fix(ollama): raise BadRequestError on malformed tool call JSON arguments

Wrap json.loads() in transform_request so callers get a descriptive
litellm.BadRequestError instead of a raw json.JSONDecodeError when an
Ollama-compatible model returns truncated or otherwise invalid JSON in
tool call arguments.

Fixes #25985

litellm/llms/ollama/chat/transformation.py

@@ -1,6 +1,7 @@
 import json
 import time
 from litellm._uuid import uuid
+from litellm._logging import verbose_logger
 from typing import (
     TYPE_CHECKING,
     Any,
@@ -272,7 +273,18 @@ class OllamaChatConfig(BaseConfig):
                     if typed_tool["type"] == "function":
                         arguments = {}
                         if "arguments" in typed_tool["function"]:
-                            arguments = json.loads(typed_tool["function"]["arguments"])
+                            raw_args = typed_tool["function"]["arguments"]
+                            try:
+                                arguments = json.loads(raw_args)
+                            except json.JSONDecodeError as e:
+                                verbose_logger.error(
+                                    f"Failed to parse tool call arguments as JSON: {raw_args!r}. Error: {e}"
+                                )
+                                raise litellm.BadRequestError(
+                                    message=f"Tool call arguments contain malformed JSON: {e.msg}. Raw arguments: {raw_args!r}",
+                                    model="ollama",
+                                    llm_provider="ollama",
+                                )
                         ollama_tool_call = OllamaToolCall(
                             function=OllamaToolCallFunction(
                                 name=typed_tool["function"].get("name") or "",

tests/test_litellm/llms/ollama/test_ollama_chat_transformation.py

@@ -338,7 +338,84 @@ class TestOllamaToolCalling:
     Issue: https://github.com/BerriAI/litellm/issues/18922
     """
 
-    def test_tools_passed_directly_without_capability_check(self):
+    def test_transform_request_malformed_tool_call_arguments_raises_bad_request(self):
+        """Test that malformed JSON in tool call arguments raises BadRequestError.
+
+        Regression: json.JSONDecodeError was previously raised directly, wrapping it
+        in litellm.BadRequestError gives callers an actionable error with context.
+
+        Issue: https://github.com/BerriAI/litellm/issues/25985
+        """
+        config = OllamaChatConfig()
+
+        messages = cast(
+            list[AllMessageValues],
+            [
+                {
+                    "role": "assistant",
+                    "content": None,
+                    "tool_calls": [
+                        {
+                            "id": "call_123",
+                            "type": "function",
+                            "function": {
+                                "name": "get_weather",
+                                "arguments": '{"city": "Toky',  # truncated JSON
+                            },
+                        }
+                    ],
+                }
+            ],
+        )
+
+        import litellm
+
+        with pytest.raises(litellm.BadRequestError) as exc_info:
+            config.transform_request(
+                model="qwen3:14b",
+                messages=messages,
+                optional_params={},
+                litellm_params={},
+                headers={},
+            )
+
+        assert "malformed JSON" in str(exc_info.value)
+
+    def test_transform_request_valid_tool_call_arguments_passes(self):
+        """Test that valid JSON tool call arguments are parsed without raising an exception."""
+        config = OllamaChatConfig()
+
+        messages = cast(
+            list[AllMessageValues],
+            [
+                {
+                    "role": "assistant",
+                    "content": None,
+                    "tool_calls": [
+                        {
+                            "id": "call_123",
+                            "type": "function",
+                            "function": {
+                                "name": "get_weather",
+                                "arguments": '{"city": "Tokyo"}',
+                            },
+                        }
+                    ],
+                }
+            ],
+        )
+
+        # Should not raise any exception
+        result = config.transform_request(
+            model="qwen3:14b",
+            messages=messages,
+            optional_params={},
+            litellm_params={},
+            headers={},
+        )
+        assert "messages" in result
+
+
         """Test that tools are passed directly to Ollama without model capability checks.
 
         Previously, the code called litellm.get_model_info() which could fail