diff --git a/haystack/dataclasses/chat_message.py b/haystack/dataclasses/chat_message.py
index a305e4187c1..e18a6221db7 100644
--- a/haystack/dataclasses/chat_message.py
+++ b/haystack/dataclasses/chat_message.py
@@ -16,6 +16,20 @@
 logger = logging.getLogger(__name__)
 
 
+def _parse_openai_tool_call_arguments(raw_arguments: Any) -> dict[str, Any]:
+    """
+    Parse OpenAI-style tool call arguments into a dictionary.
+
+    A dict is returned as is, a missing, null, or empty value becomes `{}`, and anything else is parsed as a
+    JSON string.
+
+    :raises json.JSONDecodeError: If the arguments are not valid JSON.
+    """
+    if isinstance(raw_arguments, dict):
+        return raw_arguments
+    return json.loads(raw_arguments) if raw_arguments else {}
+
+
 class ChatRole(str, Enum):
     """
     Enumeration representing the roles within a chat.
@@ -815,11 +829,12 @@ def from_openai_dict_format(cls, message: dict[str, Any]) -> "ChatMessage":
                 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 {}.
+                    # Some servers also send a parsed dict instead of a JSON string.
                     raw_arguments = tc["function"].get("arguments")
                     haystack_tc = ToolCall(
                         id=tc.get("id"),
                         tool_name=tc["function"]["name"],
-                        arguments=json.loads(raw_arguments) if raw_arguments else {},
+                        arguments=_parse_openai_tool_call_arguments(raw_arguments),
                     )
                     haystack_tool_calls.append(haystack_tc)
             return cls.from_assistant(text=content, name=name, tool_calls=haystack_tool_calls)
diff --git a/releasenotes/notes/fix-openai-dict-tool-arguments-c8e1a4b27f90d3a1.yaml b/releasenotes/notes/fix-openai-dict-tool-arguments-c8e1a4b27f90d3a1.yaml
new file mode 100644
index 00000000000..4fe034a59c8
--- /dev/null
+++ b/releasenotes/notes/fix-openai-dict-tool-arguments-c8e1a4b27f90d3a1.yaml
@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    ``ChatMessage.from_openai_dict_format`` now accepts tool-call ``arguments`` that are already a dictionary
+    instead of raising a ``TypeError``. Some OpenAI-compatible servers send a parsed object rather than a JSON string.
diff --git a/test/dataclasses/test_chat_message.py b/test/dataclasses/test_chat_message.py
index b476b59c80c..5b4fd1e042e 100644
--- a/test/dataclasses/test_chat_message.py
+++ b/test/dataclasses/test_chat_message.py
@@ -1082,6 +1082,28 @@ def test_from_openai_dict_format_tool_call_with_missing_arguments(self):
         assert message.tool_call is not None
         assert message.tool_call.arguments == {}
 
+    def test_from_openai_dict_format_tool_call_with_dict_arguments(self):
+        # Some OpenAI-compatible servers already parse arguments into a dict.
+        openai_msg = {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {"id": "call_123", "function": {"name": "get_weather", "arguments": {"location": "Berlin"}}}
+            ],
+        }
+        message = ChatMessage.from_openai_dict_format(openai_msg)
+        assert message.tool_call is not None
+        assert message.tool_call.arguments == {"location": "Berlin"}
+
+    def test_from_openai_dict_format_tool_call_with_invalid_json_arguments(self):
+        openai_msg = {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [{"id": "call_1", "function": {"name": "now", "arguments": "{not-json"}}],
+        }
+        with pytest.raises(json.JSONDecodeError):
+            ChatMessage.from_openai_dict_format(openai_msg)
+
     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)