diff --git a/nanobot/providers/openai_compat_provider.py b/nanobot/providers/openai_compat_provider.py
index 4dea2d5f..bf82e538 100644
--- a/nanobot/providers/openai_compat_provider.py
+++ b/nanobot/providers/openai_compat_provider.py
@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import asyncio
+import json
 import hashlib
 import importlib.util
 import os
@@ -222,6 +223,24 @@ class OpenAICompatProvider(LLMProvider):
             return tool_call_id
         return hashlib.sha1(tool_call_id.encode()).hexdigest()[:9]
 
+    @staticmethod
+    def _normalize_tool_call_arguments(arguments: Any) -> str:
+        """Force function.arguments into a valid JSON object string."""
+        if isinstance(arguments, str):
+            stripped = arguments.strip()
+            if not stripped:
+                return "{}"
+            try:
+                parsed = json_repair.loads(stripped)
+            except Exception:
+                return "{}"
+            if isinstance(parsed, dict):
+                return json.dumps(parsed, ensure_ascii=False)
+            return "{}"
+        if isinstance(arguments, dict):
+            return json.dumps(arguments, ensure_ascii=False)
+        return "{}"
+
     def _sanitize_messages(self, messages: list[dict[str, Any]]) -> list[dict[str, Any]]:
         """Strip non-standard keys, normalize tool_call IDs."""
         sanitized = LLMProvider._sanitize_request_messages(messages, _ALLOWED_MSG_KEYS)
@@ -241,6 +260,16 @@ class OpenAICompatProvider(LLMProvider):
                         continue
                     tc_clean = dict(tc)
                     tc_clean["id"] = map_id(tc_clean.get("id"))
+                    function = tc_clean.get("function")
+                    if isinstance(function, dict):
+                        function_clean = dict(function)
+                        if "arguments" in function_clean:
+                            function_clean["arguments"] = self._normalize_tool_call_arguments(
+                                function_clean.get("arguments")
+                            )
+                        else:
+                            function_clean["arguments"] = "{}"
+                        tc_clean["function"] = function_clean
                     normalized.append(tc_clean)
                 clean["tool_calls"] = normalized
                 if clean.get("role") == "assistant":
diff --git a/tests/providers/test_litellm_kwargs.py b/tests/providers/test_litellm_kwargs.py
index ec2581cd..31bdaa55 100644
--- a/tests/providers/test_litellm_kwargs.py
+++ b/tests/providers/test_litellm_kwargs.py
@@ -584,6 +584,54 @@ def test_openai_compat_keeps_tool_calls_after_consecutive_assistant_messages() -
     assert sanitized[2]["tool_call_id"] == "3ec83c30d"
 
 
+def test_openai_compat_stringifies_dict_tool_arguments() -> None:
+    with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI"):
+        provider = OpenAICompatProvider()
+
+    sanitized = provider._sanitize_messages([
+        {"role": "user", "content": "hi"},
+        {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [
+                {
+                    "id": "call_1",
+                    "type": "function",
+                    "function": {"name": "exec", "arguments": {"cmd": "ls -la"}},
+                }
+            ],
+        },
+        {"role": "tool", "tool_call_id": "call_1", "name": "exec", "content": "ok"},
+        {"role": "user", "content": "done"},
+    ])
+
+    assert sanitized[1]["tool_calls"][0]["function"]["arguments"] == '{"cmd": "ls -la"}'
+
+
+def test_openai_compat_repairs_non_json_tool_arguments_string() -> None:
+    with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI"):
+        provider = OpenAICompatProvider()
+
+    sanitized = provider._sanitize_messages([
+        {"role": "user", "content": "hi"},
+        {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [
+                {
+                    "id": "call_1",
+                    "type": "function",
+                    "function": {"name": "exec", "arguments": "{'cmd': 'pwd'}"},
+                }
+            ],
+        },
+        {"role": "tool", "tool_call_id": "call_1", "name": "exec", "content": "ok"},
+        {"role": "user", "content": "done"},
+    ])
+
+    assert sanitized[1]["tool_calls"][0]["function"]["arguments"] == '{"cmd": "pwd"}'
+
+
 @pytest.mark.asyncio
 async def test_openai_compat_stream_watchdog_returns_error_on_stall(monkeypatch) -> None:
     monkeypatch.setenv("NANOBOT_STREAM_IDLE_TIMEOUT_S", "0")