fix(mcp): preserve schema semantics during normalization

Only normalize nullable MCP tool schemas for OpenAI-compatible providers so optional params still work without collapsing unrelated unions. Also teach local validation to honor nullable flags and add regression coverage for nullable and non-nullable schemas.

Made-with: Cursor

tests/test_mcp_tool.py

@@ -84,6 +84,69 @@ def _make_wrapper(session: object, *, timeout: float = 0.1) -> MCPToolWrapper:
     return MCPToolWrapper(session, "test", tool_def, tool_timeout=timeout)
 
 
+def test_wrapper_preserves_non_nullable_unions() -> None:
+    tool_def = SimpleNamespace(
+        name="demo",
+        description="demo tool",
+        inputSchema={
+            "type": "object",
+            "properties": {
+                "value": {
+                    "anyOf": [{"type": "string"}, {"type": "integer"}],
+                }
+            },
+        },
+    )
+
+    wrapper = MCPToolWrapper(SimpleNamespace(call_tool=None), "test", tool_def)
+
+    assert wrapper.parameters["properties"]["value"]["anyOf"] == [
+        {"type": "string"},
+        {"type": "integer"},
+    ]
+
+
+def test_wrapper_normalizes_nullable_property_type_union() -> None:
+    tool_def = SimpleNamespace(
+        name="demo",
+        description="demo tool",
+        inputSchema={
+            "type": "object",
+            "properties": {
+                "name": {"type": ["string", "null"]},
+            },
+        },
+    )
+
+    wrapper = MCPToolWrapper(SimpleNamespace(call_tool=None), "test", tool_def)
+
+    assert wrapper.parameters["properties"]["name"] == {"type": "string", "nullable": True}
+
+
+def test_wrapper_normalizes_nullable_property_anyof() -> None:
+    tool_def = SimpleNamespace(
+        name="demo",
+        description="demo tool",
+        inputSchema={
+            "type": "object",
+            "properties": {
+                "name": {
+                    "anyOf": [{"type": "string"}, {"type": "null"}],
+                    "description": "optional name",
+                },
+            },
+        },
+    )
+
+    wrapper = MCPToolWrapper(SimpleNamespace(call_tool=None), "test", tool_def)
+
+    assert wrapper.parameters["properties"]["name"] == {
+        "type": "string",
+        "description": "optional name",
+        "nullable": True,
+    }
+
+
 @pytest.mark.asyncio
 async def test_execute_returns_text_blocks() -> None:
     async def call_tool(_name: str, arguments: dict) -> object:

tests/test_tool_validation.py

@@ -455,6 +455,18 @@ def test_validate_nullable_param_accepts_none() -> None:
     assert errors == []
 
 
+def test_validate_nullable_flag_accepts_none() -> None:
+    """OpenAI-normalized nullable params should still accept None locally."""
+    tool = CastTestTool(
+        {
+            "type": "object",
+            "properties": {"name": {"type": "string", "nullable": True}},
+        }
+    )
+    errors = tool.validate_params({"name": None})
+    assert errors == []
+
+
 def test_cast_nullable_param_no_crash() -> None:
     """cast_params should not crash on nullable type (the original bug)."""
     tool = CastTestTool(