[v2] Expose request cancellation on MCPServer tool Context

从 mcp.server.mcpserver.MCPServer 及其注入的 mcp.server.mcpserver.context.Context 入手，然后比较 mcp.shared.context.BaseContext 如何暴露 cancel_requested，以及 notifications/cancelled 如何存储在每个请求的 dispatch context 中。完成的标准是：高级工具处理器可以使用受支持的取消事件，而无需访问私有的 session 或 dispatch 内部，并且为报告的复现提供覆盖。

What happened?

With mcp==2.1.0, a tool registered through the high-level mcp.server.mcpserver.MCPServer API receives mcp.server.mcpserver.Context. That context does not expose a public cancel_requested event, although the lower-level mcp.shared.context.BaseContext does.

from mcp.server.mcpserver.context import Context as MCPServerContext
from mcp.shared.context import BaseContext

assert not hasattr(MCPServerContext, "cancel_requested")
assert hasattr(BaseContext, "cancel_requested")

The server dispatcher processes notifications/cancelled and holds the event in its per-request dispatch context, but normal tool handlers cannot cooperatively observe it through the supported high-level context API. A handler that owns cleanup-sensitive work (for example, a bounded child process) must either rely only on task interruption or traverse private session/dispatch internals to relay cancellation.

Expected behavior

Expose the current request's cancellation event on the high-level tool context (for example, await ctx.cancel_requested.wait()), or document an equivalent supported API. It should allow a handler to trigger its own cooperative cleanup without using private SDK attributes.

If this is already addressed in a later release, a version pointer would also resolve the report; the reproduction above is against 2.1.0.

Reproduction steps

1. Install mcp==2.1.0.
2. Register an async tool with MCPServer that accepts its injected Context.
3. Attempt to access ctx.cancel_requested; the attribute is absent.
4. Compare with BaseContext.cancel_requested, which is present.

SDK version

mcp==2.1.0 (Python 3.12)

Area

Server / high-level MCPServer tool context API.

AI assistance disclosure: drafted with AI assistance; the reporter is responsible for this report.