MCP 2026-07-28 removed the initialize handshake and Mcp-Session-Id, and the official post calls the release breaking.
In our test of Python SDKs 1.30.0 and 2.3.0, all four client and server pairings connected, and the new flow used 3 HTTP requests against 6.
The real breaks are state keyed to sessions and the missing-resource error code, which changes from -32002 to -32602 and was code 0 on the old Python server.