diff --git a/cli/src/cli/exec.rs b/cli/src/cli/exec.rs
index 82766c5b..c56c621f 100644
--- a/cli/src/cli/exec.rs
+++ b/cli/src/cli/exec.rs
@@ -8,6 +8,8 @@ use miette::IntoDiagnostic;
 
 use usage::Spec;
 
+use crate::env;
+
 #[derive(Debug, Args)]
 #[clap(
     disable_help_flag = true,
@@ -75,9 +77,7 @@ impl Exec {
             .collect_vec();
         cmd.args(&args);
 
-        for (key, val) in &parsed.as_env() {
-            cmd.env(key, val);
-        }
+        env::apply_parsed_env(&mut cmd, &parsed.as_env());
 
         let result = cmd.spawn().into_diagnostic()?.wait().into_diagnostic()?;
 
diff --git a/cli/src/cli/shell.rs b/cli/src/cli/shell.rs
index 670c7bd9..cf8a884b 100644
--- a/cli/src/cli/shell.rs
+++ b/cli/src/cli/shell.rs
@@ -8,6 +8,8 @@ use miette::IntoDiagnostic;
 
 use usage::Spec;
 
+use crate::env;
+
 /// Execute a shell script with the specified shell
 ///
 /// Typically, this will be called by a script's shebang.
@@ -60,9 +62,7 @@ impl Shell {
             .collect_vec();
         cmd.args(&args);
 
-        for (key, val) in &parsed.as_env() {
-            cmd.env(key, val);
-        }
+        env::apply_parsed_env(&mut cmd, &parsed.as_env());
 
         let result = cmd.spawn().into_diagnostic()?.wait().into_diagnostic()?;
 
diff --git a/cli/src/env.rs b/cli/src/env.rs
index 6e415949..7fd84814 100644
--- a/cli/src/env.rs
+++ b/cli/src/env.rs
@@ -1,5 +1,231 @@
+use std::collections::{BTreeMap, HashSet};
+use std::process::Command;
+
 pub use std::env::*;
 
