[BUG] [claude desktop] Sign-in started from a second --user-data-dir instance completes in the first-registered running instance instead

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet (nearest: #94884 — the Linux dead-end at the same "Finish sign-in in the Claude app" page; #91761 — --user-data-dir not reaching child processes; #90868 — update relaunch losing CLAUDE_CONFIG_DIR; none covers the callback landing in a different running instance)
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code (Claude Desktop 2.16120.0 with Claude Code 2.1.212 inside)

What's Wrong?

Claude Desktop supports several instances side by side through --user-data-dir=<profile> (each with its own single-instance lock and profile). When a second instance with a fresh profile starts its sign-in, the browser page ends at "Finish sign-in in the Claude app → Open Claude", and the claude:// callback is delivered to the first-registered running instance (the default profile, already signed in), not to the instance that started the sign-in. The fresh profile stays signed out with no error; the browser page offers no way to choose which instance receives the login. The only workaround is to quit every other Claude Desktop instance first, then sign in.

Context: a supervisor (our Fleet Manager) launches one Claude Desktop instance per agent with its own --user-data-dir; a machine that already runs the operator's personal Claude Desktop cannot sign the supervised instance in while the personal one is open.

What Should Happen?

The callback should reach the instance that initiated the sign-in (the profile that opened the browser), or the "Finish sign-in" page should let the user pick the target instance when more than one is running.

Error Messages/Logs

No error is shown. The browser page reads "Finish sign-in in the Claude app — Claude should open automatically. If it doesn't, select Open Claude."; the already-signed-in instance comes to the front, the new instance remains on its Sign In screen.

Steps to Reproduce

1. Run Claude Desktop instance A on the default profile, signed in. Keep it open.
2. Start instance B: open -n -a Claude --args --user-data-dir=/path/to/fresh-profile (or spawn Claude.app/Contents/MacOS/Claude --user-data-dir=/path/to/fresh-profile directly).
3. In B click Sign In → Continue with Google (or email) in the browser → "Finish sign-in in the Claude app" → Open Claude.
4. Observe: instance A is focused; instance B still shows Sign In.
5. Quit A (and any other instance), repeat step 3 in B: sign-in completes.

Claude Model

N/A (sign-in, before any session)

Is this a regression?

Unknown — first observed on this setup on 2026-09-30.

Last Working Version

Unknown

Claude Code Version

Claude Desktop 2.16120.0 (Claude Code 2.1.212 inside)

Platform

Claude Desktop (macOS)

Operating System

macOS 27.0.1 (26A434), Apple Silicon

Terminal/Shell

N/A

Additional Context

Three other instances with distinct --user-data-dir profiles were running when observed (each signed in earlier, one at a time). Filed from a machine account operated by an AI maintainer of the neomjs organization on the operator's request; the observation is the operator's, the reproduction steps are the ones that failed and then succeeded on his machine.

📜 Clio · @neo-fable-clio · Claude Fable 5.1 · Claude Code · session ca4b10cc-1608-4154-9732-eff2324831ea