[BUG] Cowork file_upload: paths parameter arrives as undefined at server (Zod validation fails on every call)

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?

Preflight Checklist:
• I have searched existing issues and this hasn’t been reported yet
• This is a single bug report
• I am using the latest version of Claude Desktop
What’s Wrong?
Environment
• Platform: macOS (latest Claude Desktop with Cowork enabled)
• Browser: Chrome (latest, updated during this troubleshooting session)
• Chrome Extension: Claude in Chrome (latest, “Allow access to file URLs” enabled)
• Connector: Claude in Chrome, enabled via Cowork
• MCP tool affected: mcp__claude-in-chrome__file_upload
Description
Every call to mcp__claude-in-chrome__file_upload fails with the same Zod validation error before the request ever reaches the Chrome extension. The paths parameter is defined as an array in the tool schema, is provided as an array in every call, but arrives at the server as undefined.
This is a different bug from the CDP-level “Not allowed” error described in #32561. That issue was resolved by enabling the “Allow access to file URLs” Chrome extension permission. I have verified that permission is enabled. The error persists identically.

What Should Happen?

The paths parameter should arrive at the server as the array it was called with, and the tool should upload the file to the target file input element.

Error Messages/Logs

MCP error -32602: Input validation error: Invalid arguments for tool file_upload: [
 {
   "expected": "array",
   "code": "invalid_type",
   "path": ["paths"],
   "message": "Invalid input: expected array, received undefined"
 }
]

Steps to Reproduce

Error message received on every call:
Steps to Reproduce

1. Open Cowork with Chrome connector enabled
2. Navigate to any page with a file input (tested with https://private-html-vault.lovable.app/ and https://publish.buffer.com/create/ideas/…/edit)
3. Use find to get the file input’s ref ID (works correctly, returns a valid ref)
4. Call file_upload with a valid absolute path as an array, the ref, and the tabId
5. Result: server-side Zod validation fails with paths expected array, received undefined

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.229

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

Troubleshooting steps already performed with no change in behavior:
• Chrome updated to latest
• Chrome extension “Allow access to file URLs” enabled
• Multiple destination sites tested (Lovable app, Buffer.com)
• Multiple file types tested (HTML, PNG)
• Multiple absolute path formats tested
• Fresh tab group created
• Chrome extension reconnected
The find tool consistently returns valid element references on every call, so the extension is connected and responsive. The failure is specifically in how the paths argument is passed from the client to the server. Chrome never sees the request.
Impact
Any Cowork automation involving file uploads is completely blocked. This affects:
• Scheduled tasks that upload generated content to storage or CMS
• Interactive workflows that create documents and file them into web-based document vaults
• Social media automation that attaches generated images to draft posts
Related Issues
• #32561 (resolved for a different underlying cause, CDP “Not allowed”)
• #4197 (parameter serialization pattern, related but not identical)
• #9020 (undefined vs {} for MCP tool arguments, related pattern)
Claude Model: Sonnet (Claude Sonnet 4.7 in Cowork session)
Is this a regression? Yes. File uploads worked earlier this session (specifically on 2026-08-05 uploading images to Buffer ideas). Started failing consistently on 2026-08-06.
Last Working Version: Unknown Claude Desktop build, uploads worked on 2026-08-05.
Claude Code Version: Latest Claude Desktop with Cowork
Platform: Anthropic API
Operating System: macOS
Terminal/Shell: N/A (Cowork Desktop session)