Count Queries not working in Built-in MCP server

[saravanan30erd]

Problem Statement

I am experimenting with the Built-in MCP server for query-based search, following this blog post:
https://opensearch.org/blog/introducing-mcp-in-opensearch/

While basic search works, count-style queries do not work as expected when using SearchIndexTool, because the tool does not return search metadata, specifically hits.total.value.

Example query,

{
  "size": 0,
  "track_total_hits": true,
  "query": {
    "range": {
      "product_id": {
        "gt": 8
      }
    }
  }
}

When using "track_total_hits": true directly with OpenSearch, the response includes total hit metadata like:

"hits": {
    "total": {
      "value": 17000,
      "relation": "eq"
    },
    "max_score": null,
    "hits": []
  }

Actual behavior with Built-in MCP SearchIndexTool
But Built-in MCP SearchIndexTool only returns document-level results as described in the documentation: https://docs.opensearch.org/latest/ml-commons-plugin/agents-tools/tools/search-index-tool/

Example tool response shape:

{
"_index": "test",
"_source": {}
}

There is no hits.total.value or equivalent metadata, which makes it impossible to implement count queries using SearchIndexTool.

Implementation

I registered the SearchIndexTool using the zero-configuration example in MCP registration API described here:
https://docs.opensearch.org/latest/ml-commons-plugin/api/mcp-server-apis/register-mcp-tools/#example-request-zero-configuration-tools

It is unclear whether:

• the input_schema is missing required fields, or
• the Built-in MCP server intentionally strips search metadata from the response

There does not appear to be a documented way to request hits.total or other search metadata from SearchIndexTool.

How it worked before

Previously, I was using the Standalone MCP server instead of the Built-in MCP server.

Key differences:

• The Standalone MCP server provides a dedicated CountTool, implemented using the _count API.
• Also the Standalone MCP SearchIndexTool returns full search responses, including hits.total.value.

Summary / Issue

• Built-in MCP SearchIndexTool does not return hits.total.value
• Count-style queries (size: 0, track_total_hits: true) are not supported in practice
• There is no Built-in MCP equivalent of CountTool