feat(dashscope): preserve cache_control for explicit prompt caching (#25331)

DashScope inherits OpenAIGPTConfig which strips cache_control from
messages and tools by default. Override remove_cache_control_flag_from_messages_and_tools()
to preserve cache_control, following the same pattern used by ZAI, MiniMax, and Databricks.

Verified through 10-round multi-turn conversation tests:
- Explicit caching works correctly: cached_tokens grows each round from R4 onwards,
  with cache_creation_tokens reported on first cache build.
- Implicit caching is not affected: models that rely on implicit prefix-matching caching
  produce identical cached_tokens with and without this change, confirmed by comparing
  results against both the reverted codebase and direct API calls bypassing litellm.
- No errors or regressions observed on any model, including those that do not support
  explicit caching — the DashScope API silently ignores unrecognized cache_control fields.

Fixes #25330

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

litellm/llms/dashscope/chat/transformation.py

@@ -4,6 +4,8 @@ Translates from OpenAI's `/v1/chat/completions` to DashScope's `/v1/chat/complet
 
 from typing import Any, Coroutine, List, Literal, Optional, Tuple, Union, overload
 
+from litellm.types.llms.openai import ChatCompletionToolParam
+
 from litellm.secret_managers.main import get_secret_str
 from litellm.types.llms.openai import AllMessageValues
 
@@ -11,6 +13,18 @@ from ...openai.chat.gpt_transformation import OpenAIGPTConfig
 
 
 class DashScopeChatConfig(OpenAIGPTConfig):
+    def remove_cache_control_flag_from_messages_and_tools(
+        self,
+        model: str,
+        messages: List[AllMessageValues],
+        tools: Optional[List[ChatCompletionToolParam]] = None,
+    ) -> Tuple[List[AllMessageValues], Optional[List[ChatCompletionToolParam]]]:
+        """
+        Override to preserve cache_control for DashScope.
+        DashScope supports cache_control - don't strip it.
+        """
+        return messages, tools
+
     @overload
     def _transform_messages(
         self, messages: List[AllMessageValues], model: str, is_async: Literal[True]

tests/test_litellm/llms/dashscope/test_dashscope_chat_transformation.py

@@ -144,3 +144,47 @@ class TestDashScopeConfig:
         assert transformed_messages[0]["content"][0]["text"] == "Hello"
         assert transformed_messages[0]["content"][1]["type"] == "text"
         assert transformed_messages[0]["content"][1]["text"] == "World"
+
+    def test_dashscope_preserves_cache_control_in_messages(self):
+        """DashScope should NOT strip cache_control from messages."""
+        config = DashScopeChatConfig()
+
+        messages = [
+            {
+                "role": "system",
+                "content": "You are a helpful assistant.",
+                "cache_control": {"type": "ephemeral"},
+            },
+            {
+                "role": "user",
+                "content": "Hello, world!",
+            },
+        ]
+
+        transformed_messages, _ = config.remove_cache_control_flag_from_messages_and_tools(
+            model="dashscope/qwen-turbo", messages=messages
+        )
+
+        assert transformed_messages[0].get("cache_control") == {"type": "ephemeral"}
+
+    def test_dashscope_preserves_cache_control_in_tools(self):
+        """DashScope should NOT strip cache_control from tools."""
+        config = DashScopeChatConfig()
+
+        tools = [
+            {
+                "type": "function",
+                "function": {
+                    "name": "get_weather",
+                    "description": "Get weather information",
+                    "parameters": {"type": "object", "properties": {}},
+                },
+                "cache_control": {"type": "ephemeral"},
+            }
+        ]
+
+        _, transformed_tools = config.remove_cache_control_flag_from_messages_and_tools(
+            model="dashscope/qwen-turbo", messages=[], tools=tools
+        )
+
+        assert transformed_tools[0].get("cache_control") == {"type": "ephemeral"}