@@ -2343,12 +2343,47 @@ pub fn get_yarn_global_prefix() -> Option<String> {
 /// Version of `get_yarn_global_prefix` that accepts an injected
 /// `CommandRunner`. See `get_npm_global_prefix_with`.
 pub fn get_yarn_global_prefix_with(runner: &dyn CommandRunner) -> Option<String> {
-    parse_yarn_dir_output(
+    get_yarn_global_prefix_or(
+        runner,
+        crate::patch::redirect::yarnrc::yarn_1_0_global_folders_for_process,
+    )
+}
+
+/// [`get_yarn_global_prefix_with`] with the yarn 1.0.x global folder
+/// candidates injected. Yarn 1.0.0 - 1.0.2 has no `yarn global dir` (it
+/// exits 1 with "Invalid subcommand"), so the first candidate that holds
+/// a `node_modules` stands in for its answer when `yarn --version`
+/// reports 1.0.x (#437). The version is asked only once such a folder
+/// exists, so a yarn without the subcommand for another reason (Berry
+/// has no `global`) costs no second spawn.
+pub fn get_yarn_global_prefix_or(
+    runner: &dyn CommandRunner,
+    yarn_1_0_folders: impl FnOnce() -> Vec<PathBuf>,
+) -> Option<String> {
+    let reported = parse_yarn_dir_output(
         runner
             .run("yarn", &["global", "dir"])
             .as_deref()
             .unwrap_or(""),
-    )
+    );
+    if reported.is_some() {
+        return reported;
+    }
+    let node_modules = yarn_1_0_folders()
+        .into_iter()
+        .map(|dir| dir.join("node_modules"))
+        .find(|dir| is_dir_sync(dir))?;
+    let version = runner.run("yarn", &["--version"])?;
+    is_yarn_1_0(&version).then(|| node_modules.to_string_lossy().to_string())
+}
+
+/// Whether `yarn --version` stdout names a 1.0.x release.
+fn is_yarn_1_0(version: &str) -> bool {
+    version
+        .trim()
+        .strip_prefix("1.0.")
+        .and_then(|patch| patch.split(['-', '+']).next())
+        .is_some_and(|patch| !patch.is_empty() && patch.bytes().all(|b| b.is_ascii_digit()))
 }
 
 /// Pure parser for `yarn global dir` stdout. Returns `<dir>/node_modules`
 
@@ -567,6 +567,41 @@ pub fn configured_yarn_global_folders_for_process() -> Vec<PathBuf> {
     )
 }
 
+/// The global folder yarn 1.0.x uses by default, which it can't report:
+/// `yarn global dir` only exists from yarn 1.1.0 (#437). This is yarn
+/// 1.0.2's `GLOBAL_MODULE_DIRECTORY`, `<CONFIG_DIRECTORY>/global`, where
+/// the config directory is `%LOCALAPPDATA%\Yarn\config` on Windows (when
+/// `LOCALAPPDATA` is set) and `<userHome>/.config/yarn` otherwise.
+pub fn yarn_1_0_default_global_folder(env: &NpmConfigEnv, root_user: bool) -> Option<PathBuf> {
+    if env.windows {
+        if let Some(local) = env.var("LOCALAPPDATA") {
+            return Some(Path::new(local).join("Yarn").join("config").join("global"));
+        }
+    }
+    let user_home = RcLayout::new(env, root_user).user_home?;
+    Some(user_home.join(".config").join("yarn").join("global"))
+}
+
+/// Where yarn 1.0.x keeps this user's global packages, most likely
+/// first: any folder configured as yarn's global folder (yarn 1.0.x reads
+/// `--global-folder` from `.yarnrc`), then its default. The config is read
+/// as yarn run from the neutral probe dir sees it, like the `yarn global
+/// dir` probe it stands in for, so a project's `.yarnrc` can't answer
+/// where the machine-wide install lives.
+pub fn yarn_1_0_global_folders_for_process() -> Vec<PathBuf> {
+    let env = NpmConfigEnv::from_process();
+    let root_user = process_is_root_user();
+    let mut folders = crate::utils::process::neutral_probe_dir()
+        .map(|dir| {
+            configured_yarn_global_folders(&env, root_user, &dir, |path| {
+                crate::utils::fs::read_regular_to_string_sync(path).ok()
+            })
+        })
+        .unwrap_or_default();
+    folders.extend(yarn_1_0_default_global_folder(&env, root_user));
+    folders
+}
+
 /// Whether this process runs as uid 0 (yarn's `getuid() === 0`).
 pub fn process_is_root_user() -> bool {
     #[cfg(unix)]
 