 pub fn var_true(key: &str) -> bool {
     matches!(var(key), Ok(v) if v == "1" || v == "true")
 }
+
+/// Hand the parsed spec's variables to a command we are about to spawn.
+///
+/// On Windows this is not just `Command::env`. The executable search order there puts the
+/// system directory ahead of `PATH`, so `bash` resolves to `C:\Windows\System32\bash.exe` —
+/// the WSL launcher — on any machine with WSL installed, whatever else is on `PATH`. WSL only
+/// carries a Win32 variable across the boundary if `WSLENV` names it, so without this the
+/// script runs with every `usage_*` variable unset, silently and with no error.
+pub fn apply_parsed_env(cmd: &mut Command, env: &BTreeMap<String, String>) {
+    for (key, val) in env {
+        cmd.env(key, val);
+    }
+    if env.is_empty() {
+        return;
+    }
+    // `cfg!` rather than `#[cfg(windows)]`: CI only runs on Linux, so a `#[cfg]` block here
+    // would never be compiled, type-checked or linted anywhere. This compiles everywhere and
+    // optimizes away off Windows.
+    if cfg!(windows) {
+        let existing = var("WSLENV").ok();
+        let keys = env.keys().map(String::as_str);
+        cmd.env("WSLENV", append_to_wslenv(existing.as_deref(), keys));
+    }
+}
+
+/// Whether an entry already in `WSLENV` delivers its value to WSL unchanged.
+///
+/// Only a bare name or `/u` does. Measured against WSL rather than read off the flag list,
+/// because two of them lose the value outright in this direction:
+///
+/// | entry     | `FOO=bar`        | `FOO=C:\Windows` |
+/// | --------- | ---------------- | ---------------- |
+/// | `FOO`     | `bar`            | `C:\Windows`     |
+/// | `FOO/u`   | `bar`            | `C:\Windows`     |
+/// | `FOO/w`   | *unset*          | *unset*          |
+/// | `FOO/uw`  | *unset*          | *unset*          |
+/// | `FOO/p`   | *unset*          | `/mnt/c/Windows` |
+/// | `FOO/l`   | *unset*          | `/mnt/c/Windows` |
+///
+/// `/w` is the other direction only, and `/p` and `/l` translate the value as a path — which
+/// drops anything that is not one. usage's values are arbitrary strings off a command line, so
+/// a `/p` entry would silently swallow almost all of them.
+fn carries_value_verbatim(entry: &str) -> bool {
+    match entry.split_once('/') {
+        None => true,
+        Some((_, flags)) => !flags.is_empty() && flags.chars().all(|flag| flag == 'u'),
+    }
+}
+
+/// Add `keys` to a `WSLENV` value, preserving whatever was already there.
+///
+/// `WSLENV` is a `:`-separated list of *variable names*, each optionally suffixed with flags.
+/// Names are added bare: usage has no idea whether a given value is a path, and `/p` would
+/// silently rewrite anything that merely looks like one. Unflagged names copy the value
+/// verbatim, so a script sees the same bytes it would on Unix.
+///
+/// Existing entries are never rewritten or dropped — a name the caller configured is theirs.
+/// But an existing entry for a name usage is about to set does not stop usage adding its own
+/// bare one unless it [carries the value verbatim](carries_value_verbatim): a `usage_foo/p`
+/// inherited from somewhere would otherwise mean the script sees nothing at all. Listing the
+/// name twice is how it is fixed rather than a problem to avoid — WSL takes the entry that
+/// transfers, so `FOO/p:FOO` arrives as plain `FOO`.
+///
+/// Takes the current value as an argument instead of reading the environment so it stays a
+/// pure function, testable on every platform rather than only where it does anything.
+pub fn append_to_wslenv<'a>(
+    existing: Option<&str>,
+    keys: impl IntoIterator<Item = &'a str>,
+) -> String {
+    let mut entries: Vec<&str> = vec![];
+    let mut names: HashSet<&str> = HashSet::new();
+
+    for entry in existing.unwrap_or_default().split(':') {
+        // Absorbs a leading, trailing or doubled `:`, either inherited or left by a caller
+        // that built the list by naive concatenation.
+        if entry.is_empty() {
+            continue;
+        }
+        if carries_value_verbatim(entry) {
+            names.insert(entry.split('/').next().unwrap_or(entry));
+        }
+        entries.push(entry);
+    }
+
+    for key in keys {
+        // A name carrying `:` or `/` would not just fail to transfer, it would corrupt the
+        // rest of the list and take the caller's own entries down with it. `as_env` derives
+        // names from `to_snake_case`, which cannot produce either, so this is a guard against
+        // that changing out from under us rather than a case we expect.
+        if key.is_empty() || key.contains(':') || key.contains('/') {
+            continue;
+        }
+        if names.insert(key) {
+            entries.push(key);
+        }
+    }
+
+    entries.join(":")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn append(existing: Option<&str>, keys: &[&str]) -> String {
+        append_to_wslenv(existing, keys.iter().copied())
+    }
+
+    #[test]
+    fn wslenv_adds_keys_in_order() {
+        assert_eq!(append(None, &["usage_workspace"]), "usage_workspace");
+        assert_eq!(
+            append(None, &["usage_workspace", "usage_region"]),
+            "usage_workspace:usage_region"
+        );
+    }
+
+    #[test]
+    fn wslenv_appends_after_existing_entries() {
+        assert_eq!(append(Some("FOO"), &["usage_a"]), "FOO:usage_a");
+    }
+
+    #[test]
+    fn wslenv_leaves_existing_flags_untouched() {
+        assert_eq!(
+            append(Some("FOO/p:BAR/l"), &["usage_a"]),
+            "FOO/p:BAR/l:usage_a"
+        );
+    }
+
+    #[test]
+    fn wslenv_does_not_repeat_an_existing_name() {
+        assert_eq!(
+            append(Some("usage_a"), &["usage_a", "usage_b"]),
+            "usage_a:usage_b"
+        );
+    }
+
+    #[test]
+    fn wslenv_treats_a_direction_only_entry_as_covering_the_name() {
+        // `/u` is this direction — Win32 invoking WSL — so the value already arrives intact.
+        assert_eq!(append(Some("usage_a/u"), &["usage_a"]), "usage_a/u");
+    }
+
+    #[test]
+    fn wslenv_adds_its_own_entry_beside_one_that_would_lose_the_value() {
+        // `/p` translates the value as a path and drops anything that is not one; `/w` is the
+        // other direction entirely. Neither would deliver a parsed argument, so usage adds a
+        // bare entry after it — WSL then takes the one that transfers.
+        for flags in ["/p", "/l", "/w", "/uw"] {
+            let existing = format!("usage_a{flags}");
+            assert_eq!(
+                append(Some(&existing), &["usage_a"]),
+                format!("{existing}:usage_a"),
+                "an inherited {existing} must not swallow the value"
+            );
+        }
+    }
+
+    #[test]
+    fn carries_value_verbatim_only_for_bare_names_and_u() {
+        assert!(carries_value_verbatim("FOO"));
+        assert!(carries_value_verbatim("FOO/u"));
+        for entry in [
+            "FOO/p", "FOO/l", "FOO/w", "FOO/uw", "FOO/wu", "FOO/pu", "FOO/",
+        ] {
+            assert!(!carries_value_verbatim(entry), "{entry}");
+        }
+    }
+
+    #[test]
+    fn wslenv_drops_empty_segments() {
+        assert_eq!(append(Some(""), &["usage_a"]), "usage_a");
+        assert_eq!(append(Some("::FOO::"), &["usage_a"]), "FOO:usage_a");
+    }
+
+    #[test]
+    fn wslenv_with_no_keys_returns_existing() {
+        assert_eq!(append(Some("FOO"), &[]), "FOO");
+        assert_eq!(append(None, &[]), "");
+    }
+
+    #[test]
+    fn wslenv_skips_keys_that_would_corrupt_the_list() {
+        assert_eq!(append(None, &["ok", "bad:name"]), "ok");
+        assert_eq!(append(None, &["ok", "bad/p"]), "ok");
+        assert_eq!(append(None, &["", "ok"]), "ok");
+    }
+
+    #[test]
+    fn wslenv_dedups_within_the_new_keys() {
+        assert_eq!(append(None, &["a", "a"]), "a");
+    }
+
+    #[test]
+    fn wslenv_adds_no_flags() {
+        // Values are arbitrary strings, not known to be paths, so they must cross verbatim.
+        assert!(!append(None, &["usage_a"]).contains('/'));
+    }
+
+    #[test]
+    fn parsed_env_keys_are_safe_for_wslenv() {
+        // `append_to_wslenv` skips names containing `:` or `/`. Nothing usage-lib produces
+        // should ever hit that path; if `as_env` starts generating such names, variables
+        // would go missing on Windows, so pin the invariant here.
+        let spec: usage::Spec = r#"
+            arg "<some file>"
+            flag "--dry-run"
+            "#
+        .parse()
+        .unwrap();
+        let args = ["test", "x", "--dry-run"].map(String::from);
+        let env = usage::parse(&spec, &args).unwrap().as_env();
+
+        assert!(!env.is_empty());
+        for key in env.keys() {
+            assert!(
+                !key.is_empty() && !key.contains(':') && !key.contains('/'),
+                "as_env produced a key that cannot go in WSLENV: {key}"
+            );
+        }
+    }
+}