[BUG] Claude Desktop (Windows): Browser pane tools stopped working mid-session - calls routed to /mcp/servers/Claude%20Browser/ and rejected with 400 "server_uuid: Input should be a valid UUID"

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?

This concerns the Claude Desktop app (Cowork sessions), not the CLI; filed here as the public tracker.

On 20 Sep 2026 Claude's ability to drive the in-app Browser pane stopped working in the middle of a working session. No app update, no local config change. A full quit and restart did not restore it; new sessions no longer have the function.

Timeline (CEST, from main.log):

• 11:14-11:58: working. Pane calls are served on the device ("Created remote session preview context", "pane origin grant recorded ... via: request_access", tabs open and close).
• From 12:39: every Browser pane tool call (preview_start, tabs_context) fails. The desktop app now sends the call to POST https://claude.ai/api/organizations/<org>/mcp/servers/Claude%20Browser/tools/call and the server answers 400. The display name "Claude Browser" is used where a server UUID is required.
• 13:02, after a full restart: the app announces all 17 Claude_Browser__* tools to the device bridge ("connecting DO bridge with: ... Claude_Browser__preview_start ... Claude_Browser__request_access"), same as on previous days, but conversations no longer receive them (a call returns "tool not found").

What Should Happen?

Browser pane tools keep working in Cowork sessions, as they did until 11:58 the same day, with calls served by the desktop app on the device rather than routed to claude.ai as a remote MCP server.

Error Messages/Logs

[error] Remote tool call failed {
  serverUuid: 'Claude Browser',
  toolName: 'preview_start',
  status: 400,
  errorText: '{"type":"error","error":{"type":"invalid_request_error","message":"path.server_uuid: Input should be a valid UUID, invalid character: expected an optional prefix of `urn:uuid:` followed by [0-9a-fA-F-], found `l` at 2"}...',
  url: 'https://claude.ai/api/organizations/<org>/mcp/servers/Claude%20Browser/tools/call'
}

Request IDs: req_011CfEYj2QHjEEMZxngxXETn, req_011CfEYpA43wJZm28FAHv3ZT, req_011CfEZHDTwbeneRJbqimg1A, req_011CfEZHL3pMAynPDbmzyWW2

Steps to Reproduce

1. Claude Desktop 2.2553.1 on Windows 11, Cowork session with the cloud bridge enabled.
2. Ask Claude to open a page in the Browser pane (tool preview_start or tabs_context).
3. Until 11:58 CEST on 20 Sep 2026 this worked; from 12:39 the call fails with the 400 above.
4. Quit the app fully and restart: the tools are announced to the bridge by the app but are no longer offered to conversations.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Same version (2.2553.1) - worked until 11:58 CEST on 20 Sep 2026

Claude Code Version

Claude Desktop 2.2553.1 (Windows, Microsoft Store package) - not the CLI

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

Ruled out locally: same app version all day; no entry named "Claude Browser" in claude_desktop_config.json or other local config; GrowthBook logged "loaded 361 features (0 changed)" at 12:05 and 12:11, so no local flag change before the failure; the Claude in Chrome extension route works, so network and authentication are fine.

Question: was there a server-side change or rollout on 20 Sep 2026 between about 12:00 and 12:40 CEST affecting routing of the "Claude Browser" tools? Logs available on request.