[BUG] Cowork unresponsive — LocalAgentModeSessions.start fails validation on every session attempt (macOS 15.6.1)

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?

Note: This issue is being submitted by Claude (AI assistant) on behalf of the user.

Cowork is completely unresponsive. The VM boots successfully, API is reachable, SDK installs, but the Cowork UI shows nothing — no response, no error message, just blank/unresponsive. Every attempt to start a Cowork session fails with:

Uncaught (in promise) Error: Error invoking remote method '$eipc_message$_1f28a184-5d05-4af7-bed8-f3c9d44564b0_$_claude.web_$_LocalAgentModeSessions_$_start': Error: Argument "info" at position 0 to method "start" in interface "LocalAgentModeSessions" failed to pass validation

This error repeats on every Cowork session attempt. Regular Chat mode works fine.

Key Diagnostic Findings

Everything backend/VM-side is healthy:

• VM boots successfully (startup complete in ~124s)
• guest_vsock_connect completes
• Network status: CONNECTED
• API reachability: REACHABLE
• SDK installed: v2.1.34
• coworkd log: clean, no errors
• curl to api.anthropic.com returns 405 (expected)
• TLS certificate is legitimate (Google Trust Services, not intercepted)
• No conversation_uuid validation errors in logs (this is NOT the local_ UUID prefix bug from #18644/#18974/#20016)

The bug is in the Electron frontend IPC layer — the app constructs an invalid session payload before communicating with the VM.

What Should Happen?

Cowork session should start and display Claude's response.

Error Messages/Logs

From ~/Library/Logs/Claude/claude.ai-web.log:

2026-02-12 09:04:02 [error] Uncaught (in promise) Error: Error invoking remote method '$eipc_message$_..._LocalAgentModeSessions_$_start': Error: Argument "info" at position 0 to method "start" in interface "LocalAgentModeSessions" failed to pass validation
2026-02-12 09:05:14 [error] (same error)
2026-02-12 09:13:45 [error] (same error)
2026-02-12 09:13:47 [error] (same error x2)
2026-02-12 09:25:35 [error] (same error)
2026-02-12 09:25:36 [error] (same error)
2026-02-12 09:36:03 [error] (same error — persists after full cache clear)
2026-02-12 09:36:05 [error] (same error)

Also present:

2026-02-12 09:03:52 [error] Uncaught Error: Minified React error #418
2026-02-11 21:28:19 [error] [REACT_QUERY_CLIENT] QueryClient error: Error: Wiggle artifact not found

Steps to Reproduce

1. Open Claude Desktop v1.1.2685 on macOS 15.6.1
2. Click the Cowork tab
3. Enter any task/message
4. Observe: UI is unresponsive, no output, no visible error
5. Check ~/Library/Logs/Claude/claude.ai-web.log — LocalAgentModeSessions.start validation error on every attempt

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

1.1.2685 (Claude Desktop)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

Troubleshooting Already Attempted (all failed)

• Force quit and restart Claude Desktop
• Cleared all Electron caches: Cache/, GPUCache/, Code Cache/, Session Storage/, Local Storage/, IndexedDB/, databases/
• Deleted VM bundle (rm -rf ~/Library/Application Support/Claude/vm_bundles/claudevm.bundle) — VM re-downloaded fresh, same error
• Cleared Electron preferences (defaults delete com.anthropic.claude)
• Full app data wipe (rm -rf ~/Library/Application Support/Claude) + app uninstall + fresh download from claude.ai/download — same error on clean install
• Verified no corrupted local session data (~/Library/Application Support/Claude/local-agent-mode-sessions/ does not exist)
• Verified no network interception (TLS cert is Google Trust Services, not FortiClient/AVG)
• Verified no VPN tunnel interference (utun interfaces present but not blocking)

Environment Details

• macOS: 15.6.1 (Sequoia), Build 24G90, Apple Silicon
• Claude Desktop: 1.1.2685
• Claude Code VM SDK: 2.1.34
• Network extensions active: FortiClient VPN (com.fortinet.forticlient.macos.vpn.nwextension), AVG AntiVirus (com.avg.Antivirus.SystemExtension) — neither appears to be intercepting traffic based on TLS certificate inspection

Related Issues

Not the same as #18644 / #18974 / #20016 (those were conversation_uuid / local_ prefix UUID validation errors — no such error present in these logs)

Symptom is similar to #18974 (Cowork hangs, backend works) but the underlying error is different.