[Bug]: Windows: html_preview/html_render fail with "Internal error" when Group Policy blocks chrome-headless-shell.exe in %USERPROFILE%\.t3\tools #21686

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Use T3 Code (Beta) desktop on a managed Windows PC where Group Policy (AppLocker / Software Restriction Policies) only allows executables from approved locations. That's common in corporate and university environments. On this PC, T3 Code itself is installed in an approved folder (C:\GBW_MyPrograms\t3-code-desktop\).
2. Have an agent call html_preview or html_render.
3. T3 downloads Chrome for Testing's headless shell to %USERPROFILE%\.t3\tools\chrome-headless-shell\win64\<version>\chrome-headless-shell.exe and tries to spawn it.

Expected behavior

The preview browser should launch from a location the PC's policy already allows, for example:

• next to the installed T3 Code app (<install dir>\resources\...), which policy already trusts because T3 Code itself runs from there; and/or
• a user-configurable path, such as an env var or setting (T3CODE_PREVIEW_BROWSER_PATH) or a configurable tools directory, so it can point to an approved location or an already-installed Chrome/Edge.

When the launch is blocked, T3 should report a clear error that says Windows policy blocked the browser and where it is installed.

Actual behavior

Windows blocks the executable because it is unsigned and runs from the user profile ("This program is blocked by group policy"). Node's spawn fails with spawn UNKNOWN, and both html_preview and html_render return only Internal error. The agent then falls back to Mermaid/text and tells the user the "preview service returned an internal error".

The tools directory is always path.join(baseDir, "tools", "chrome-headless-shell", ...). The only way to change it is T3CODE_HOME, and that moves all of T3's state, not just the browser.

Impact

Major degradation or frequent failure: inline HTML visuals never work on policy-managed Windows machines.

Version or commit

T3 Code (Beta) desktop 0.1.0; chrome-headless-shell 154.0.8037.92

Environment

Windows 11 Enterprise 10.0.26200 (domain-managed, AppLocker/SRP path rules); Claude Code provider

Logs or stack traces

HtmlRender.preview / HtmlRender.publish failed:
Error: spawn UNKNOWN
    at ChildProcess.spawn (node:internal/child_process:458:11)
    at Module.spawn (node:child_process:813:9)
    at headlessChrome.captureHtmlScreenshot (.../resources/server.asar/apps/server/dist/binCli-*.mjs)
    at HtmlRender.preview (...)
    at McpServer.@effect/mcp/2025-06-18/tools/call

# Running the exe directly from PowerShell:
This command cannot be run due to the error: This program is blocked by group policy.
Get-AuthenticodeSignature chrome-headless-shell.exe -> Status: NotSigned

Workaround

Move the browser into an approved folder and leave a directory junction at the path T3 expects. AppLocker evaluates the resolved path, so the launch is allowed:

Move-Item "$env:USERPROFILE\.t3\tools\chrome-headless-shell" "C:\<approved dir>\t3-chrome-headless-shell"
New-Item -ItemType Junction -Path "$env:USERPROFILE\.t3\tools\chrome-headless-shell" -Target "C:\<approved dir>\t3-chrome-headless-shell"

After that, html_preview and html_render work. A T3 update or re-install of the tools folder could break this, so a supported option would be better.