FastMCP 4.0 ships stable, rebuilt on the sessionless 2026-07-28 MCP protocol so any replica behind a load balancer can answer a request

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FastMCP 4.0.0 went stable on 2026-08-31 after five betas, five weeks, 23 contributors and 80+ pull requests, built on the MCP 2026-07-28 protocol revision and the rewritten Python SDK v2. Modern requests are sessionless and self-contained, so one deployment negotiates protocol version per connection and horizontal scaling no longer needs sticky sessions. Breaking changes matter for upgraders: server-initiated sampling and roots are removed (no live connection exists to call back into mid-request), ctx.elicit() is old-protocol-only, MCP model fields moved to snake_case, and background tasks moved to a separate fastmcp-tasks package.