diff --git a/hive-subagent-mcp/src/session.rs b/hive-subagent-mcp/src/session.rs
index 18868a2c..78d998fc 100644
--- a/hive-subagent-mcp/src/session.rs
+++ b/hive-subagent-mcp/src/session.rs
@@ -3012,11 +3012,34 @@ mod tests {
 
     #[test]
     fn a_signal_url_reaches_the_subagent_and_a_status_check_renders_no_config() {
+        // `turn` actually writes an --mcp-config file (crate::mcp_config::build),
+        // which resolves `hive_agent_sock::paths::harness_dir` — normally the
+        // container's injected `HYPERHIVE_HARNESS_DIR`, unset in a plain `cargo
+        // test` sandbox. Point it at a scratch dir for just this test rather
+        // than depending on ambient environment; only this test's call path
+        // reads the var, so there's no cross-test race to sequence around
+        // (contrast `otel_attrs_append_ambient_value_or_stand_alone` above).
+        let dir = std::env::temp_dir().join(format!(
+            "hive-subagent-mcp-test-harness-dir-{}",
+            std::process::id()
+        ));
+        // SAFETY: test-only env mutation; no other test reads this var.
+        unsafe {
+            std::env::set_var("HYPERHIVE_HARNESS_DIR", &dir);
+        }
+
         // The signal surface is how `goal_reached` is callable at all, so a
         // spawned turn's config has to carry it; `status` builds a config
         // purely to resolve the store and has no subagent to hand it to.
         let turn = build_config("n", None, None, None, None, Some(&signal_url()));
         let checked = build_config("n", None, None, None, None, None);
+
+        // SAFETY: see above.
+        unsafe {
+            std::env::remove_var("HYPERHIVE_HARNESS_DIR");
+        }
+        let _ = std::fs::remove_dir_all(&dir);
+
         assert!(
             turn.mcp_config.is_some(),
             "a turn's config must carry an --mcp-config with the signal surface in it"