diff --git a/mini_agent/llm/openai_client.py b/mini_agent/llm/openai_client.py
index b8f891c..6e8a35b 100644
--- a/mini_agent/llm/openai_client.py
+++ b/mini_agent/llm/openai_client.py
@@ -144,13 +144,19 @@ def _convert_messages(self, messages: list[Message]) -> tuple[str | None, list[d
                 if msg.tool_calls:
                     tool_calls_list = []
                     for tool_call in msg.tool_calls:
+                        arguments_json = tool_call.function.arguments_json
+                        if arguments_json is None:
+                            # Fall back to deterministic dump if raw string missing
+                            arguments_json = json.dumps(
+                                tool_call.function.arguments, separators=(",", ":"), sort_keys=True
+                            )
                         tool_calls_list.append(
                             {
                                 "id": tool_call.id,
                                 "type": "function",
                                 "function": {
                                     "name": tool_call.function.name,
-                                    "arguments": json.dumps(tool_call.function.arguments),
+                                    "arguments": arguments_json,
                                 },
                             }
                         )
@@ -223,8 +229,9 @@ def _parse_response(self, response: Any) -> LLMResponse:
         tool_calls = []
         if response.tool_calls:
             for tool_call in response.tool_calls:
-                # Parse arguments from JSON string
-                arguments = json.loads(tool_call.function.arguments)
+                # Parse arguments from JSON string while preserving the raw text
+                raw_arguments = tool_call.function.arguments
+                arguments = json.loads(raw_arguments) if raw_arguments else {}
 
                 tool_calls.append(
                     ToolCall(
@@ -233,6 +240,7 @@ def _parse_response(self, response: Any) -> LLMResponse:
                         function=FunctionCall(
                             name=tool_call.function.name,
                             arguments=arguments,
+                            arguments_json=raw_arguments,
                         ),
                     )
                 )
diff --git a/mini_agent/schema/schema.py b/mini_agent/schema/schema.py
index d3b032f..910c5ca 100644
--- a/mini_agent/schema/schema.py
+++ b/mini_agent/schema/schema.py
@@ -15,7 +15,8 @@ class FunctionCall(BaseModel):
     """Function call details."""
 
     name: str
-    arguments: dict[str, Any]  # Function arguments as dict
+    arguments: dict[str, Any]  # Parsed function arguments
+    arguments_json: str | None = None  # Raw JSON string (for deterministic replay)
 
 
 class ToolCall(BaseModel):