Vscode Dev Workbench Agent Skill

Running vscode.dev Against Local VS Code Sources

The vscode-dev repo is the vscode.dev server. When run locally with ?vscode-quality=dev, it serves the VS Code web workbench (or Agents window at /agents) from the sibling microsoft/vscode checkout. This is the fastest way to validate web-only changes to the workbench without shipping an Insiders build.

Layout assumption

vscode-dev and vscode must be sibling folders:

<workRoot>/
  vscode/          # microsoft/vscode checkout
  vscode-dev/      # microsoft/vscode-dev checkout

If your paths differ, check server/ in vscode-dev for the source root resolution — the /vscode-sources/* route maps to ../vscode.

Start the dev server

Critical: Run npm run dev from the vscode-dev folder, NOT from vscode. The vscode repo has no dev script and will fail with npm error Missing script: "dev". Terminal tools that simplify/strip leading cd into separate commands will silently keep the cwd of a previous terminal — always use an absolute pushd or verify with pwd before npm run dev.

cd /path/to/vscode-dev     # NOT /path/to/vscode
npm run dev                # runs watch + nodemon; serves https://127.0.0.1:3000

If you're driving this through an agent/terminal tool, prefer:

pushd /absolute/path/to/vscode-dev >/dev/null && pwd && npm run dev

On first start you may see one crash like Cannot find module './indexes' — it's the watcher racing the first build. nodemon restarts automatically once out/ finishes compiling. The server is ready when curl -sk -o /dev/null -w "%{http_code}" https://127.0.0.1:3000/ returns 200.

URLs

• https://127.0.0.1:3000/?vscode-quality=dev — main workbench, local dev sources
• https://127.0.0.1:3000/agents?vscode-quality=dev — Agents window, local dev sources
• https://127.0.0.1:3000/?vscode-version=<commit> — pinned production commit
• Add &vscode-log=trace for verbose client logging

Interacting via the integrated browser

Use open_browser_page and the standard browser tools.

Enter inserts a newline in the chat input

The chat input is a Monaco editor — page.keyboard.press('Enter') inserts a newline. To send, click the Send button (a[aria-label^="Send"]) or use the send keybinding.

Hard-reloading after a rebuild

The service worker caches client assets aggressively. A plain reload can still serve stale modules:

await page.evaluate(async () => {
  const regs = await navigator.serviceWorker?.getRegistrations() ?? [];
  await Promise.all(regs.map(r => r.unregister()));
  const keys = await caches?.keys() ?? [];
  await Promise.all(keys.map(k => caches.delete(k)));
});
await page.reload({ waitUntil: 'domcontentloaded' });

Simulating mobile (only when explicitly requested)

The integrated browser panel clamps width, so page.setViewportSize() and CDP setDeviceMetricsOverride narrow the viewport only as far as the panel allows. User-Agent override and touch emulation work fine:

const client = await page.context().newCDPSession(page);
await client.send('Emulation.setUserAgentOverride', {
  userAgent: 'Mozilla/5.0 (iPhone; CPU iPhone OS 17_0 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/17.0 Mobile/15E148 Safari/604.1',
  platform: 'iPhone'
});
await client.send('Emulation.setTouchEmulationEnabled', { enabled: true, maxTouchPoints: 5 });
await client.send('Emulation.setDeviceMetricsOverride', {
  width: 393, height: 852, deviceScaleFactor: 3, mobile: true,
  screenOrientation: { type: 'portraitPrimary', angle: 0 }
});
await page.reload();

For a true mobile viewport, drive a standalone Playwright script with devices['iPhone 14 Pro'] instead of the integrated browser. If a mobile-responsive overlay intercepts pointer events during automation, fall back to { force: true } on click().

Known-noise console messages (ignore)

• Canceled: Canceled at clipboardService.js — cancelled permission probes on hover.
• NotAllowedError: Failed to execute 'write' on 'Clipboard' — web clipboard requires a user gesture.
• [WebTunnelAgentHost] Failed to list tunnels — only fires when not signed in.
• The web worker extension host is started in a same-origin iframe! — expected in dev.
• Unrecognized feature: 'local-network-access' — dev manifest warning.
• [LEAKED DISPOSABLE] stacks — GC-based tracker; only real if reproducible across reloads.

Troubleshooting

| Symptom | Cause | Fix |

| --------------------------------------------------------- | ------------------------------------------ | ---------------------------------------------------------- |

| Cannot find module './indexes' on first run | nodemon started before TS compile finished | Wait; it auto-restarts |

| Session not found: <uuid> when sending chat | Reopened a cloud/tunnel-backed session | Start a fresh session (⌘N) in the Agents window |

| Workspace picker opens native dialog and hangs automation | Select Folder… needs a real file dialog | Pick a workspace URL scheme instead, or skip in automation |

| Stale UI after editing vscode/ sources | Service worker cache | Unregister SWs + clear caches (snippet above) |

Testing the Agents window against a local mock agent host

If the scenario touches the Agents window (/agents route), you almost always need the mock agent host running. Without it, the Agents window will sit on the sign-in / tunnel-discovery screen and block any real interaction. Start it in addition to the dev server — it's a second terminal, not a replacement.

vscode-dev supports a ?mock-agent-host=ws://… URL parameter that short-circuits tunnel discovery and wires the Agents window to a raw WebSocket. Pair it with the mock agent host binary from microsoft/vscode:

cd /path/to/vscode
node out/vs/platform/agentHost/node/agentHostServerMain.js \
  --enable-mock-agent --quiet --without-connection-token --port 8765
# Listens on ws://localhost:8765

Prerequisite: out/ in the vscode repo must be populated by the VS Code - Build task (or npm run watch). If out/vs/platform/agentHost/node/agentHostServerMain.js is missing, start that task first.

--enable-mock-agent registers the ScriptedMockAgent from src/vs/platform/agentHost/test/node/mockAgent.ts with one pre-existing session. Seed additional sessions via the VSCODE_AGENT_HOST_MOCK_SEED_SESSIONS env var, using a comma-separated list of session URIs (for example, VSCODE_AGENT_HOST_MOCK_SEED_SESSIONS=mock://pre-1,mock://pre-2). Scripted prompts include hello, use-tool, error, permission, write-file, run-safe-command, slow, client-tool, subagent, etc. (see mockAgent.ts for the full list).

Then open:

https://127.0.0.1:3000/agents?vscode-quality=dev&mock-agent-host=ws://localhost:8765&vscode-log=trace

Expect these logs in order:

• [MockAgentHost] Using local mock agent host at ws://localhost:8765/
• [WebTunnelAgentHost] Found 1 tunnel(s) with agent host support
• [WebTunnelAgentHost] Connecting to tunnel 'mock-agent-host' (mock)
• [WebTunnelAgentHost] Protocol handshake completed with tunnel:mock
• [RemoteAgentHost] Registered agent mock from tunnel:mock as remote-tunnel__mock-mock

This bypasses GitHub auth and the /agents/api/hosts endpoint entirely, so it works offline. The fake tunnel on the vscode-dev side must advertise a protocolvN tag ≥ TUNNEL_MIN_PROTOCOL_VERSION in src/vs/platform/agentHost/common/tunnelAgentHost.ts (currently 5); otherwise WebTunnelAgentHostService filters it out and you'll see Found 0 tunnel(s) with agent host support.