diff --git a/hive-agent/src/paths.rs b/hive-agent/src/paths.rs
index fdc4e968..dc4ceddc 100644
--- a/hive-agent/src/paths.rs
+++ b/hive-agent/src/paths.rs
@@ -131,3 +131,38 @@ pub fn claude_dir() -> PathBuf {
     }
     PathBuf::from("/root/.claude")
 }
+
+/// Run `f` with `HYPERHIVE_HARNESS_DIR` pointing at a fresh temp dir, then
+/// restore the previous value. Tests that build a `Bus` or read the harness
+/// state files go through this: the variable is process-wide, so the calls
+/// are serialised on a lock.
+#[cfg(test)]
+pub fn with_harness_dir<T>(f: impl FnOnce() -> T) -> T {
+    use std::ffi::OsString;
+    use std::sync::Mutex;
+
+    static LOCK: Mutex<()> = Mutex::new(());
+
+    struct Restore(Option<OsString>);
+    impl Drop for Restore {
+        fn drop(&mut self) {
+            // SAFETY: still holding `LOCK`, the only writer of this variable.
+            unsafe {
+                match self.0.take() {
+                    Some(v) => std::env::set_var("HYPERHIVE_HARNESS_DIR", v),
+                    None => std::env::remove_var("HYPERHIVE_HARNESS_DIR"),
+                }
+            }
+        }
+    }
+
+    let _lock = LOCK
+        .lock()
+        .unwrap_or_else(std::sync::PoisonError::into_inner);
+    let dir = tempfile::tempdir().unwrap();
+    // Declared after `_lock` so the variable is restored before the lock drops.
+    let _restore = Restore(std::env::var_os("HYPERHIVE_HARNESS_DIR"));
+    // SAFETY: serialised by `LOCK`.
+    unsafe { std::env::set_var("HYPERHIVE_HARNESS_DIR", dir.path()) };
+    f()
+}
diff --git a/hive-agent/src/swarm_inference.rs b/hive-agent/src/swarm_inference.rs
index 3be47c2d..eebb3d50 100644
--- a/hive-agent/src/swarm_inference.rs
+++ b/hive-agent/src/swarm_inference.rs
@@ -320,34 +320,29 @@ mod tests {
     /// persisted files, so clearing it, or a restart, finds the local value.
     #[test]
     fn the_swarm_value_is_not_persisted_over_the_local_one() {
-        let dir = tempfile::tempdir().unwrap();
-        let prev = std::env::var_os("HYPERHIVE_HARNESS_DIR");
-        // SAFETY: no other test in this crate reads or sets this variable.
-        unsafe { std::env::set_var("HYPERHIVE_HARNESS_DIR", dir.path()) };
-        let bus = Bus::new();
-        bus.set_model("sonnet");
-        bus.set_effort("low");
-        let target = Harness {
-            bus: bus.clone(),
-            nix_model: None,
-            nix_effort: None,
-        };
-        let mut o = Overridden::default();
-        o.apply(&set(Some("opus"), Some(Effort::Max)), &target);
-        let swarm = (bus.model(), bus.effort());
-        let swarm_sources = (bus.model_source(), bus.effort_source());
-        let files = (
-            crate::harness_state::load_model(),
-            crate::harness_state::load_effort(),
-        );
-        o.apply(&AgentInference::default(), &target);
-        let cleared = (bus.model(), bus.effort());
-        let cleared_sources = (bus.model_source(), bus.effort_source());
-        match prev {
-            // SAFETY: as above.
-            Some(v) => unsafe { std::env::set_var("HYPERHIVE_HARNESS_DIR", v) },
-            None => unsafe { std::env::remove_var("HYPERHIVE_HARNESS_DIR") },
-        }
+        let (swarm, swarm_sources, files, cleared, cleared_sources) =
+            crate::paths::with_harness_dir(|| {
+                let bus = Bus::new();
+                bus.set_model("sonnet");
+                bus.set_effort("low");
+                let target = Harness {
+                    bus: bus.clone(),
+                    nix_model: None,
+                    nix_effort: None,
+                };
+                let mut o = Overridden::default();
+                o.apply(&set(Some("opus"), Some(Effort::Max)), &target);
+                let swarm = (bus.model(), bus.effort());
+                let swarm_sources = (bus.model_source(), bus.effort_source());
+                let files = (
+                    crate::harness_state::load_model(),
+                    crate::harness_state::load_effort(),
+                );
+                o.apply(&AgentInference::default(), &target);
+                let cleared = (bus.model(), bus.effort());
+                let cleared_sources = (bus.model_source(), bus.effort_source());
+                (swarm, swarm_sources, files, cleared, cleared_sources)
+            });
         assert_eq!(swarm, pair("opus", "max"));
         assert_eq!(files, (Some("sonnet".to_owned()), Some("low".to_owned())));
         assert_eq!(cleared, pair("sonnet", "low"));
diff --git a/hive-agent/src/turn.rs b/hive-agent/src/turn.rs
index 4828d753..156e83ef 100644
--- a/hive-agent/src/turn.rs
+++ b/hive-agent/src/turn.rs
@@ -983,16 +983,18 @@ mod tests {
     }
 
     fn compact_report(result: Result<(), &str>) -> (bool, Vec<String>) {
-        let bus = Bus::new();
-        let mut rx = bus.subscribe();
-        let ok = report_compact(&bus, result);
-        let mut notes = Vec::new();
-        while let Ok(ev) = rx.try_recv() {
-            if let LiveEvent::Note { text } = ev.event {
-                notes.push(text);
+        crate::paths::with_harness_dir(|| {
+            let bus = Bus::new();
+            let mut rx = bus.subscribe();
+            let ok = report_compact(&bus, result);
+            let mut notes = Vec::new();
+            while let Ok(ev) = rx.try_recv() {
+                if let LiveEvent::Note { text } = ev.event {
+                    notes.push(text);
+                }
             }
-        }
-        (ok, notes)
+            (ok, notes)
+        })
     }
 
     #[test]