MCP Drops Its Handshake, Breaks an Error Code July 28

MCP's July 28 release candidate drops the initialize/initialized handshake, per the spec posted at blog.modelcontextprotocol.io. A July analysis from andrew.ooo puts MCP-backed agents in production at 78% of enterprise AI teams. It puts the Fortune 500 figure, companies running their own MCP servers, at 28%. Those are single-source numbers, so treat the decimals as directional.

The Mcp-Session-Id header goes with the handshake. That header is why remote servers have needed sticky sessions, shared session stores, or gateway packet inspection just to route requests.

I've built that workaround. You end up pinning at the ALB with cookies MCP clients don't reliably send. Or you stand up Redis to hold a session map that only exists because the protocol demanded continuity it didn't need. Ripping that out is real infrastructure gone, not a paper cut.

Three features get a 12-month deprecation window: Roots, Sampling, and Logging. Sampling stings most. It was the only sanctioned way for a server to ask the client's model for a completion. Twelve months sounds generous. Start migrating well before month eleven, not after.

The change most teams will miss: missing-resource errors move from MCP's own -32002 code to the JSON-RPC standard -32602. Any client branching on -32002 silently stops matching. That's a one-line grep across your codebase, and it's the single highest-value thing to check before the July 28 cutover.

But the corroborating signal is everywhere. SnapLogic GA'd its MCP Server on July 14. Coveo announced a hosted server the same window. TechCrunch covered the stateless shift as mainstream news, not protocol trivia. Deprecating three features while a quarter of the Fortune 500 depends on this protocol is a real test of MCP's governance. It has to move fast without breaking the people who already bet on it.