fix: mcp tool with array type should include items

# What does this PR do?


## Test Plan

llama_stack/providers/inline/agents/meta_reference/responses/streaming.py

@@ -50,6 +50,37 @@ from .utils import convert_chat_choice_to_response_message, is_function_tool_cal
 logger = get_logger(name=__name__, category="agents::meta_reference")
 
 
+def convert_tooldef_to_chat_tool(tool_def):
+    """Convert a ToolDef to OpenAI ChatCompletionToolParam format.
+
+    Args:
+        tool_def: ToolDef from the tools API
+
+    Returns:
+        ChatCompletionToolParam suitable for OpenAI chat completion
+    """
+    from openai.types.chat import ChatCompletionToolParam
+
+    from llama_stack.models.llama.datatypes import ToolDefinition, ToolParamDefinition
+    from llama_stack.providers.utils.inference.openai_compat import convert_tooldef_to_openai_tool
+
+    internal_tool_def = ToolDefinition(
+        tool_name=tool_def.name,
+        description=tool_def.description,
+        parameters={
+            param.name: ToolParamDefinition(
+                param_type=param.parameter_type,
+                description=param.description,
+                required=param.required,
+                default=param.default,
+                items=param.items,
+            )
+            for param in tool_def.parameters
+        },
+    )
+    return convert_tooldef_to_openai_tool(internal_tool_def)
+
+
 class StreamingResponseOrchestrator:
     def __init__(
         self,
@@ -556,23 +587,7 @@ class StreamingResponseOrchestrator:
                     continue
                 if not always_allowed or t.name in always_allowed:
                     # Add to chat tools for inference
-                    from llama_stack.models.llama.datatypes import ToolDefinition, ToolParamDefinition
-                    from llama_stack.providers.utils.inference.openai_compat import convert_tooldef_to_openai_tool
-
-                    tool_def = ToolDefinition(
-                        tool_name=t.name,
-                        description=t.description,
-                        parameters={
-                            param.name: ToolParamDefinition(
-                                param_type=param.parameter_type,
-                                description=param.description,
-                                required=param.required,
-                                default=param.default,
-                            )
-                            for param in t.parameters
-                        },
-                    )
-                    openai_tool = convert_tooldef_to_openai_tool(tool_def)
+                    openai_tool = convert_tooldef_to_chat_tool(t)
                     if self.ctx.chat_tools is None:
                         self.ctx.chat_tools = []
                     self.ctx.chat_tools.append(openai_tool)

tests/unit/providers/inline/agents/meta_reference/responses/__init__.py

@@ -0,0 +1,5 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.

tests/unit/providers/inline/agents/meta_reference/responses/test_streaming.py

@@ -0,0 +1,42 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from llama_stack.apis.tools import ToolDef, ToolParameter
+from llama_stack.providers.inline.agents.meta_reference.responses.streaming import (
+    convert_tooldef_to_chat_tool,
+)
+
+
+def test_convert_tooldef_to_chat_tool_preserves_items_field():
+    """Test that array parameters preserve the items field during conversion.
+
+    This test ensures that when converting ToolDef with array-type parameters
+    to OpenAI ChatCompletionToolParam format, the 'items' field is preserved.
+    Without this fix, array parameters would be missing schema information about their items.
+    """
+    tool_def = ToolDef(
+        name="test_tool",
+        description="A test tool with array parameter",
+        parameters=[
+            ToolParameter(
+                name="tags",
+                parameter_type="array",
+                description="List of tags",
+                required=True,
+                items={"type": "string"},
+            )
+        ],
+    )
+
+    result = convert_tooldef_to_chat_tool(tool_def)
+
+    assert result["type"] == "function"
+    assert result["function"]["name"] == "test_tool"
+
+    tags_param = result["function"]["parameters"]["properties"]["tags"]
+    assert tags_param["type"] == "array"
+    assert "items" in tags_param, "items field should be preserved for array parameters"
+    assert tags_param["items"] == {"type": "string"}