What
mcp serve runs Claude Code as an MCP server, a program that other apps can connect to and use as a source of tools. It now looks for a session id in the extra details sent with each tool call, under the key anthropic/session. A session is one Claude Code conversation. If the value is not a valid session id, Claude Code logs a warning and records events under its own id instead.
Why
An app connecting to Claude Code this way can now link a call's events to a particular session.
Names in the bundleclaude mcp serveanthropic/session
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed since
Anthropic's documentation has since written up claude mcp serve, on Claude Code changelog.
* Changed fork subagents (`subagent_type: "fork"`) to be on by default in `claude -p`, the Agent SDK, the desktop app and VS Code, as they already are in the terminal; `CLAUDE_CODE_FORK_SUBAGENT=0` turns them off, and `claude mcp serve` ne…changelog see the edit
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agrees
Anthropic's documentation has since written up claude mcp serve, on Claude Code changelog.