diff --git a/crates/socket-patch-core/src/crawlers/pkg_managers.rs b/crates/socket-patch-core/src/crawlers/pkg_managers.rs
index 404434443..3aa4f4491 100644
--- a/crates/socket-patch-core/src/crawlers/pkg_managers.rs
+++ b/crates/socket-patch-core/src/crawlers/pkg_managers.rs
@@ -219,15 +219,16 @@ pub(crate) struct EnvFiles {
 
 impl YarnEnv {
     pub(crate) fn current(scope: crate::utils::repo_root::ReadScope) -> Self {
+        let vars = yarn_process_vars();
         Self {
             // Yarn takes the last `yarn_node_linker` spelling in the
             // environment (each one overwrites the setting) but the first
             // `yarn_rc_filename` (`getRcFilename` returns on a match).
-            node_linker: yarn_env_key(std::env::vars_os(), "YARN_NODE_LINKER", true),
-            pnpm_store_folder: yarn_env_key(std::env::vars_os(), "YARN_PNPM_STORE_FOLDER", true),
-            rc_filename: yarn_env_key(std::env::vars_os(), "YARN_RC_FILENAME", false)
+            node_linker: yarn_env_key(vars.iter().cloned(), "YARN_NODE_LINKER", true),
+            pnpm_store_folder: yarn_env_key(vars.iter().cloned(), "YARN_PNPM_STORE_FOLDER", true),
+            rc_filename: yarn_env_key(vars.iter().cloned(), "YARN_RC_FILENAME", false)
                 .unwrap_or_else(|| ".yarnrc.yml".to_string()),
-            home: crate::utils::fs::home_dir(),
+            home: yarn_home(),
             vars: None,
             env_files: Default::default(),
             scope,
@@ -240,7 +241,11 @@ impl YarnEnv {
         scope: crate::utils::repo_root::ReadScope,
         project_root: &Path,
     ) -> Self {
-        let env_list = yarn_env_key(std::env::vars_os(), "YARN_INJECT_ENVIRONMENT_FILES", true);
+        let env_list = yarn_env_key(
+            yarn_process_vars().into_iter(),
+            "YARN_INJECT_ENVIRONMENT_FILES",
+            true,
+        );
         Self::current(scope).with_env_files(project_root, env_list.as_deref())
     }
 
@@ -331,7 +336,7 @@ impl YarnEnv {
     fn base_var(&self, name: &str) -> Option<String> {
         match &self.vars {
             Some(vars) => vars.get(name).cloned(),
-            None => std::env::var(name).ok(),
+            None => yarn_process_var(name),
         }
     }
 
@@ -360,6 +365,56 @@ impl YarnEnv {
     }
 }
 
+/// The home folder whose rc file [`YarnEnv::current`] reads last.
+#[cfg(not(test))]
+fn yarn_home() -> Option<PathBuf> {
+    crate::utils::fs::home_dir()
+}
+
+/// In unit tests, only a `HOME`/`USERPROFILE` a live `EnvGuard` sets: a
+/// developer's `~/.yarnrc.yml` can't change what the tests see (#1919).
+#[cfg(test)]
+fn yarn_home() -> Option<PathBuf> {
+    crate::utils::fs::home_from_env(|key| crate::test_env::guarded_var(key).map(Into::into))
+}
+
+/// The process environment the `YARN_*` lookups of [`YarnEnv::current`]
+/// and [`YarnEnv::for_project`] read.
+#[cfg(not(test))]
+fn yarn_process_vars() -> Vec<(std::ffi::OsString, std::ffi::OsString)> {
+    std::env::vars_os().collect()
+}
+
+/// In unit tests, only the variables a live `EnvGuard` holds: a
+/// developer's `YARN_NODE_LINKER` (or `YARN_PNPM_STORE_FOLDER`,
+/// `YARN_RC_FILENAME`, `YARN_INJECT_ENVIRONMENT_FILES`) can't change what
+/// the tests see (#1919), as `utils::go_env` does for Go. Windows keeps a
+/// key's existing spelling when a guard sets it in another case, so there
+/// the upper-case spelling a guard uses counts too.
+#[cfg(test)]
+fn yarn_process_vars() -> Vec<(std::ffi::OsString, std::ffi::OsString)> {
+    let held = |key: &str| {
+        crate::test_env::guarded_var(key).is_some()
+            || (cfg!(windows) && crate::test_env::guarded_var(&key.to_ascii_uppercase()).is_some())
+    };
+    std::env::vars_os()
+        .filter(|(key, _)| key.to_str().is_some_and(held))
+        .collect()
+}
+
+/// The variable a `${NAME}` reference in a yarn rc or env file expands
+/// from when [`YarnEnv::vars`] is `None`.
+#[cfg(not(test))]
+fn yarn_process_var(name: &str) -> Option<String> {
+    std::env::var(name).ok()
+}
+
+/// In unit tests, only a variable a live `EnvGuard` sets (#1919).
+#[cfg(test)]
+fn yarn_process_var(name: &str) -> Option<String> {
+    crate::test_env::guarded_var(name)
+}
+
 /// The yarn environment setting `key` (upper case) in `vars`: yarn
 /// lower-cases every environment key before matching it, so
 /// `yarn_node_linker` sets the linker as `YARN_NODE_LINKER` does (#1100;
@@ -704,6 +759,45 @@ impl YarnPnpLoader {
 mod tests {
     use super::*;
 
+    /// The unit tests read a `YARN_*` variable or the home folder only
+    /// while a guard sets it, so a developer's `YARN_NODE_LINKER` or
+    /// `~/.yarnrc.yml` can't flip a test (#1919), while a test that sets
+    /// one still sees it.
+    #[test]
+    #[serial_test::serial]
+    fn yarn_env_sees_only_guarded_variables() {
+        use crate::utils::repo_root::ReadScope;
+        let home = tempfile::tempdir().unwrap();
+        {
+            let _linker = crate::test_env::EnvGuard::many([
+                (
+                    "YARN_NODE_LINKER",
+                    Some(std::ffi::OsStr::new("node-modules")),
+                ),
+                (
+                    "YARN_PNPM_STORE_FOLDER",
+                    Some(std::ffi::OsStr::new("store")),
+                ),
+                ("YARN_RC_FILENAME", Some(std::ffi::OsStr::new(".alt.yml"))),
+                ("SOCKET_YARN_TEST_VAR", Some(std::ffi::OsStr::new("x"))),
+                ("HOME", Some(home.path().as_os_str())),
+            ]);
+            let env = YarnEnv::current(ReadScope::Workspace);
+            assert_eq!(env.node_linker.as_deref(), Some("node-modules"));
+            assert_eq!(env.pnpm_store_folder.as_deref(), Some("store"));
+            assert_eq!(env.rc_filename, ".alt.yml");
+            assert_eq!(env.home.as_deref(), Some(home.path()));
+            assert_eq!(env.var("SOCKET_YARN_TEST_VAR").as_deref(), Some("x"));
+        }
+        let env = YarnEnv::current(ReadScope::Workspace);
+        assert_eq!(env.node_linker, None);
+        assert_eq!(env.home, None);
+        assert_eq!(env.pnpm_store_folder, None);
+        assert_eq!(env.rc_filename, ".yarnrc.yml");
+        assert_eq!(env.var("SOCKET_YARN_TEST_VAR"), None);
+        assert_eq!(env.var("PATH"), None, "an unguarded host variable");
+    }
+
     /// The public entry points only add the host's yarn env, which is
     /// asked for a loader file alone: without one they agree with the
     /// hermetic variants the other tests use.