FastMCP 4.0 went stable on a sessionless protocol, so any replica behind a load balancer can answer.

Released August 31 after five betas, five weeks, 23 contributors and 80+ PRs, built on the MCP 2026-07-28 revision and the rewritten Python SDK v2 (GitHub). Modern requests are sessionless and self-contained, so protocol version is negotiated per connection and horizontal scaling no longer needs sticky sessions. Breaking changes to plan for: server-initiated sampling and roots are removed because 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. The repo also moved from jlowin/fastmcp to PrefectHQ/fastmcp at 27,471 stars, and every historical release URL now resolves to the new path.