Python: Preserve MCP array items schema in Pydantic field generation (#2382)

* mcp tool parameter fix

* small fixes

Author: giles17

python/packages/core/agent_framework/_mcp.py

@@ -278,18 +278,30 @@ def resolve_type(prop_details: dict[str, Any]) -> type:
         python_type = resolve_type(prop_details)
         description = prop_details.get("description", "")
 
+        # Build field kwargs (description, array items schema, etc.)
+        field_kwargs: dict[str, Any] = {}
+        if description:
+            field_kwargs["description"] = description
+
+        # Preserve array items schema if present
+        if prop_details.get("type") == "array" and "items" in prop_details:
+            items_schema = prop_details["items"]
+            if items_schema and items_schema != {}:
+                field_kwargs["json_schema_extra"] = {"items": items_schema}
+
         # Create field definition for create_model
         if prop_name in required:
-            field_definitions[prop_name] = (
-                (python_type, Field(description=description)) if description else (python_type, ...)
-            )
+            if field_kwargs:
+                field_definitions[prop_name] = (python_type, Field(**field_kwargs))
+            else:
+                field_definitions[prop_name] = (python_type, ...)
         else:
             default_value = prop_details.get("default", None)
-            field_definitions[prop_name] = (
-                (python_type, Field(default=default_value, description=description))
-                if description
-                else (python_type, default_value)
-            )
+            field_kwargs["default"] = default_value
+            if field_kwargs and any(k != "default" for k in field_kwargs):
+                field_definitions[prop_name] = (python_type, Field(**field_kwargs))
+            else:
+                field_definitions[prop_name] = (python_type, default_value)
 
     return create_model(f"{tool.name}_input", **field_definitions)
 

python/packages/core/tests/core/test_mcp.py

@@ -483,6 +483,36 @@ def test_get_input_model_from_mcp_tool_with_ref_schema():
     assert dumped == {"params": {"customer_id": 251}}
 
 
+def test_get_input_model_from_mcp_tool_with_simple_array():
+    """Test array with simple items schema (items schema should be preserved in json_schema_extra)."""
+    tool = types.Tool(
+        name="simple_array_tool",
+        description="Tool with simple array",
+        inputSchema={
+            "type": "object",
+            "properties": {
+                "tags": {
+                    "type": "array",
+                    "description": "List of tags",
+                    "items": {"type": "string"},  # Simple string array
+                }
+            },
+            "required": ["tags"],
+        },
+    )
+    model = _get_input_model_from_mcp_tool(tool)
+
+    # Create an instance
+    instance = model(tags=["tag1", "tag2", "tag3"])
+    assert instance.tags == ["tag1", "tag2", "tag3"]
+
+    # Verify JSON schema still preserves items for simple types
+    json_schema = model.model_json_schema()
+    tags_property = json_schema["properties"]["tags"]
+    assert "items" in tags_property
+    assert tags_property["items"]["type"] == "string"
+
+
 def test_get_input_model_from_mcp_prompt():
     """Test creation of input model from MCP prompt."""
     prompt = types.Prompt(