diff --git a/src-gossamer/src/boj/commands.rs b/src-gossamer/src/boj/commands.rs
index e3e40fe..a95c733 100644
--- a/src-gossamer/src/boj/commands.rs
+++ b/src-gossamer/src/boj/commands.rs
@@ -106,6 +106,7 @@ mod tests {
 
     #[test]
     fn boj_url_default_and_override() {
+        let _env = crate::test_env::lock();
         std::env::remove_var("BOJ_URL");
         assert_eq!(boj_url(), DEFAULT_BOJ_URL);
         let custom = "http://boj.local:9999/v2";
diff --git a/src-gossamer/src/boj_live.rs b/src-gossamer/src/boj_live.rs
index 46b43c1..1127734 100644
--- a/src-gossamer/src/boj_live.rs
+++ b/src-gossamer/src/boj_live.rs
@@ -122,6 +122,7 @@ mod tests {
     /// Verify the endpoint builder respects BOJ_URL and strips trailing slashes.
     #[test]
     fn boj_endpoint_default_and_override() {
+        let _env = crate::test_env::lock();
         std::env::remove_var("BOJ_URL");
         let ep = boj_endpoint();
         assert_eq!(ep.base_url, DEFAULT_BOJ_URL);
diff --git a/src-gossamer/src/governance/commands.rs b/src-gossamer/src/governance/commands.rs
index aa2c38a..cd49d85 100644
--- a/src-gossamer/src/governance/commands.rs
+++ b/src-gossamer/src/governance/commands.rs
@@ -134,12 +134,14 @@ mod tests {
 
     #[test]
     fn boj_url_default() {
+        let _env = crate::test_env::lock();
         std::env::remove_var("BOJ_URL");
         assert_eq!(boj_url(), DEFAULT_BOJ_URL);
     }
 
     #[test]
     fn boj_url_override() {
+        let _env = crate::test_env::lock();
         let custom = "http://nesy.local:8800/v2";
         std::env::set_var("BOJ_URL", custom);
         assert_eq!(boj_url(), custom);
diff --git a/src-gossamer/src/hypatia/commands.rs b/src-gossamer/src/hypatia/commands.rs
index bfcb72d..bf09ac0 100644
--- a/src-gossamer/src/hypatia/commands.rs
+++ b/src-gossamer/src/hypatia/commands.rs
@@ -117,6 +117,7 @@ mod tests {
     /// Verify the default Hypatia URL is the expected localhost endpoint.
     #[test]
     fn smoke_hypatia_url_default() {
+        let _env = crate::test_env::lock();
         // Unset the env var to ensure we get the hardcoded default.
         std::env::remove_var("HYPATIA_URL");
         let url = hypatia_url();
@@ -126,6 +127,7 @@ mod tests {
     /// Verify that HYPATIA_URL env var overrides the default.
     #[test]
     fn smoke_hypatia_url_override() {
+        let _env = crate::test_env::lock();
         std::env::set_var("HYPATIA_URL", "http://custom-host:9999/api/v2");
         let url = hypatia_url();
         assert_eq!(url, "http://custom-host:9999/api/v2");
diff --git a/src-gossamer/src/lib.rs b/src-gossamer/src/lib.rs
index 97328a3..c1c5ac0 100644
--- a/src-gossamer/src/lib.rs
+++ b/src-gossamer/src/lib.rs
@@ -95,6 +95,23 @@ pub mod watcher;
 pub mod wiring_inspector;
 pub mod workspace;
 
+/// Serialises the tests that set or remove process environment variables
+/// (`BOJ_URL`, `OVERLAY_URL`, `HYPATIA_URL`, `TYPELL_URL`). `cargo test` runs
+/// tests on parallel threads and the environment is process-wide, so without
+/// this lock one test can read the value another has just set.
+#[cfg(test)]
+pub(crate) mod test_env {
+    use std::sync::{Mutex, MutexGuard};
+
+    static LOCK: Mutex<()> = Mutex::new(());
+
+    /// Hold the guard for the whole test. A test that panics while holding it
+    /// poisons the lock; the next test takes it anyway.
+    pub(crate) fn lock() -> MutexGuard<'static, ()> {
+        LOCK.lock().unwrap_or_else(|e| e.into_inner())
+    }
+}
+
 /// Cross-module integration tests. These exercise the public command-backed
 /// functions end-to-end. They live in the lib (not `tests/`) so they run via
 /// `cargo test --lib` without cargo also force-building the GTK-linked
diff --git a/src-gossamer/src/overlay/commands.rs b/src-gossamer/src/overlay/commands.rs
index 1adfe9c..33c4426 100644
--- a/src-gossamer/src/overlay/commands.rs
+++ b/src-gossamer/src/overlay/commands.rs
@@ -273,6 +273,7 @@ mod tests {
 
     #[test]
     fn overlay_url_default_and_override() {
+        let _env = crate::test_env::lock();
         std::env::remove_var("OVERLAY_URL");
         assert_eq!(overlay_url(), DEFAULT_OVERLAY_URL);
         let custom = "http://overlay.local:9999/v2";
diff --git a/src-gossamer/src/typell/commands.rs b/src-gossamer/src/typell/commands.rs
index f4043d5..90a8369 100644
--- a/src-gossamer/src/typell/commands.rs
+++ b/src-gossamer/src/typell/commands.rs
@@ -104,6 +104,7 @@ mod tests {
 
     #[test]
     fn typell_url_default_and_override() {
+        let _env = crate::test_env::lock();
         std::env::remove_var("TYPELL_URL");
         assert_eq!(typell_url(), DEFAULT_TYPELL_URL);
         let custom = "http://typell.local:9999/v2";