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MCP 2026-07-28: sessions removed, SDK split into server and client packages

Summary

The MCP specification dated 2026-07-28 makes the protocol stateless: the initialize / initialized handshake and the Mcp-Session-Id header are removed, so any instance behind a plain round-robin load balancer can serve any request. The monolithic @modelcontextprotocol/sdk npm package is retired and split into @modelcontextprotocol/server and @modelcontextprotocol/client. Server-to-client calls that needed a permanently open stream are replaced by Multi-Round-Trip Requests, deprecating sampling/createMessage, elicitation/create, and roots/list.

Key Details

  • Stateless by default — per-request identity moves into a _meta field; capability discovery becomes an optional server/discover RPC. This is what unblocks serverless and autoscaled remote MCP deployments, which sticky sessions and shared Redis/Postgres session stores only ever worked around.
  • The old package name still installs. npm install @modelcontextprotocol/sdk resolves to v1 without erroring, so the symptom is "the new API methods don't exist" rather than a failed install. sessionIdGenerator no longer exists in v2.
  • Cacheable list results (ttlMs, cacheScope) stop every reconnect from re-fetching the tool catalog — previously invisible overhead that also destabilised prompt caches and burned context tokens.
  • Migration codemod exists but is incomplete: npx @modelcontextprotocol/codemod@2.0.0-beta.2 v1-to-v2 . handles the mechanical rewrite, warns that zod >= 4.2.0 is required, and does not fix package.json — miss the warning and the server starts, then fails on the first tool call.
  • Auth hardened: iss required per RFC 9207, credentials bound to their issuer, Dynamic Client Registration deprecated for Client ID Metadata Documents, application_type required so localhost redirects stop being rejected for CLI and desktop clients.
  • Local stdio users feel none of this — the pain was always remote, shared, multi-instance servers.

Why Rolf Thinks This Matters

Further Reading