fix(anthropic): order messages after system and tools in /v1/messages request body

The /v1/messages transform built the upstream body with messages as the first key, so the part that
changes every turn led the request and the stable system and tools prefix followed it. On Vertex AI
that ordering makes follow-up turns miss the prompt cache much more often. #36079 moves messages last
for the chat completions path, and this applies the same ordering to the Anthropic Messages path,
which Vertex AI inherits

litellm/llms/anthropic/pass_through/messages/transformation.py

@@ -693,12 +693,12 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
             messages = strip_advisor_blocks_from_messages(messages)
 
         anthropic_messages_request: Final[AnthropicMessagesRequest] = AnthropicMessagesRequest(
-            messages=strip_encrypted_reasoning_blocks_from_anthropic_messages(
-                drop_non_string_tool_result_texts(messages, getattr(litellm_params, "drop_params", None))
-            ),
             max_tokens=max_tokens,
             model=model,
             **anthropic_messages_optional_request_params,
+            messages=strip_encrypted_reasoning_blocks_from_anthropic_messages(
+                drop_non_string_tool_result_texts(messages, getattr(litellm_params, "drop_params", None))
+            ),
         )
         return dict(anthropic_messages_request)
 

tests/unit/llms/anthropic/pass_through/messages/test_anthropic_experimental_pass_through_messages_transformation.py

@@ -364,3 +364,21 @@ def test_drop_params_forwards_a_non_object_message_item_as_sent_on_the_native_re
 
     assert request["messages"][1] == 7
     assert request["messages"][2]["content"][0]["content"] == [{"type": "text", "text": "sunny"}]
+
+
+def test_messages_request_body_ends_with_messages_after_system_and_tools() -> None:
+    messages: Final = [{"role": "user", "content": "Hello"}]
+    request: Final = AnthropicMessagesConfig().transform_anthropic_messages_request(
+        model="claude-opus-5-5",
+        messages=messages,
+        anthropic_messages_optional_request_params={
+            "max_tokens": 256,
+            "system": [{"type": "text", "text": "You are terse."}],
+            "tools": [{"name": "get_weather", "input_schema": {"type": "object", "properties": {}}}],
+        },
+        litellm_params=GenericLiteLLMParams(),
+        headers={},
+    )
+
+    assert list(request)[-1] == "messages", f"key order sent upstream: {list(request)}"
+    assert request["messages"] == messages

tests/unit/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py

@@ -784,3 +784,22 @@ def test_vertex_claude_4_8_plus_cost_map_entries_carry_mid_conversation_system_f
         and info.get("supports_mid_conversation_system") is not True
     ]
     assert missing == []
+
+
+def test_vertex_messages_request_body_puts_system_and_tools_before_messages() -> None:
+    result: Final = VertexAIPartnerModelsAnthropicMessagesConfig().transform_anthropic_messages_request(
+        model="claude-opus-5-5",
+        messages=[{"role": "user", "content": "Hello"}],
+        anthropic_messages_optional_request_params={
+            "max_tokens": 256,
+            "system": [{"type": "text", "text": "You are terse."}],
+            "tools": [{"name": "get_weather", "input_schema": {"type": "object", "properties": {}}}],
+        },
+        litellm_params=GenericLiteLLMParams(),
+        headers={},
+    )
+
+    keys: Final = list(result)
+    assert keys.index("system") < keys.index("messages"), f"key order sent to Vertex: {keys}"
+    assert keys.index("tools") < keys.index("messages"), f"key order sent to Vertex: {keys}"
+    assert result["messages"] == [{"role": "user", "content": "Hello"}]