@@ -610,3 +610,69 @@ fn global_folder_values_read_like_yarn() {
         paths(&["/opt/nq"])
     );
 }
+
+/// #437: yarn 1.0.x can't report its global folder, so it is derived the
+/// way yarn 1.0.2's `constants.js` does: `<userHome>/.config/yarn/global`
+/// (`userHome` is `/usr/local/share` for a root user that isn't under
+/// fakeroot), or `%LOCALAPPDATA%\Yarn\config\global` on Windows.
+#[test]
+fn yarn_1_0_default_global_folder_follows_yarn_1_0_2() {
+    let vars = |pairs: &[(&str, &str)]| -> Vec<(String, String)> {
+        pairs
+            .iter()
+            .map(|(k, v)| (k.to_string(), v.to_string()))
+            .collect()
+    };
+    let unix = NpmConfigEnv::from_parts(vars(&[("HOME", "/home/u")]), None, false);
+    assert_eq!(
+        yarn_1_0_default_global_folder(&unix, false),
+        Some(PathBuf::from("/home/u/.config/yarn/global"))
+    );
+    assert_eq!(
+        yarn_1_0_default_global_folder(&unix, true),
+        Some(PathBuf::from("/usr/local/share/.config/yarn/global"))
+    );
+    let fakeroot = NpmConfigEnv::from_parts(
+        vars(&[("HOME", "/home/u"), ("FAKEROOTKEY", "1")]),
+        None,
+        false,
+    );
+    assert_eq!(
+        yarn_1_0_default_global_folder(&fakeroot, true),
+        Some(PathBuf::from("/home/u/.config/yarn/global"))
+    );
+    assert_eq!(
+        yarn_1_0_default_global_folder(&NpmConfigEnv::from_parts(Vec::new(), None, false), false),
+        None,
+        "no home, no folder"
+    );
+
+    let windows = NpmConfigEnv::from_parts(
+        vars(&[
+            ("USERPROFILE", r"C:\Users\u"),
+            ("LOCALAPPDATA", r"C:\Users\u\AppData\Local"),
+        ]),
+        None,
+        true,
+    );
+    assert_eq!(
+        yarn_1_0_default_global_folder(&windows, false),
+        Some(
+            Path::new(r"C:\Users\u\AppData\Local")
+                .join("Yarn")
+                .join("config")
+                .join("global")
+        )
+    );
+    // Without LOCALAPPDATA yarn 1.0.x falls back to the home layout.
+    let windows = NpmConfigEnv::from_parts(vars(&[("USERPROFILE", r"C:\Users\u")]), None, true);
+    assert_eq!(
+        yarn_1_0_default_global_folder(&windows, false),
+        Some(
+            Path::new(r"C:\Users\u")
+                .join(".config")
+                .join("yarn")
+                .join("global")
+        )
+    );
+}
@@ -10,9 +10,9 @@ use socket_patch_core::crawlers::npm_crawler::{
     build_npm_purl, bun_global_dir_from_env, find_store_peer_variant_copies, get_bun_global_prefix,
     get_bun_global_prefix_with, get_npm_global_prefix, get_npm_global_prefix_with,
     get_pnpm_global_prefix, get_pnpm_global_prefix_with, get_yarn_global_prefix,
-    get_yarn_global_prefix_with, parse_bun_ls_global_output, parse_npm_root_output,
-    parse_package_name, parse_pnpm_root_output, parse_yarn_dir_output, read_package_json,
-    resolve_bun_global_prefix_with,
+    get_yarn_global_prefix_or, get_yarn_global_prefix_with, parse_bun_ls_global_output,
+    parse_npm_root_output, parse_package_name, parse_pnpm_root_output, parse_yarn_dir_output,
+    read_package_json, resolve_bun_global_prefix_with,
 };
 use socket_patch_core::crawlers::types::CrawlerOptions;
 use socket_patch_core::crawlers::NpmCrawler;
@@ -352,6 +352,130 @@ fn get_yarn_global_prefix_with_mock_runner_success() {
     );
 }
 
