Public plugin marketplaces fail on Windows ARM64 with fatal: unable to access NUL

[brandonthomas]

Short summary

No response

Affected version or release

GitHub Copilot app 1.1.13; bundled Copilot CLI file version 1.0.80

Installation context

GitHub Copilot app on Windows ARM64, OS build 26200

What happened?

The Copilot app cannot refresh either public built-in plugin marketplace:

• github/copilot-plugins
• github/awesome-copilot

Both fail during git clone with:

fatal: unable to access 'NUL': Invalid argument

Errors:

Failed to fetch GitHub marketplace github/copilot-plugins: Command failed: git clone --depth 1 --progress https://github.com/github/copilot-plugins.git C:\Users<user>\AppData\Local\copilot\marketplaces\github-copilot-plugins
fatal: unable to access 'NUL': Invalid argument

Failed to fetch GitHub marketplace github/awesome-copilot: Command failed: git clone --depth 1 --progress --branch marketplace https://github.com/github/awesome-copilot.git C:\Users<user>\AppData\Local\copilot\marketplaces\github-awesome-copilot
fatal: unable to access 'NUL': Invalid argument

These repositories are public.

Steps to reproduce

1. Open GitHub Copilot app on Windows ARM64.
2. Open plugin settings.
3. Refresh marketplaces or select Update plugins.
4. Both built-in marketplaces fail with fatal: unable to access 'NUL': Invalid argument.

Expected behavior

The app should clone and refresh both public built-in marketplaces successfully.

Additional context

The failure appears isolated to Git launched by the Copilot app:

• Both repositories clone successfully with Git from a terminal.
• Standalone Copilot CLI 1.0.81-9 successfully registers GitHub marketplaces.
• Local Git status, fetch, and ls-remote operations succeed.
• Refreshing app GitHub credentials and restarting did not help.
• Renaming %LOCALAPPDATA%\copilot\marketplaces forced a clean cache; the error persisted and only lock files remained.
• Renaming %LOCALAPPDATA%\github-copilot-sdk forced a fresh bundled runtime download; the error persisted.
• The fresh CLI binary exactly matched the previous SHA-256, ruling out runtime corruption.
• The app launches %LOCALAPPDATA%\github-copilot-sdk\cli\1.0.80\copilot.exe --server --stdio --no-auto-update.

This rules out stale marketplace caches, installed plugins, repository authentication, general Git/network failure, and a corrupted bundled CLI download.

[brandonthomas]

Additional root-cause findings

I traced this beyond the marketplace and credential layers. This appears to be an app-side regression exposing long-standing Git for Windows behavior, rather than a regression caused by updating Git or the standalone Copilot CLI.

The app-launched process sets:

GIT_CONFIG_NOSYSTEM=1
GIT_CONFIG_GLOBAL=NUL
GIT_TERMINAL_PROMPT=0
GCM_INTERACTIVE=Never

Setting only GIT_CONFIG_GLOBAL=NUL reproduces the exact failure with a public repository. Changing the value to lowercase nul, /dev/null, or a real empty file succeeds. Both the app-bundled Git 2.53.0.windows.4 and installed Git 2.55.0.windows.3 behave identically, so this is not new to Git 2.53.

The Git failure path is:

1. config.c reads GIT_CONFIG_GLOBAL as the global configuration path.
2. config.c checks that path with access_or_die().
3. wrapper.c emits fatal: unable to access 'NUL': Invalid argument when that check returns EINVAL.
4. Git for Windows mingw_access() special-cases only lowercase nul and /dev/null, using case-sensitive strcmp() calls. Uppercase NUL falls through to the failing Windows path handling.

That special case was introduced in [git-for-windows/git@9160068](git-for-windows/git@9160068) on April 16, 2021 because Microsoft's Universal C Runtime does not support access("NUL", flags). Therefore, an updated Git dependency does not explain why this started recently.

Binary inspection locates the originating app code in the first-party Rust module github_app::session::cli, with the embedded private source path src-tauri\src\session\cli.rs. The relevant Git environment strings occur together in that module. The implementation source is not present in the public github/app repository, so I cannot identify the private commit that introduced or activated this behavior.

App 1.1.12 already contains the Git-environment and bundled_git experiment scaffolding. Combined with app 1.1.13 making Customize generally available, this may be a feature/experiment rollout exposing a latent bug rather than a newly updated dependency. [#3194](#3194) independently reports the same error through the new Customize interface on app 1.1.13.

The fact that standalone Copilot CLI 1.0.81-9 works does not by itself prove that it contains a hotfix: when run standalone, it is not launched with the app's GIT_CONFIG_GLOBAL=NUL override.

Recommended fix

The app should use a real empty temporary Git configuration file instead of the Windows null device. Lowercase nul and /dev/null work with current Git for Windows, but a real file is less dependent on implementation-specific device handling. Git for Windows could separately harden mingw_access() by treating NUL case-insensitively.