diff --git a/src/openharness/tools/mcp_tool.py b/src/openharness/tools/mcp_tool.py
index 095db7e..ed79dd9 100644
--- a/src/openharness/tools/mcp_tool.py
+++ b/src/openharness/tools/mcp_tool.py
@@ -29,13 +29,23 @@ class McpToolAdapter(BaseTool):
             output = await self._manager.call_tool(
                 self._tool_info.server_name,
                 self._tool_info.name,
-                arguments.model_dump(mode="json"),
+                arguments.model_dump(mode="json", exclude_none=True),
             )
         except McpServerNotConnectedError as exc:
             return ToolResult(output=str(exc), is_error=True)
         return ToolResult(output=output)
 
 
+_JSON_TYPE_MAP: dict[str, type] = {
+    "string": str,
+    "integer": int,
+    "number": float,
+    "boolean": bool,
+    "array": list,
+    "object": dict,
+}
+
+
 def _input_model_from_schema(tool_name: str, schema: dict[str, object]) -> type[BaseModel]:
     properties = schema.get("properties", {})
     if not isinstance(properties, dict):
@@ -44,8 +54,12 @@ def _input_model_from_schema(tool_name: str, schema: dict[str, object]) -> type[
     fields = {}
     required = set(schema.get("required", [])) if isinstance(schema.get("required", []), list) else set()
     for key in properties:
-        default = ... if key in required else None
-        fields[key] = (object | None, Field(default=default))
+        prop = properties[key] if isinstance(properties[key], dict) else {}
+        py_type = _JSON_TYPE_MAP.get(str(prop.get("type", "")), object)
+        if key in required:
+            fields[key] = (py_type, Field(default=...))
+        else:
+            fields[key] = (py_type | None, Field(default=None))
     return create_model(f"{tool_name.title().replace('-', '_')}Input", **fields)
 
 
diff --git a/tests/test_tools/test_mcp_tool.py b/tests/test_tools/test_mcp_tool.py
new file mode 100644
index 0000000..655ed9f
--- /dev/null
+++ b/tests/test_tools/test_mcp_tool.py
@@ -0,0 +1,94 @@
+"""Tests for MCP tool adapters — input model generation and argument serialization."""
+
+import pytest
+from pydantic import ValidationError
+
+from openharness.tools.mcp_tool import _input_model_from_schema
+
+
+class TestInputModelFromSchema:
+    """Verify _input_model_from_schema maps JSON Schema types correctly."""
+
+    def test_required_string_rejects_none(self):
+        schema = {
+            "type": "object",
+            "properties": {"query": {"type": "string"}},
+            "required": ["query"],
+        }
+        Model = _input_model_from_schema("search", schema)
+        with pytest.raises(ValidationError):
+            Model(query=None)
+
+    def test_required_string_accepts_value(self):
+        schema = {
+            "type": "object",
+            "properties": {"query": {"type": "string"}},
+            "required": ["query"],
+        }
+        Model = _input_model_from_schema("search", schema)
+        m = Model(query="zigzag")
+        assert m.query == "zigzag"
+
+    def test_optional_string_defaults_to_none(self):
+        schema = {
+            "type": "object",
+            "properties": {
+                "query": {"type": "string"},
+                "wing": {"type": "string"},
+            },
+            "required": ["query"],
+        }
+        Model = _input_model_from_schema("search", schema)
+        m = Model(query="test")
+        assert m.wing is None
+
+    def test_exclude_none_omits_optional_keeps_required(self):
+        schema = {
+            "type": "object",
+            "properties": {
+                "query": {"type": "string"},
+                "wing": {"type": "string"},
+                "limit": {"type": "integer"},
+            },
+            "required": ["query"],
+        }
+        Model = _input_model_from_schema("search", schema)
+        m = Model(query="test")
+        dumped = m.model_dump(mode="json", exclude_none=True)
+        assert dumped == {"query": "test"}
+
+    def test_all_json_types_mapped(self):
+        schema = {
+            "type": "object",
+            "properties": {
+                "name": {"type": "string"},
+                "count": {"type": "integer"},
+                "score": {"type": "number"},
+                "active": {"type": "boolean"},
+                "tags": {"type": "array"},
+                "meta": {"type": "object"},
+            },
+            "required": ["name", "count", "score", "active", "tags", "meta"],
+        }
+        Model = _input_model_from_schema("full", schema)
+        m = Model(name="x", count=1, score=0.5, active=True, tags=["a"], meta={"k": "v"})
+        dumped = m.model_dump(mode="json")
+        assert dumped == {
+            "name": "x", "count": 1, "score": 0.5,
+            "active": True, "tags": ["a"], "meta": {"k": "v"},
+        }
+
+    def test_empty_schema_creates_valid_model(self):
+        Model = _input_model_from_schema("empty", {"type": "object"})
+        m = Model()
+        assert m.model_dump(mode="json") == {}
+
+    def test_model_rejects_null_for_required_integer(self):
+        schema = {
+            "type": "object",
+            "properties": {"limit": {"type": "integer"}},
+            "required": ["limit"],
+        }
+        Model = _input_model_from_schema("limited", schema)
+        with pytest.raises(ValidationError):
+            Model(limit=None)