Fix extract_cacheable_prefix to handle string content with message-level cache_control

litellm/router_utils/prompt_caching_cache.py

@@ -76,6 +76,22 @@ def extract_cacheable_prefix(messages: List[AllMessageValues]) -> List[AllMessag
 
         for msg_idx, message in enumerate(messages):
             content = message.get("content")
+
+            # Check for cache_control at message level (when content is a string)
+            # This handles the case where cache_control is a sibling of string content:
+            # {"role": "user", "content": "...", "cache_control": {"type": "ephemeral"}}
+            message_level_cache_control = message.get("cache_control")
+            if (
+                message_level_cache_control is not None
+                and isinstance(message_level_cache_control, dict)
+                and message_level_cache_control.get("type") == "ephemeral"
+            ):
+                last_cacheable_message_idx = msg_idx
+                # Set to None to indicate the entire message content is cacheable
+                # (not a specific content block index within a list)
+                last_cacheable_content_idx = None
+
+            # Also check for cache_control within content blocks (when content is a list)
             if not isinstance(content, list):
                 continue
 

tests/router_unit_tests/test_router_prompt_caching.py

@@ -185,3 +185,89 @@ def create_messages(user_content: str) -> list[AllMessageValues]:
         assert (
             model_id_1 == model_id_2 == model_id_3
         ), f"All requests should route to same deployment, but got: {model_id_1}, {model_id_2}, {model_id_3}"
+
+
+def test_extract_cacheable_prefix_with_string_content_and_message_level_cache_control():
+    """
+    Test that extract_cacheable_prefix correctly handles messages where:
+    - content is a string (not a list of content blocks)
+    - cache_control is a sibling key at the message level
+
+    This is a valid message format per LiteLLM's ChatCompletionUserMessage type:
+    {"role": "user", "content": "...", "cache_control": {"type": "ephemeral"}}
+
+    Regression test for issue #19228.
+    """
+    # Test case 1: Single message with string content and message-level cache_control
+    messages_string_content = [
+        {"role": "system", "content": "You are a helpful assistant"},
+        {
+            "role": "user",
+            "content": "This is a large message that should be cached",
+            "cache_control": {"type": "ephemeral", "ttl": "5m"},
+        },
+    ]
+
+    result = PromptCachingCache.extract_cacheable_prefix(messages_string_content)
+
+    # Should return both messages (system + user with cache_control)
+    assert len(result) == 2, f"Expected 2 messages, got {len(result)}"
+    assert result[0]["role"] == "system"
+    assert result[1]["role"] == "user"
+    assert result[1]["content"] == "This is a large message that should be cached"
+    assert result[1].get("cache_control") == {"type": "ephemeral", "ttl": "5m"}
+
+
+def test_extract_cacheable_prefix_with_string_content_no_cache_control():
+    """
+    Test that extract_cacheable_prefix returns empty list when:
+    - content is a string
+    - no cache_control is present
+    """
+    messages_no_cache = [
+        {"role": "system", "content": "You are a helpful assistant"},
+        {"role": "user", "content": "Hello"},
+    ]
+
+    result = PromptCachingCache.extract_cacheable_prefix(messages_no_cache)
+
+    # Should return empty list (no cacheable content)
+    assert len(result) == 0, f"Expected 0 messages, got {len(result)}"
+
+
+def test_extract_cacheable_prefix_mixed_string_and_list_content():
+    """
+    Test that extract_cacheable_prefix handles messages with a mix of:
+    - String content with message-level cache_control
+    - List content with block-level cache_control
+
+    The last cache_control (regardless of format) should determine the cacheable prefix.
+    """
+    # Message with string content + cache_control, followed by message with list content + cache_control
+    messages_mixed = [
+        {"role": "system", "content": "You are a helpful assistant"},
+        {
+            "role": "user",
+            "content": "First cached message",
+            "cache_control": {"type": "ephemeral"},
+        },
+        {
+            "role": "user",
+            "content": [
+                {
+                    "type": "text",
+                    "text": "Second cached message in list format",
+                    "cache_control": {"type": "ephemeral"},
+                }
+            ],
+        },
+        {"role": "user", "content": "This should not be in the prefix"},
+    ]
+
+    result = PromptCachingCache.extract_cacheable_prefix(messages_mixed)
+
+    # Should include first 3 messages (up to and including the last cache_control)
+    assert len(result) == 3, f"Expected 3 messages, got {len(result)}"
+    assert result[0]["role"] == "system"
+    assert result[1]["content"] == "First cached message"
+    assert isinstance(result[2]["content"], list)