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MCP SSE message

Introduced 3.0

This is an experimental feature and is not recommended for use in a production environment. For updates on the progress of the feature or if you want to leave feedback, join the discussion on the OpenSearch forum.

This endpoint handles standard message interactions for the Model Context Protocol (MCP). It enables communication with the MCP server in OpenSearch through Server-Sent Events (SSE).

Most users won’t need to interact with this API directly when using a standard MCP client.

Endpoints

POST /_plugins/_ml/mcp/sse/message

Request body fields

The following table lists the available request fields.

Field Data type Required/Optional Description
jsonrpc String   The JSON-RPC version.
id String   A unique ID for the request.
method String   The operation to perform, such as tools/call.
params Object Required The top-level container for request parameters.
params.name String Required The name of the tool to call.
params.arguments Object Required The arguments to pass to the tool.
params.arguments.input Object Required The input parameters for the tool. The parameters are dependent on the tool type. For information about specific tool types, see Tools.

Example request

The SSE Message API provides direct, low-level access to tools using the JSON-RPC (remote procedure call) protocol structure. This differs from the agent framework approach, where tools are configured using parameters during agent registration. When using this API directly, you’ll structure your request with params and arguments fields according to the JSON-RPC specification, bypassing the agent framework entirely:

POST /_plugins/_ml/mcp/sse/message
{
  "jsonrpc": "2.0",
  "id": "110",
  "method": "tools/call",
  "params": {
      "name": "ListIndexTool1",
      "arguments": {
          "indices": ["test"]
      }
  }
}

Example response

OpenSearch sends an SSE data stream to the client:

event: message
data: {
  "jsonrpc": "2.0",
  "id": "100",
  "result": {
    "tools": [
      {
        "name": "ListIndexTool",
        "description": "This is my first list index tool",
        "inputSchema": {
          "type": "object",
          "properties": {
            "indices": {
              "type": "array",
              "items": { "type": "string" },
              "description": "A comma-separated list of OpenSearch index names. For example: [\"index1\", \"index2\"]. Use [] (an empty array) to list all indices in the cluster."
            }
          },
          "additionalProperties": false
        }
      }
    ]
  }
}
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