MCP Python SDK v2 just killed session state. 84% of packages have no upper bound.

The Model Context Protocol just went stateless.

MCP Python SDK v2.0.0a1 shipped June 11.

The initialize handshake? Gone.

The Mcp-Session-Id header? Gone.

Every agent framework that depends on MCP just inherited a ticking migration bomb.

Here is the structural reality:

The MCP 2026-07-28 spec removes the protocol-level session entirely.

No more sticky routing.

No more shared session stores.

Every request must now carry its own identity in _meta.

Any server instance can handle any request.

This is not a feature update.

This is a protocol architecture rewrite.

What breaks:

The initialize/initialized handshake is removed (SEP-2575).

Mcp-Session-Id header is removed (SEP-2567).

FastMCP is renamed to MCPServer.

Server handlers move from decorators to constructor parameters.

Return values are no longer auto-wrapped.

Type names change to snake_case with stricter validation.

The error code for missing resources changes from -32002 to -32602.

Roots, Sampling, and Logging are deprecated.

The scale of the blast radius:

84% of the 10,000+ PyPI packages that depend on mcp declare no upper bound.

When stable v2 ships on July 28, every unpinned dependency resolves to v2 automatically.

Your agent framework breaks without warning.

Your MCP servers break without warning.

Your tool integrations break without warning.

The timeline:

June 11 — v2.0.0a1 alpha (now)
June 30 — Beta with full 2026-07-28 spec support
July 27 — Stable v2.0.0
July 28 — MCP 2026-07-28 spec ships

If you maintain a package that depends on mcp, add an upper bound today.

mcp>=1.27,<2

Do it now.

Before your users resolve to v2 and your entire tool chain silently degrades.

The migration guide is the most complete documentation for v2 right now.

The protocol now requires Mcp-Method and Mcp-Name headers on every request.

Load balancers can route without inspecting the body.

Tool list results carry ttlMs and cacheScope.

This is the infrastructure layer that every agent framework sits on.

If you are running MCP servers in production, audit your session handling now.

Every handler must become self-contained.

Every piece of state must move to explicit handles.

The session is dead.

Long live the request.

SOURCE: https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.0.0a1
VERIFIED: GitHub Release, MCP Blog (blog.modelcontextprotocol.io), PyPI
SIGNAL: The entire MCP ecosystem just got a forced migration deadline. Every agent framework using MCP tool-use must migrate before July 28 or break silently.