fix(mcp): preserve nullable schema coercion

tests/run_agent/test_tool_arg_coercion.py

@@ -67,7 +67,7 @@ class TestCoerceNumber:
     def test_inf_stays_string_for_integer_only(self):
         """Infinity should not be converted to int."""
         result = _coerce_number("inf")
-        assert result == float("inf")
+        assert result == "inf"
 
     def test_negative_float(self):
         assert _coerce_number("-2.5") == -2.5
@@ -255,6 +255,35 @@ class TestCoerceToolArgs:
             result = coerce_tool_args("test_tool", args)
             assert result["config"] == {"max": 50}
 
+    def test_coerces_string_null_for_nullable_object_arg(self):
+        """Models often emit literal "null" for optional MCP object args."""
+        schema = self._mock_schema({
+            "setting": {
+                "type": "object",
+                "additionalProperties": True,
+                "nullable": True,
+                "default": None,
+            },
+        })
+        with patch("model_tools.registry.get_schema", return_value=schema):
+            args = {"setting": "null"}
+            result = coerce_tool_args("test_tool", args)
+            assert result["setting"] is None
+
+    def test_coerces_string_null_for_nullable_array_arg(self):
+        schema = self._mock_schema({
+            "stages": {
+                "type": "array",
+                "items": {"type": "object"},
+                "nullable": True,
+                "default": None,
+            },
+        })
+        with patch("model_tools.registry.get_schema", return_value=schema):
+            args = {"stages": "null"}
+            result = coerce_tool_args("test_tool", args)
+            assert result["stages"] is None
+
     def test_invalid_json_array_preserved_as_string(self):
         """If the string isn't valid JSON, pass it through — let the tool decide."""
         schema = self._mock_schema({"items": {"type": "array"}})

tests/tools/test_mcp_tool.py

@@ -285,6 +285,7 @@ class TestSchemaConversion:
 
         assert schema["properties"]["workdir"] == {
             "type": "string",
+            "nullable": True,
             "default": None,
             "description": "Optional working directory",
         }
@@ -314,6 +315,7 @@ class TestSchemaConversion:
         assert schema["properties"]["filters"]["items"] == {
             "type": "object",
             "properties": {"field": {"type": "string"}},
+            "nullable": True,
         }
 
     def test_convert_mcp_schema_survives_missing_inputschema_attribute(self):