/chrome "Select Browser" fails with `JSON Parse error: Unexpected identifier "Browser"` — extension launches native host but immediately drops (EOF), never registers as connected

Summary

Running /chrome → Select Browser in the Claude Code CLI fails with:

Couldn't list connected browsers: JSON Parse error: Unexpected identifier "Browser"

Investigation shows two distinct bugs:

1. UI bug — list_connected_browsers returns the plain-text string Browser extension is not connected…, and the "Select Browser" flow feeds that string to JSON.parse(), which throws Unexpected identifier "Browser". The user should see the real message, not a JSON parse error.
2. Connection bug (root cause) — the Chrome extension launches the native messaging host but immediately drops the connection (Chrome disconnected (EOF received)), so no browser ever registers as connected.

Environment

• macOS (Darwin 25.5.0). Default browser is Arc; Chrome is secondary and used directly (not relevant to the failure).
• Claude Code CLI 2.1.173 (~/.local/share/claude/versions/2.1.173).
• Chrome latest; "Claude in Chrome" extension fcoeoabgfenejglbffodgkkbkcdhcgfn (verified via a clean uninstall/reinstall).
• Native-host component reports version 0.1.0.
• Both Claude Desktop and Claude Code CLI are installed.

Evidence the plumbing is correct

• Chrome launches the CLI native host: PID … = ~/.local/share/claude/versions/2.1.173 --chrome-native-host, with Google Chrome as the parent process.
• The host creates and listens on /tmp/claude-mcp-browser-bridge-<user>/<pid>.sock.
• The native messaging manifest com.anthropic.claude_code_browser_extension.json lists the installed extension id in allowed_origins.
• The running CLI session and the launched host are the same install (2.1.173) — there is no version split between client and host.
• The extension is signed into claude.ai with the same account as the CLI.

Evidence of the failure

The native-host log repeatedly shows, within seconds of each other:

[INFO chrome-native-host] Chrome native host starting (version 0.1.0)
[INFO chrome-native-host] Socket server listening for connections
[INFO chrome-native-host] Chrome disconnected (EOF received)
[INFO chrome-native-host] Shutting down chrome native host

The extension connects to the host, then immediately disconnects (EOF), so no browser is ever registered. Both list_connected_browsers and switch_browser return Browser extension is not connected… instantly (switch_browser does not push a Connect prompt or wait). This was reproduced on both the Desktop native host (com.anthropic.claude_browser_extension) and the CLI native host (com.anthropic.claude_code_browser_extension), which points to an extension-side handshake drop rather than a host- or config-specific issue.

Things tried that did NOT help

• Quit Claude Desktop completely.
• Full Chrome restart (Cmd+Q) — multiple times.
• Re-verified the extension is logged into claude.ai with the same account as the CLI.
• Temporarily disabled the Desktop native-messaging manifest to force the extension onto the CLI host (the extension does fall back and the CLI host launches — but it still EOF-drops).
• Clean uninstall + reinstall of the "Claude in Chrome" extension.
• Restarted the Claude Code CLI session.

Expected

list_connected_browsers lists the connected Chrome instance, and /chrome → Select Browser lets me pick it.

Actual

Extension launches the native host then immediately disconnects (EOF); no browser registers; the CLI surfaces a misleading JSON Parse error: Unexpected identifier "Browser" because it JSON-parses a plain-text error string.