[BUG] claude://cowork/new?file= deeplink reuses the prior deeplink's session instead of opening a new one (file gets attached to existing session)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Summary

The working-folder carryover reported in #53402 is fixed from the project-folder perspective. However, a related defect remains for the file parameter: firing claude://cowork/new?file=fileA and then claude://cowork/new?file=fileB does not open a second session. Instead, fileB is attached to the same session that the first deeplink opened, so that session ends up with both fileA and fileB attached.
Each claude://cowork/new invocation is documented and expected to create a new session. The second deeplink should open a fresh session scoped to only fileB.

Why this is a bug

The /new contract is a fresh session. A second claude://cowork/new invocation reusing the first deeplink's session violates that contract.
The file parameter is documented in the singular (the file to attach to the new session), implying the new session is authoritatively scoped to that file — not that the file is merged into a pre-existing deeplink session.
UX impact: Particularly problematic for "Open in Claude Cowork" buttons in third-party tools. The caller fires a deeplink expecting an isolated session for its file, but the file lands in whatever session a previous deeplink happened to open.
Safety impact: A file the user (or the calling tool) never intended to bring into a given session remains in scope, expanding the blast radius of any prompt run there — especially relevant given Cowork's agentic, file-writing nature.

Reference
https://support.claude.com/en/articles/14729294-open-claude-desktop-with-a-link
Related: #53402 (working-folder carryover; fixed for the project-folder case).

What Should Happen?

Firing claude://cowork/new?file=fileB after a prior claude://cowork/new?file=fileA deeplink should open a new session whose attached files are only fileB. fileA and the prior session should not be reused.

Error Messages/Logs

Steps to Reproduce

1. Launch Claude Desktop.
2. Fire claude://cowork/new?file=/Users/me/docs/fileA.md → a new session opens with fileA.md attached. ✅
3. Without quitting the app, fire claude://cowork/new?file=/Users/me/docs/fileB.md.
4. Actual: No new session is created. fileB.md is attached to the session opened in step 2, which now has both fileA.md and fileB.md attached.
5. Expected: A new session opens with only fileB.md attached.

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

Claude 1.9659.4 (8cc655)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_