Symptom#

After cargo test (or any test run), the developer's real tool configuration has changed: for example a plugin and a marketplace installed in their real Claude Code, pointing at a temporary folder that no longer exists.

Cause#

The code under test runs an external command with side effects (here claude plugin marketplace add and claude plugin install) through a program name that defaults to the real command unless an environment variable names a fake. Tests that forget the fake, often older tests written before the command was added, run the real command against the real $HOME.

Fix#

Make the program fail closed in test builds, so a forgotten fake fails instead of reaching the real tool:

pub fn external_program() -> String {
    if cfg!(test) {
        return "/nonexistent/program-in-unit-tests".into();
    }
    std::env::var("MYTOOL_PROGRAM").unwrap_or_else(|_| "program".into())
}

Do the same for every path taken from the environment (XDG_DATA_HOME, XDG_CACHE_HOME, HOME), and give integration tests their own HOME, XDG_*, working directory and a fake script on PATH. Clean up anything that leaked with the tool's own commands, not by editing its files.

Evidence#

In rkb (project rkb), a doctor unit test called the install plan without the fake and ran the real claude: it left rkb@rkb, a rkb marketplace and ~/.claude/plugins/cache/rkb/rkb/0.1.0-664efa454a8a in the real Claude Code. claude plugin uninstall rkb@rkb and claude plugin marketplace remove rkb removed them; after the cfg!(test) guard, repeated full test runs left the real config unchanged. Sharing the real cache folder also made a test fail now and then in parallel runs.