diff --git a/litellm/llms/anthropic/pass_through/messages/transformation.py b/litellm/llms/anthropic/pass_through/messages/transformation.py
index 35db54147c5..582b58e8817 100644
--- a/litellm/llms/anthropic/pass_through/messages/transformation.py
+++ b/litellm/llms/anthropic/pass_through/messages/transformation.py
@@ -706,12 +706,12 @@ 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)
 
diff --git a/tests/unit/llms/anthropic/pass_through/messages/test_anthropic_experimental_pass_through_messages_transformation.py b/tests/unit/llms/anthropic/pass_through/messages/test_anthropic_experimental_pass_through_messages_transformation.py
index 457d5d5e1c8..fe434d023c3 100644
--- a/tests/unit/llms/anthropic/pass_through/messages/test_anthropic_experimental_pass_through_messages_transformation.py
+++ b/tests/unit/llms/anthropic/pass_through/messages/test_anthropic_experimental_pass_through_messages_transformation.py
@@ -416,3 +416,21 @@ 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
diff --git a/tests/unit/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py b/tests/unit/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py
index ae9b9684f2d..fa7c9b383bf 100644
--- a/tests/unit/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py
+++ b/tests/unit/llms/vertex_ai/vertex_ai_partner_models/anthropic/test_vertex_ai_partner_models_anthropic_messages_config.py
@@ -823,3 +823,22 @@ 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"}]