FastMCP 4 is GA

FastMCP 4 is now generally available, adding support for the MCP 2026-07-28 protocol revision while preserving compatibility with older clients and allowing most FastMCP 3 applications to upgrade without code changes. The new protocol replaces connection-bound sessions with independent requests that can be handled by any server replica, while FastMCP offers configurable user- and session-based storage for applications that need persistent state. Major additions include interactive tools for collecting follow-up input, background tasks for long-running work, a negotiated extension system, argument completion, expanded authentication and authorization options, and infrastructure features such as cache hints and routing headers. The release also introduces dependency injection for tool arguments, ClientGroup for combining and routing tools across multiple MCP servers running different protocol versions, typed-output and schema improvements, faster startup, and Python 3.14 compatibility. FastMCP 4 removes server-initiated sampling and roots for the new protocol, relocates background tasks to an optional package, deprecates some older APIs and string-based local stdio client detection, and incorporates security, durability, and compatibility fixes contributed through an extended beta period.

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