MCP Just Deleted Its Own Handshake — and Every Request Got 118% Bigger

The Model Context Protocol (MCP) maintainers will publish version 2026-07-28 on July 28, described as the largest revision since launch, which deletes the initialize handshake and Mcp-Session-Id header. A measurement of the new stateless wire format shows a single tools/call request grew from 190 bytes to 414 bytes, a 118% increase on every request, with the break-even point against the handshake it replaces being less than one call.

Member-only story MCP Just Deleted Its Own Handshake — and Every Request Got 118% Bigger I measured the new Model Context Protocol wire format against the old one this morning, and the same tools/call went from 190 bytes to 414 bytes. That is a 118% increase on every single request, and the break-even point against the handshake it replaces is less than one call. On July 28 — two days from now — the MCP maintainers publish 2026-07-28 , which they describe in their own release candidate post as "the largest revision of the protocol since launch." Six Specification Enhancement Proposals work together to delete the initialize handshake and the Mcp-Session-Id header that has anchored every MCP connection since the protocol shipped. The official MCP Registry counted 9,652 latest server records as of late May. Every one of them is built on a lifecycle that is about to stop being the lifecycle. I installed the beta SDK, ported a small server to the new model, and measured the thing nobody in the announcement posts has measured: what the stateless rewrite actually costs on the wire. The engineering case for the change is strong. The bill is real too, and it is not the one you would guess. What Actually Got Deleted Under 2025-11-25 , calling a tool over Streamable HTTP means establishing a session first. You POST an initialize …