MCP SSE session
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.
The SSE Session API creates a Server-Sent Events (SSE) session between a client and the Model Context Protocol (MCP) server in OpenSearch. The session establishes a persistent connection that allows the server to push updates to the client.
Most users won’t need to interact with this API directly when using a standard MCP client library, which handles session management automatically.
URL construction methods
The SSE Session API supports two different methods of URL construction to accommodate various client implementations.
Default URL construction
When append_to_base_url is set to false (default), the API returns a relative path that clients must append to their base URL.
The Java MCP client accepts a baseURI (for example, http://localhost:9200) when creating the HTTP SSE connection. The default SSE URI is /sse, so the full SSE URL becomes baseUri + /sse and the message endpoint is constructed as baseUri + sse.data.
Complete path URL construction
When append_to_base_url is set to true, the API returns a complete path that includes the plugin prefix.
The Python MCP client accepts an endpoint as the SSE endpoint (for example, http://localhost:8000/_plugins/_ml/mcp/sse) and concatenates it with sse.data. Setting append_to_base_url=true ensures that the correct message endpoint is constructed as /_plugins/_ml/mcp/sse/message.
Endpoints
GET /_plugins/_ml/mcp/sse
Path parameters
| Parameter | Type | Required/Optional | Description |
|---|---|---|---|
append_to_base_url | Boolean | Optional | Controls how the SSE message endpoint URL is constructed. Default is false. See URL construction methods. |
Example request: Default URL construction
GET /_plugins/_ml/mcp/sse
Example response: Default URL construction
OpenSearch sends an SSE data stream to the client:
event: endpoint
data: /sse/message?sessionId=e2d65bb9-e82e-473a-b050-b69dc67ca9dd
Example request: Complete path URL construction
GET /_plugins/_ml/mcp/sse?append_to_base_url=true
Example response: Complete path URL construction
OpenSearch sends an SSE data stream to the client:
event: endpoint
data: /_plugins/_ml/mcp/sse/message?sessionId=e2d65bb9-e82e-473a-b050-b69dc67ca9dd