)
     if chat is None:
         raise HTTPException(status_code=404, detail=f"chat not found: {chat_id}")