feat(OpenAICompatProvider): enhance tool call handling with provider-specific fields

tests/providers/test_litellm_kwargs.py

@@ -29,6 +29,29 @@ def _fake_chat_response(content: str = "ok") -> SimpleNamespace:
     return SimpleNamespace(choices=[choice], usage=usage)
 
 
+def _fake_tool_call_response() -> SimpleNamespace:
+    """Build a minimal chat response that includes Gemini-style provider fields."""
+    function = SimpleNamespace(
+        name="exec",
+        arguments='{"cmd":"ls"}',
+        provider_specific_fields={"inner": "value"},
+    )
+    tool_call = SimpleNamespace(
+        id="call_123",
+        index=0,
+        function=function,
+        provider_specific_fields={"thought_signature": "signed-token"},
+    )
+    message = SimpleNamespace(
+        content=None,
+        tool_calls=[tool_call],
+        reasoning_content=None,
+    )
+    choice = SimpleNamespace(message=message, finish_reason="tool_calls")
+    usage = SimpleNamespace(prompt_tokens=10, completion_tokens=5, total_tokens=15)
+    return SimpleNamespace(choices=[choice], usage=usage)
+
+
 def test_openrouter_spec_is_gateway() -> None:
     spec = find_by_name("openrouter")
     assert spec is not None
@@ -110,6 +133,37 @@ async def test_standard_provider_passes_model_through() -> None:
     assert call_kwargs["model"] == "deepseek-chat"
 
 
+@pytest.mark.asyncio
+async def test_openai_compat_preserves_provider_specific_fields_on_tool_calls() -> None:
+    """Gemini thought signatures must survive parsing so they can be sent back."""
+    mock_create = AsyncMock(return_value=_fake_tool_call_response())
+    spec = find_by_name("gemini")
+
+    with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI") as MockClient:
+        client_instance = MockClient.return_value
+        client_instance.chat.completions.create = mock_create
+
+        provider = OpenAICompatProvider(
+            api_key="test-key",
+            api_base="https://generativelanguage.googleapis.com/v1beta/openai/",
+            default_model="google/gemini-3.1-pro-preview",
+            spec=spec,
+        )
+        result = await provider.chat(
+            messages=[{"role": "user", "content": "run exec"}],
+            model="google/gemini-3.1-pro-preview",
+        )
+
+    assert len(result.tool_calls) == 1
+    tool_call = result.tool_calls[0]
+    assert tool_call.provider_specific_fields == {"thought_signature": "signed-token"}
+    assert tool_call.function_provider_specific_fields == {"inner": "value"}
+
+    serialized = tool_call.to_openai_tool_call()
+    assert serialized["provider_specific_fields"] == {"thought_signature": "signed-token"}
+    assert serialized["function"]["provider_specific_fields"] == {"inner": "value"}
+
+
 def test_openai_model_passthrough() -> None:
     """OpenAI models pass through unchanged."""
     spec = find_by_name("openai")