fix: normalize tool-call arguments for strict providers

Ensure assistant tool-call function.arguments is always emitted as valid JSON text so strict OpenAI-compatible backends (including Alibaba code models) do not reject requests. Add regressions for dict and malformed-string argument payloads in message sanitization.

Made-with: Cursor

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")