The Model Context Protocol shipped its 2026-07-28 revision on July 28 — described by its maintainers and by AWS as the largest revision since launch. If you run a retrieval server, an internal search connector, or anything that hands documents to an agent, this one is not optional reading. It contains backward-incompatible changes.
What actually changed
MCP is now stateless at the protocol layer. Six Specification Enhancement Proposals combine to remove the `initialize`/`initialized` exchange (SEP-2575) and the `Mcp-Session-Id` header (SEP-2567). Protocol version and capabilities move onto each request. The practical consequence, per the C# SDK v2.0 notes, is that connection-scoped setup is replaced by self-contained requests, so ordinary HTTP infrastructure can route MCP traffic and servers can sit on serverless or edge runtimes.
For interactive flows that used to depend on a live session, the replacement is Multi Round-Trip Requests (MRTR), which lets a server call back to a client without a long-lived connection. `roots`, `sampling`, and `logging` are deprecated, marked obsolete in the SDKs with pointers to replacements.
Why this bites retrieval teams specifically
Most non-trivial knowledge servers kept something in the session: a warm index handle, a per-user credential exchanged once at init, pagination cursors over a large result set, a reranker cache keyed by conversation. All of that was implicitly scoped to a connection that no longer exists. You now have to either make each request self-describing — cursor in the request, tenant identity in the token — or move that state into a store you own, with the latency and consistency cost that implies. The upside is real: horizontal scaling without sticky routing, and cheap cold-start deployment for connectors that are queried in bursts.
Auth got stricter in ways that matter for multi-tenant enterprise search. SEP-2468 requires including and validating the `iss` parameter in authorization responses, which The Register notes closes OAuth mixup attacks — a live risk when one client federates several MCP servers across different providers. There's also a formal move away from Dynamic Client Registration toward client metadata documents (CIMD), and the Enterprise-Managed Authorization extension is now stable.
The part worth planning around
Capabilities like MCP Apps and Tasks now ship under a versioned extensions framework rather than as core surface area, alongside a deprecation policy guaranteeing a minimum twelve months between deprecation and removal. Paired with a conformance-suite requirement, that's the maintainers signaling this break is meant to be the last foundational one.
Tier 1 SDKs — Python, TypeScript, Go, C# — have shipped or are shipping support; AWS says AgentCore Gateway can move to the new version with a single `UpdateGateway` call. Anthropic put MCP past 400M monthly SDK downloads, roughly 4x growth this year, so the migration surface is large.
If you maintain a connector, audit it for session-scoped state this week, not next quarter.