Unclear It is not clear what sets the modern flag, so SDK users cannot tell how to turn this mode on.
What
With the SDK, which is the kit for building programs on top of Claude Code, you can run MCP servers inside your own program. MCP (Model Context Protocol) is the standard Claude Code uses for outside tools. Previously every such server connected the same way. Now Claude Code checks a per-server modern flag. When it is set, Claude Code connects using capabilities it already discovered and a store for cached responses, and skips the initialize handshake that normally opens the connection.
Why
This is early support for a newer, stateless version of the MCP protocol for SDK servers, where a server does not need to remember earlier exchanges.
Names in the bundlemodern
Documented inapi/build-with-claude/working-with-messages
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The writer flagged doubt
It is not clear what sets the `modern` flag, so SDK users cannot tell how to turn this mode on.