+/// Sets environment variables for a test and puts the old values back on
+/// drop, a panic included.
+struct EnvGuard(Vec<(&'static str, Option<std::ffi::OsString>)>);
+
+impl EnvGuard {
+    fn set(vars: &[(&'static str, &Path)]) -> Self {
+        let saved = vars
+            .iter()
+            .map(|(k, v)| {
+                let old = std::env::var_os(k);
+                std::env::set_var(k, v);
+                (*k, old)
+            })
+            .collect();
+        EnvGuard(saved)
+    }
+}
+
+impl Drop for EnvGuard {
+    fn drop(&mut self) {
+        for (k, v) in self.0.drain(..) {
+            match v {
+                Some(v) => std::env::set_var(k, v),
+                None => std::env::remove_var(k),
+            }
+        }
+    }
+}
+
+/// #437: yarn 1.0.0 - 1.0.2 has no `yarn global dir` (it exits 1), so
+/// the folder yarn 1.0.x itself uses stands in for the answer it can't
+/// give. Real process environment: HOME points at a scratch home and
+/// FAKEROOTKEY keeps a root test runner on the plain-home layout.
+#[cfg(unix)]
+#[test]
+#[serial_test::serial]
+fn get_yarn_global_prefix_with_falls_back_for_yarn_1_0() {
+    let home = tempfile::tempdir().unwrap();
+    let global = home.path().join(".config/yarn/global/node_modules");
+    std::fs::create_dir_all(global.join("left-pad")).unwrap();
+    let _env = EnvGuard::set(&[("HOME", home.path()), ("FAKEROOTKEY", Path::new("1"))]);
+    let runner = common::MockCommandRunner::new()
+        .with_response("yarn", &["global", "dir"], None)
+        .with_response("yarn", &["--version"], Some("1.0.2\n"));
+    assert_eq!(
+        get_yarn_global_prefix_with(&runner).map(PathBuf::from),
+        Some(global.clone())
+    );
+
+    // A project's `.yarnrc` in the cwd names a global folder that exists:
+    // it can't stand in for the machine-wide answer, which yarn run from
+    // the neutral probe dir (HOME) never sees.
+    let project = tempfile::tempdir().unwrap();
+    std::fs::create_dir_all(project.path().join("g/node_modules")).unwrap();
+    std::fs::write(project.path().join(".yarnrc"), "--global-folder \"./g\"\n").unwrap();
+    let cwd = std::env::current_dir().unwrap();
+    std::env::set_current_dir(project.path()).unwrap();
+    let got = get_yarn_global_prefix_with(&runner);
+    std::env::set_current_dir(cwd).unwrap();
+    assert_eq!(got.map(PathBuf::from), Some(global));
+}
+
+/// The fallback is for yarn 1.0.x only: a newer yarn whose
+/// `yarn global dir` failed, or no yarn at all, still finds nothing. The
+/// first candidate folder that exists wins, and none existing finds
+/// nothing without asking yarn for its version.
+#[test]
+#[serial_test::parallel]
+fn get_yarn_global_prefix_or_falls_back_only_for_yarn_1_0() {
+    let tmp = tempfile::tempdir().unwrap();
+    let configured = tmp.path().join("configured");
+    let default = tmp.path().join("default");
+    std::fs::create_dir_all(default.join("node_modules")).unwrap();
+    let folders = || vec![configured.clone(), default.clone()];
+    let want = default.join("node_modules");
+    for version in ["1.0.0", "1.0.1\n", "1.0.2", "1.0.2-rc.1"] {
+        let runner = common::MockCommandRunner::new()
+            .with_response("yarn", &["global", "dir"], None)
+            .with_response("yarn", &["--version"], Some(version));
+        assert_eq!(
+            get_yarn_global_prefix_or(&runner, folders).map(PathBuf::from),
+            Some(want.clone()),
+            "{version:?}"
+        );
+    }
+    for version in [
+        Some("1.1.0"),
+        Some("1.22.22"),
+        Some("1.0"),
+        Some("4.5.0"),
+        None,
+    ] {
+        let runner = common::MockCommandRunner::new()
+            .with_response("yarn", &["global", "dir"], None)
+            .with_response("yarn", &["--version"], version);
+        assert_eq!(
+            get_yarn_global_prefix_or(&runner, folders),
+            None,
+            "{version:?}"
+        );
+    }
+    // A configured global folder that exists comes before the default.
+    std::fs::create_dir_all(configured.join("node_modules")).unwrap();
+    let runner = common::MockCommandRunner::new()
+        .with_response("yarn", &["global", "dir"], None)
+        .with_response("yarn", &["--version"], Some("1.0.2"));
+    assert_eq!(
+        get_yarn_global_prefix_or(&runner, folders).map(PathBuf::from),
+        Some(configured.join("node_modules"))
+    );
+    // No candidate folder: no version probe is needed (the mock has no
+    // answer for it either way).
+    let runner = common::MockCommandRunner::new().with_response("yarn", &["global", "dir"], None);
+    assert_eq!(get_yarn_global_prefix_or(&runner, Vec::new), None);
+    // A yarn that answers `yarn global dir` is taken at its word.
+    let runner = common::MockCommandRunner::new()
+        .with_response("yarn", &["global", "dir"], Some("/other/global\n"))
+        .with_response("yarn", &["--version"], Some("1.0.2"));
+    assert_eq!(
+        get_yarn_global_prefix_or(&runner, folders).map(PathBuf::from),
+        Some(Path::new("/other/global").join("node_modules"))
+    );
+}
+
 #[test]
 #[serial_test::parallel]
 fn get_pnpm_global_prefix_with_mock_runner_success() {