[BUG] VS Code 1.123 Agents Window: "Error during execution" + Extension Host unresponsive — "No chat session found for resource: [object Object]" (Windows, Vertex AI)

Environment

• OS: Windows 11
• VS Code: 1.123 (June 2026 preview)
• Claude Code extension: latest
• Auth: Vertex AI (GCP) via enterprise CLI wrapper (claudeProcessWrapper)
• Feature: VS Code 1.123 Agents Window (new preview feature — distinct from Chat panel Agent Handoff)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong

Using the new VS Code 1.123 Agents Window with Claude Code fails immediately with "Error during execution". The regular Claude Code panel and CLI both work correctly — the issue is isolated to the Agents Window.

Unlike #62335, the marketplace config is clean (no corrupted "local" entry). The Claude Code extension output log shows zero new entries when attempting to send via the Agents Window — the error happens above the extension layer.

Logs

exthost.log at the exact moment of sending from Agents Window:

2026-06-04 22:32:35.545 [warning] No chat session found for resource: [object Object]
2026-06-04 22:32:35.547 [warning] No chat session found for resource: [object Object]
2026-06-04 22:32:35.547 [warning] No chat session found for resource: [object Object]

Developer Console shows Extension Host becoming unresponsive immediately after:

localProcessExtensionHost.ts:317 Extension Host
INFO UNRESPONSIVE extension host: starting to profile NOW
INFO Extension host (LocalProcess pid: 22304) is responsive.
INFO Extension host (LocalProcess pid: 22304) is unresponsive.

Claude VSCode.log — no new entries at all when using Agents Window:

2026-06-04 22:31:12.177 [info] AuthManager initialized
2026-06-04 22:31:12.177 [info] Claude code extension is now active?
2026-06-04 22:31:12.177 [info] MCP Server running on port 65267 (localhost only)

Root Cause Hypothesis

The [object Object] warning suggests VS Code's Agents Window is passing a resource object that isn't being serialized to a string — a serialization bug in either VS Code 1.123's agent routing or the Claude Code extension's integration with VS Code's Chat Participant API.

The Claude Code extension appears to not register itself as a VS Code chat participant (vscode.chat.createChatParticipant()), which the Agents Window likely requires. The existing panel works because it uses a different code path (spawning a child process) rather than VS Code's native agent protocol.

Steps to Reproduce

1. Install VS Code 1.123 (June 2026 build) on Windows
2. Install Claude Code extension with claudeProcessWrapper set to enterprise CLI wrapper, disableLoginPrompt: true
3. Open the Agents Window (new VS Code 1.123 preview feature)
4. Type any prompt and send
5. Observe: "Error during execution" immediately, no logs in Claude Code output

What Should Happen

Submitting a prompt via the VS Code 1.123 Agents Window should invoke Claude Code and return a response.

Additional Context

Related to #62335 but distinct — different root cause (no marketplace corruption), different VS Code feature (Agents Window vs Chat panel), and specific exthost.log evidence pointing to a session lookup serialization failure.