[FEATURE] Use sparse checkout for plugin installs — only fetch shipping assets, not entire repo

[ViktorHofer]

Problem Statement

When GitHub Copilot CLI installs a plugin, it performs a full git clone of the source repository. This means every file in the repo — tests, CI configs, development tooling, documentation source, images, etc. — is downloaded to the user's machine, even though only the plugin directory is a shipping asset.

This has two problems:

1. Performance: Full clones are unnecessarily slow and disk-heavy, especially for monorepos or repos with large non-plugin content (media, test fixtures, etc.).

2. Shipping asset boundary (more important): Files outside the plugin directory are development artifacts — tests, .github/ workflows, .config/, contributor docs, build scripts, etc. These are not intended to be distributed to end users. Fetching them conflates "installing a plugin" with "cloning a development environment." Plugin consumers should only receive what plugin authors intend to ship.

Proposed Solution

Use Git's partial clone + sparse checkout to fetch only the plugin subdirectory:

# 1. Partial clone — no blobs downloaded yet
git clone --filter=blob:none --sparse --depth=1 <repo-url>
cd <repo>

# 2. Sparse checkout — only materialize the plugin path
git sparse-checkout set <plugin-path>

Flag	Effect
--filter=blob:none	Partial clone — skips downloading file content until needed
--sparse	Enables sparse checkout (only root files initially)
--depth=1	Shallow — only the latest commit, no history

This is well-supported by GitHub's servers and requires Git 2.25+.

Alternative Solutions

Approach	Downside
Full clone (status quo)	Downloads non-shipping content; slower
--depth=1 only (shallow clone)	Still downloads all files across the tree
Download tarball of subdirectory via GitHub API	Loses git-based update/ref mechanism

Additional Context

• Git partial clone + sparse checkout is production-ready and used at scale (e.g., by large monorepos at Microsoft, Google).
• GitHub's servers fully support --filter=blob:none and --filter=tree:0.
• A corresponding issue has been filed for Claude Code: [FEATURE] Use sparse checkout for plugin installs — only fetch shipping assets, not entire repo anthropics/claude-code#40864