fix: handle empty or missing tool-call arguments in from_openai_dict_format

`ChatMessage.from_openai_dict_format` did `json.loads(tc["function"]["arguments"])`,
assuming `arguments` is always present and a valid JSON string. Zero-argument tool
calls from OpenAI-compatible servers (vLLM, llama.cpp, Ollama, LM Studio, ...)
send an empty string, null, or omit the key entirely, which raised
`json.JSONDecodeError` or `KeyError` — even though the method docstring courts
these 'shallow OpenAI-compatible APIs'. The message validator only checks that a
`function` field exists, so nothing guarded this.

Treat empty/null/missing arguments as an empty dict, leaving valid-JSON parsing
unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: chuenchen309

haystack/dataclasses/chat_message.py

@@ -792,10 +792,13 @@ def from_openai_dict_format(cls, message: dict[str, Any]) -> "ChatMessage":
             if tool_calls:
                 haystack_tool_calls = []
                 for tc in tool_calls:
+                    # Zero-argument tool calls from OpenAI-compatible servers may send an
+                    # empty string, null, or omit `arguments` entirely; treat all as {}.
+                    raw_arguments = tc["function"].get("arguments")
                     haystack_tc = ToolCall(
                         id=tc.get("id"),
                         tool_name=tc["function"]["name"],
-                        arguments=json.loads(tc["function"]["arguments"]),
+                        arguments=json.loads(raw_arguments) if raw_arguments else {},
                     )
                     haystack_tool_calls.append(haystack_tc)
             return cls.from_assistant(text=content, name=name, tool_calls=haystack_tool_calls)

releasenotes/notes/fix-openai-toolcall-empty-arguments-53c5621622bc1866.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed `ChatMessage.from_openai_dict_format` crashing on zero-argument tool
+    calls. OpenAI-compatible servers (vLLM, llama.cpp, Ollama, LM Studio, ...) may
+    send an empty string or omit the `arguments` field for a tool call that takes
+    no arguments; this previously raised `json.JSONDecodeError` or `KeyError`.
+    Such tool calls are now parsed into a `ToolCall` with empty `arguments`.

test/dataclasses/test_chat_message.py

@@ -966,6 +966,27 @@ def test_from_openai_dict_format_assistant_message_with_tool_calls(self):
         assert tool_call.tool_name == "get_weather"
         assert tool_call.arguments == {"location": "Berlin"}
 
+    def test_from_openai_dict_format_tool_call_with_empty_arguments(self):
+        # OpenAI-compatible servers (vLLM, llama.cpp, Ollama, ...) emit an empty
+        # string for a zero-argument tool call; it must not crash.
+        openai_msg = {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [{"id": "call_1", "function": {"name": "now", "arguments": ""}}],
+        }
+        message = ChatMessage.from_openai_dict_format(openai_msg)
+        assert message.tool_call == ToolCall(id="call_1", tool_name="now", arguments={})
+
+    def test_from_openai_dict_format_tool_call_with_missing_arguments(self):
+        # Some servers omit the `arguments` key entirely for zero-argument calls.
+        openai_msg = {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [{"id": "call_1", "function": {"name": "now"}}],
+        }
+        message = ChatMessage.from_openai_dict_format(openai_msg)
+        assert message.tool_call.arguments == {}
+
     def test_from_openai_dict_format_tool_message(self):
         openai_msg = {"role": "tool", "content": "The weather is sunny", "tool_call_id": "call_123"}
         message = ChatMessage.from_openai_dict_format(openai_msg)