Unclear Which experiment flag controls the Chrome setup offer, and whether any shipped client declares either entry, is not known.
What
When a program connects to Claude Code, for example through the Agent SDK or the stream-json mode, it first sends an initialize message listing what it can handle. Two optional entries can now go in that list:
forwardNestedSubagentToolCalls: Claude Code also sends the tool calls and tool results of subagents (helper agents) that other subagents start, and of skills a subagent forks. Each one carriesparent_tool_use_id, so the client can tell where it belongs.rendersChromeSetupOffer: the client can show an offer to set up Claude in Chrome as a setup card. That offer only appears after a Claude in Chrome call reports the extension is not connected, and only when an experiment flag for the session is on.
A client that sends neither entry behaves as before.
Why
Existing clients see no change. Clients that opt in can show the full tree of nested subagent work and offer Chrome setup inside their own interface.
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Which experiment flag controls the Chrome setup offer, and whether any shipped client declares either entry, is not known.