diff --git a/crates/core/src/tuning.rs b/crates/core/src/tuning.rs
index dd2b742..70ec908 100644
--- a/crates/core/src/tuning.rs
+++ b/crates/core/src/tuning.rs
@@ -63,24 +63,38 @@ pub fn inference_thread_count(_workload: Workload, _gpu_offloaded: bool) -> usiz
 mod tests {
     use super::*;
 
+    /// Serialises tests that read/write `MAGNOTIA_INFERENCE_THREADS` so
+    /// they don't race under cargo's parallel test runner.
+    /// Mirrors the pattern used by `power::with_override`.
+    fn with_thread_env_lock<R>(body: impl FnOnce() -> R) -> R {
+        use std::sync::Mutex;
+        static THREAD_ENV_LOCK: Mutex<()> = Mutex::new(());
+        let _lock = THREAD_ENV_LOCK.lock().expect("tuning thread-env lock poisoned");
+        body()
+    }
+
     #[test]
     fn matches_existing_clamp_when_no_clamps_apply() {
         // With no env override, no battery, no gpu_offload, helper
         // should return physical-core count clamped to [2, 8].
         // We can't pin physical exactly without mocking num_cpus; just
         // assert the result is in range.
-        std::env::remove_var("MAGNOTIA_INFERENCE_THREADS");
-        let n = inference_thread_count(Workload::Llm, false);
-        assert!(n >= MIN_INFERENCE_THREADS && n <= MAX_INFERENCE_THREADS,
-                "got {n}, expected within [{MIN_INFERENCE_THREADS}, {MAX_INFERENCE_THREADS}]");
+        with_thread_env_lock(|| {
+            std::env::remove_var("MAGNOTIA_INFERENCE_THREADS");
+            let n = inference_thread_count(Workload::Llm, false);
+            assert!(n >= MIN_INFERENCE_THREADS && n <= MAX_INFERENCE_THREADS,
+                    "got {n}, expected within [{MIN_INFERENCE_THREADS}, {MAX_INFERENCE_THREADS}]");
+        });
     }
 
     #[test]
     fn env_var_bypasses_clamps() {
-        std::env::set_var("MAGNOTIA_INFERENCE_THREADS", "10");
-        let n = inference_thread_count(Workload::Llm, true);
-        assert_eq!(n, 10);
-        std::env::remove_var("MAGNOTIA_INFERENCE_THREADS");
+        with_thread_env_lock(|| {
+            std::env::set_var("MAGNOTIA_INFERENCE_THREADS", "10");
+            let n = inference_thread_count(Workload::Llm, true);
+            assert_eq!(n, 10);
+            std::env::remove_var("MAGNOTIA_INFERENCE_THREADS");
+        });
     }
 
     #[test]