fix truncation logic (anthropics#177)

Author: x5a

computer-use-demo/computer_use_demo/loop.py

@@ -113,9 +113,9 @@ async def sampling_loop(
         if enable_prompt_caching:
             betas.append(PROMPT_CACHING_BETA_FLAG)
             _inject_prompt_caching(messages)
-            # Is it ever worth it to bust the cache with prompt caching?
-            if image_truncation_threshold:
-                image_truncation_threshold = 0
+            # Because cached reads are 10% of the price, we don't think it's
+            # ever sensible to break the cache by truncating images
+            only_n_most_recent_images = 0
             system["cache_control"] = {"type": "ephemeral"}
 
         if only_n_most_recent_images: