MCP SDK 1.23 introduced some changes related to the 11/25/25 MCP protocol update that break some patches/workarounds that FastMCP had implemented previously. In particular, OAuth changes in the new protocol changed some implementation details that FastMCP patched; as such 1.23 is not necessarily a breaking SDK change but it is "breaking" for certain FastMCP behaviors.
As a precaution, this release pins mcp<1.23. FastMCP 2.14 will introduce 11/25/25 support (and require mcp>=1.23).