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Server variants
Negotiate stable, principal-bound legacy server views.
Server variants are legacy-only. Configure protocol 2025-11-25, GET SSE, session deletion, and extensions.variants. Modern protocol options reject the extension.
During initialize, the adapter authenticates the request, applies visible, applies rank, limits discovery, and stores the negotiated list against MCP-Session-Id. The first default variant must be stable unless the client requests experimental results. io.modelcontextprotocol/server-variant request metadata takes precedence over MCP-Server-Variant.
The selected variant binds tool, resource, and prompt registries. It also binds pagination cursors and resource subscriptions. Requests from another principal cannot reuse the session. resources/unsubscribe closes its provider iterator. Session deletion closes streams and subscriptions.
Use core.subscriptions.provider with resources: true for resource updates. Connect the session's GET /mcp SSE stream to receive variant-tagged notifications.