VS Code Agent Host Harness: ENOPRO, no filesystem provider for claude-code:/agents/*

Claude Code extension 2.1.197, VS Code Agent Window (Agent Host Harnesses, 1.126+), macOS.

Repro is dead simple: open the Agents Window, pick the Claude agent host, send any message.

What happens: it sits there "working," throws a vague error, then goes blank. No response, nothing.

Save yourself the detour I took assuming this was auth or subscription related: I'm on Copilot Pro+, I signed out and back in from inside the Agents window itself, I updated VS Code. Same failure every time, byte for byte.

DevTools console has the real error, every single run:

ERR CodeExpectedError: ENOPRO: No file system provider found for resource 'claude-code:/agents/Explore'
    at Ere.withProvider (fileService.ts:150:1)
    at async Ere.withReadProvider (fileService.ts:160:1)
    at async Ere.readFile (fileService.ts:541:1)
    at async U7t.parseNew (promptsServiceImpl.ts:694:1)
    at async kKe.getCustomAgents (customizationHarnessService.ts:613:1)
    at async upe.refreshCustomPromptModes (chatModes.ts:246:1)
    at async chatModes.ts:223:1

Looks like VS Code's chat-modes code tries to read Claude's subagent definitions (Explore, here) from a virtual path at claude-code:/agents/..., and nothing ever registers a filesystem provider for the claude-code: scheme. So it fails, every time, regardless of anything on my end.

The classic sidebar works fine. This is only broken in the new Agent Window.

Possibly related, different symptom, same feature: microsoft/vscode#323726 (/code-review gets routed to VS Code's built-in agent instead of Claude's own).