fix(sap): default FunctionObj.arguments for streaming tool calls

Streaming tool-call deltas from Claude/SAP often omit `arguments` on
intermediate chunks (name only). FunctionObj required the field, so
pydantic ValidationError crashed the proxy on multi-turn streamed tool use.

Default arguments to "" and add unit tests covering FunctionObj,
MessageToolCall, and SAPAssistantMessage.

Fixes #46405

litellm/llms/sap/chat/models.py

@@ -65,7 +65,9 @@ class ImageContent(LiteLLMBaseModel):
 
 class FunctionObj(LiteLLMBaseModel):
     name: str
-    arguments: str
+    # Default empty string: streaming tool-call deltas often omit arguments
+    # until the final chunk (OpenAI/Anthropic streaming semantics).
+    arguments: str = ""
 
 
 class FunctionTool(LiteLLMBaseModel):

tests/unit/llms/sap/chat/test_sap_models.py

@@ -2,6 +2,8 @@ import pytest
 from pydantic import ValidationError
 
 from litellm.llms.sap.chat.models import (
+    FunctionObj,
+    MessageToolCall,
     SAPAssistantMessage,
     SAPMessage,
     SAPToolChatMessage,
@@ -168,6 +170,58 @@ class TestSAPAssistantMessage:
         assert msg.content.cache_control.type == "ephemeral"
 
 
+class TestFunctionObj:
+    def test_name_and_arguments_accepted(self):
+        obj = FunctionObj.model_validate({"name": "get_weather", "arguments": '{"city":"Berlin"}'})
+        assert obj.name == "get_weather"
+        assert obj.arguments == '{"city":"Berlin"}'
+
+    def test_missing_arguments_defaults_to_empty_string(self):
+        # Streaming intermediate tool-call chunks often omit arguments (name only).
+        obj = FunctionObj.model_validate({"name": "get_weather"})
+        assert obj.name == "get_weather"
+        assert obj.arguments == ""
+
+    def test_explicit_empty_arguments_accepted(self):
+        obj = FunctionObj.model_validate({"name": "get_weather", "arguments": ""})
+        assert obj.arguments == ""
+
+    def test_missing_name_rejected(self):
+        with pytest.raises(ValidationError):
+            FunctionObj.model_validate({"arguments": "{}"})
+
+
+class TestMessageToolCall:
+    def test_tool_call_with_arguments(self):
+        tc = MessageToolCall.model_validate({
+            "id": "call_1",
+            "type": "function",
+            "function": {"name": "get_weather", "arguments": "{}"},
+        })
+        assert tc.function.name == "get_weather"
+        assert tc.function.arguments == "{}"
+
+    def test_tool_call_without_arguments_for_streaming_delta(self):
+        tc = MessageToolCall.model_validate({
+            "id": "call_1",
+            "type": "function",
+            "function": {"name": "get_weather"},
+        })
+        assert tc.function.arguments == ""
+
+    def test_assistant_message_streaming_tool_call_without_arguments(self):
+        msg = SAPAssistantMessage.model_validate({
+            "tool_calls": [{
+                "id": "call_1",
+                "type": "function",
+                "function": {"name": "get_weather"},
+            }]
+        })
+        assert len(msg.tool_calls) == 1
+        assert msg.tool_calls[0].function.name == "get_weather"
+        assert msg.tool_calls[0].function.arguments == ""
+
+
 class TestSAPToolChatMessage:
     def test_role_is_always_tool(self):
         msg = SAPToolChatMessage.model_validate({"tool_call_id": "call_1", "content": "ok"})